[Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Sergey Kaplun via Tarantool-patches]

When there is only one frame (so-called dummy frame) with a dummy thread
on stack bottom at L->base - 1 - LJ_FR2, stack dump does not show stack
slots from bottom to top, because the frame link is already pointed to
stack bottom (dummy thread mentioned above). Its output looks like the
following:

| 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| 0x7fb512ac38            [   M]
| 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| 0x7fb512ab20            [  T ]
| 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L

This patch unrolled first loop iteration. Stack dump first inspect all
fields for the top frame and then continue if there are other frames.
After patch the output looks like the following:

| 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| 0x7fb512ac38            [   M]
| 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| 0x7fb512ab20            [  T ]
| 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
| 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
| 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
---

Branch: https://github.com/tarantool/luajit/tree/skaplun/fix-luajit-gdb

Steps to test behaviour:
0) Clone branch
1) Build tarantool with GC64 or not (I've tested both cases)
2) Run the following script </tmp/net_box.lua> by Tarantool via gdb.

| $ cat /tmp/netbox.lua
| box.cfg{log_level = 50, listen = "127.0.0.1:3802"}
| box.schema.user.grant('guest','read,write,execute','universe', nil, {if_not_exists=true})
| local net_box = require"net.box"
| local cn = net_box:connect("127.0.0.1:3802")
| local function f()
|   return setmetatable({}, {__serialize = function() error[[err in ser]] end})
| end
| _G.f = f
| local r, st = cn:call("f")
| print(r,st)

| $ gdb --args ./tarantool /tmp/netbox.lua
| ...
| (gdb) source ~/builds_workspace/luajit/fix-luajit-gdb/src/luajit-gdb.py
| ...

3) Set breakpoint to ./src/lua/msgpack.c:234, and run

| (gdb) b ./src/lua/msgpack.c:234

I'm not sure about your sources version, so you can just set breakpoin to
mp_encode(), run and continue (need two entries). We are interested in
the line #234:

| 233|.......if (luaL_tofield(L, cfg, opts, lua_gettop(L), &field) < 0)
| 234|.......|.......return luaT_error(L);

And dump the stack:

| (gdb) lj-stack L
| 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| 0x7fb512ac38            [   M]
| 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| 0x7fb512ab20            [  T ]
| 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
| 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
| 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L

 src/luajit-gdb.py | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/luajit-gdb.py b/src/luajit-gdb.py
index f1fd6230..a3dad585 100644
--- a/src/luajit-gdb.py
+++ b/src/luajit-gdb.py
@@ -424,16 +424,26 @@ def dump_stack(L, base=None, top=None):
         )
     ]) + '\n'
 
+    # Set framelink for the top frame. Dump frame slots even for dummy frame.
     slot = top - 1
     framelink = base - (1 + LJ_FR2)
+    while slot > framelink + LJ_FR2:
+        dump += dump_stack_slot(L, slot, base, top)
+        slot -= 1
 
+    # If there are other frames -- continue.
     while framelink > mref('TValue *', L['stack']):
-        while slot > framelink + LJ_FR2:
-            dump += dump_stack_slot(L, slot, base, top)
-            slot -= 1
+        assert slot == framelink + LJ_FR2, "Invalid slot during frame unwind"
         dump += dump_framelink(L, framelink)
         framelink = frame_prev(framelink + LJ_FR2) - LJ_FR2
         slot -= 1 + LJ_FR2
+        while slot > framelink + LJ_FR2:
+            dump += dump_stack_slot(L, slot, base, top)
+            slot -= 1
+
+    assert slot == framelink + LJ_FR2, "Invalid slot after frame unwind"
+    # Skip nil slot for the last frame for 2-slot frames.
+    slot -= LJ_FR2
 
     dump += '{fr}{padding} [S   ] FRAME: dummy L'.format(
         fr = slot,
-- 
2.31.0

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Sergey Kaplun via Tarantool-patches]

Hi!

I've changed the commit head to the following:
| gdb: fix dump stack without function frame

Branch is force-pushed.

-- 
Best regards,
Sergey Kaplun

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Igor Munkin via Tarantool-patches]

Sergey,

Thanks for the patch! Nice catch, actually! I wonder, how much impact
this bug made while digging the crash reports. Anyway, LGTM, but
consider several nits below.

On 18.06.21, Sergey Kaplun wrote:
> When there is only one frame (so-called dummy frame) with a dummy thread
> on stack bottom at L->base - 1 - LJ_FR2, stack dump does not show stack
> slots from bottom to top, because the frame link is already pointed to
> stack bottom (dummy thread mentioned above). Its output looks like the

Too much information in a single sentence. I consider the following info
vital to understand the patch.
* Dummy frame is the particular and kinda "initial"  coroutine state,
  when the framelink slot (i.e. L->base - (1 + LJ_FR2)) is the most
  bottom slot of the guest stack (i.e. L->stack).
* Since coroutine stack unwinding is implemented via precondition loop,
  no slots are dumped from L->top to L->base in case when only dummy
  frame is on the stack.
* Python is a crap language, and seems like postcondition loop violates
  The Zen (this is too hard for users, isn't it?), so Guido decided not
  to introduce such construction in the language at all.

  #ifdef offtop
    I dare to guess, it was just too hard to distinguish precondition
    from postcondition in "colon&spaces" paradigm; glad Larry Wall
    decided to use braces for blocks, and Roberto just left Pascalish
    syntax for this. I'm upset with the GDB guys choice every time we're
    back to developing this extension.
  #endif

comment.yield()

> following:
> 
> | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | 0x7fb512ac38            [   M]
> | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | 0x7fb512ab20            [  T ]
> | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L

Minor: I guess, you can align the output. At least I see no reason not
to do this.

> 

comment.resume()

Hence, there is no other option except unroll the first iteration and
reorder stack slots dump with moving framelinks downwards to the
coroutine stack bottom.

> This patch unrolled first loop iteration. Stack dump first inspect all
> fields for the top frame and then continue if there are other frames.
> After patch the output looks like the following:
> 
> | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | 0x7fb512ac38            [   M]
> | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | 0x7fb512ab20            [  T ]
> | 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> | 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L

Feel free to align the output here too.

> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/skaplun/fix-luajit-gdb
> 
> Steps to test behaviour:
> 0) Clone branch
> 1) Build tarantool with GC64 or not (I've tested both cases)
> 2) Run the following script </tmp/net_box.lua> by Tarantool via gdb.
> 
> | $ cat /tmp/netbox.lua
> | box.cfg{log_level = 50, listen = "127.0.0.1:3802"}
> | box.schema.user.grant('guest','read,write,execute','universe', nil, {if_not_exists=true})
> | local net_box = require"net.box"
> | local cn = net_box:connect("127.0.0.1:3802")
> | local function f()
> |   return setmetatable({}, {__serialize = function() error[[err in ser]] end})
> | end
> | _G.f = f
> | local r, st = cn:call("f")
> | print(r,st)
> 
> | $ gdb --args ./tarantool /tmp/netbox.lua
> | ...
> | (gdb) source ~/builds_workspace/luajit/fix-luajit-gdb/src/luajit-gdb.py
> | ...
> 
> 3) Set breakpoint to ./src/lua/msgpack.c:234, and run
> 
> | (gdb) b ./src/lua/msgpack.c:234
> 
> I'm not sure about your sources version, so you can just set breakpoin to
> mp_encode(), run and continue (need two entries). We are interested in
> the line #234:
> 
> | 233|.......if (luaL_tofield(L, cfg, opts, lua_gettop(L), &field) < 0)
> | 234|.......|.......return luaT_error(L);
> 
> And dump the stack:
> 
> | (gdb) lj-stack L
> | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | 0x7fb512ac38            [   M]
> | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | 0x7fb512ab20            [  T ]
> | 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> | 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
> 
>  src/luajit-gdb.py | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/src/luajit-gdb.py b/src/luajit-gdb.py
> index f1fd6230..a3dad585 100644
> --- a/src/luajit-gdb.py
> +++ b/src/luajit-gdb.py
> @@ -424,16 +424,26 @@ def dump_stack(L, base=None, top=None):
>          )
>      ]) + '\n'
>  
> +    # Set framelink for the top frame. Dump frame slots even for dummy frame.

It would be also great to drop a few words why the iteration is unrolled
(the lack of postcondition loops in Python) via XXX comment.

>      slot = top - 1
>      framelink = base - (1 + LJ_FR2)

I suggest to split this part into two blocks via blank line:
* variable initialization
* unrolled iteration

> +    while slot > framelink + LJ_FR2:
> +        dump += dump_stack_slot(L, slot, base, top)
> +        slot -= 1
>  
> +    # If there are other frames -- continue.

Again, this is not the particular reason. As for me the main reason you
can't implement stack unwinding via do-while construction.

>      while framelink > mref('TValue *', L['stack']):
> -        while slot > framelink + LJ_FR2:
> -            dump += dump_stack_slot(L, slot, base, top)
> -            slot -= 1
> +        assert slot == framelink + LJ_FR2, "Invalid slot during frame unwind"

Please, use parentheses for assert call.

>          dump += dump_framelink(L, framelink)
>          framelink = frame_prev(framelink + LJ_FR2) - LJ_FR2
>          slot -= 1 + LJ_FR2
> +        while slot > framelink + LJ_FR2:
> +            dump += dump_stack_slot(L, slot, base, top)
> +            slot -= 1
> +
> +    assert slot == framelink + LJ_FR2, "Invalid slot after frame unwind"

Ditto.

> +    # Skip nil slot for the last frame for 2-slot frames.
> +    slot -= LJ_FR2
>  
>      dump += '{fr}{padding} [S   ] FRAME: dummy L'.format(
>          fr = slot,
> -- 
> 2.31.0
> 

-- 
Best regards,
IM

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Sergey Kaplun via Tarantool-patches]

Igor,

On 20.07.21, Igor Munkin wrote:
> Sergey,
> 
> Thanks for the patch! Nice catch, actually! I wonder, how much impact
> this bug made while digging the crash reports. Anyway, LGTM, but
> consider several nits below.

Actually, not too much. It is the unique situation, when we have naked
guest Lua stack without any function call.

Branch is force-pushed with the changes below.

> 
> On 18.06.21, Sergey Kaplun wrote:
> > When there is only one frame (so-called dummy frame) with a dummy thread
> > on stack bottom at L->base - 1 - LJ_FR2, stack dump does not show stack
> > slots from bottom to top, because the frame link is already pointed to
> > stack bottom (dummy thread mentioned above). Its output looks like the
> 
> Too much information in a single sentence. I consider the following info
> vital to understand the patch.
> * Dummy frame is the particular and kinda "initial"  coroutine state,
>   when the framelink slot (i.e. L->base - (1 + LJ_FR2)) is the most
>   bottom slot of the guest stack (i.e. L->stack).
> * Since coroutine stack unwinding is implemented via precondition loop,
>   no slots are dumped from L->top to L->base in case when only dummy
>   frame is on the stack.
> * Python is a crap language, and seems like postcondition loop violates
>   The Zen (this is too hard for users, isn't it?), so Guido decided not
>   to introduce such construction in the language at all.
> 
>   #ifdef offtop
>     I dare to guess, it was just too hard to distinguish precondition
>     from postcondition in "colon&spaces" paradigm; glad Larry Wall
>     decided to use braces for blocks, and Roberto just left Pascalish
>     syntax for this. I'm upset with the GDB guys choice every time we're
>     back to developing this extension.

I suppose that it just was rather simple and one of the most popular
languages at the moment of making decision.

>   #endif
> 
> comment.yield()
> 
> > following:
> > 
> > | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> > | 0x7fb512ac38            [   M]
> > | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> > | 0x7fb512ab20            [  T ]
> > | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L
> 
> Minor: I guess, you can align the output. At least I see no reason not
> to do this.
> 
> > 
> 
> comment.resume()
> 
> Hence, there is no other option except unroll the first iteration and
> reorder stack slots dump with moving framelinks downwards to the
> coroutine stack bottom.
> 
> > This patch unrolled first loop iteration. Stack dump first inspect all
> > fields for the top frame and then continue if there are other frames.
> > After patch the output looks like the following:
> > 
> > | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> > | 0x7fb512ac38            [   M]
> > | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> > | 0x7fb512ab20            [  T ]
> > | 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> > | 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> > | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
> 
> Feel free to align the output here too.

Thanks for your comments!
The new commit message version:

| Dummy frame is the "initial" coroutine state, when the framelink slot (i.e.
| L->base - (1 + LJ_FR2)) is the bottom slot of the guest stack
| (i.e. L->stack).  Since coroutine stack unwinding is implemented via
| precondition loop, lj-stack doesn't dump the slots for the dummy frame, since
| the framelink points to the stack bottom.
|
| The output looks like the following:
|
| | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| | 0x7fb512ac38              [   M]
| | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| | 0x7fb512ab20              [  T ]
| | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L
|
| Python doesn't provide post-condition (do-while) syntax construction,
| that fits better for this case, so the unwinding of the topmost frame is just manually
| unrolled.
|
| As a result of the patch the output looks like the following:
|
| | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| | 0x7fb512ac38              [   M]
| | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| | 0x7fb512ab20              [  T ]
| | 0x7fb512ab18              [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
| | 0x7fb512ab10              [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
| | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L

> 
> > ---
> > 
> > Branch: https://github.com/tarantool/luajit/tree/skaplun/fix-luajit-gdb
> > 
> > Steps to test behaviour:
> > 0) Clone branch
> > 1) Build tarantool with GC64 or not (I've tested both cases)
> > 2) Run the following script </tmp/net_box.lua> by Tarantool via gdb.
> > 
> > | $ cat /tmp/netbox.lua
> > | box.cfg{log_level = 50, listen = "127.0.0.1:3802"}
> > | box.schema.user.grant('guest','read,write,execute','universe', nil, {if_not_exists=true})
> > | local net_box = require"net.box"
> > | local cn = net_box:connect("127.0.0.1:3802")
> > | local function f()
> > |   return setmetatable({}, {__serialize = function() error[[err in ser]] end})
> > | end
> > | _G.f = f
> > | local r, st = cn:call("f")
> > | print(r,st)
> > 
> > | $ gdb --args ./tarantool /tmp/netbox.lua
> > | ...
> > | (gdb) source ~/builds_workspace/luajit/fix-luajit-gdb/src/luajit-gdb.py
> > | ...
> > 
> > 3) Set breakpoint to ./src/lua/msgpack.c:234, and run
> > 
> > | (gdb) b ./src/lua/msgpack.c:234
> > 
> > I'm not sure about your sources version, so you can just set breakpoin to
> > mp_encode(), run and continue (need two entries). We are interested in
> > the line #234:
> > 
> > | 233|.......if (luaL_tofield(L, cfg, opts, lua_gettop(L), &field) < 0)
> > | 234|.......|.......return luaT_error(L);
> > 
> > And dump the stack:
> > 
> > | (gdb) lj-stack L
> > | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> > | 0x7fb512ac38            [   M]
> > | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> > | 0x7fb512ab20            [  T ]
> > | 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> > | 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> > | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
> > 
> >  src/luajit-gdb.py | 16 +++++++++++++---
> >  1 file changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/src/luajit-gdb.py b/src/luajit-gdb.py
> > index f1fd6230..a3dad585 100644
> > --- a/src/luajit-gdb.py
> > +++ b/src/luajit-gdb.py
> > @@ -424,16 +424,26 @@ def dump_stack(L, base=None, top=None):
> >          )
> >      ]) + '\n'
> >  
> > +    # Set framelink for the top frame. Dump frame slots even for dummy frame.
> 
> It would be also great to drop a few words why the iteration is unrolled
> (the lack of postcondition loops in Python) via XXX comment.
> 
> >      slot = top - 1
> >      framelink = base - (1 + LJ_FR2)
> 
> I suggest to split this part into two blocks via blank line:
> * variable initialization
> * unrolled iteration
> 
> > +    while slot > framelink + LJ_FR2:
> > +        dump += dump_stack_slot(L, slot, base, top)
> > +        slot -= 1
> >  
> > +    # If there are other frames -- continue.
> 
> Again, this is not the particular reason. As for me the main reason you
> can't implement stack unwinding via do-while construction.

Adjusted this chunk considering your suggestions.

===================================================================
diff --git a/src/luajit-gdb.py b/src/luajit-gdb.py
index a3dad585..fab276ed 100644
--- a/src/luajit-gdb.py
+++ b/src/luajit-gdb.py
@@ -424,14 +424,22 @@ def dump_stack(L, base=None, top=None):
         )
     ]) + '\n'
 
-    # Set framelink for the top frame. Dump frame slots even for dummy frame.
     slot = top - 1
     framelink = base - (1 + LJ_FR2)
+
+    # XXX: Lua stack unwinding algorithm consists of the following steps:
+    # 1. dump all data slots in the (framelink, top) interval
+    # 2. check whether there are remaining frames
+    # 3. if there are no slots further, stop the unwinding loop
+    # 4. otherwise, resolve the next framelink and top and go to (1)
+    #
+    # Postcondition (i.e. do-while) loops is the most fitting idiom for such
+    # case, but Python doesn't provide such lexical construction. Hence step (1)
+    # is unrolled for the topmost stack frame.
     while slot > framelink + LJ_FR2:
         dump += dump_stack_slot(L, slot, base, top)
         slot -= 1
 
-    # If there are other frames -- continue.
     while framelink > mref('TValue *', L['stack']):
         assert slot == framelink + LJ_FR2, "Invalid slot during frame unwind"
         dump += dump_framelink(L, framelink)
@@ -442,7 +450,7 @@ def dump_stack(L, base=None, top=None):
             slot -= 1
 
     assert slot == framelink + LJ_FR2, "Invalid slot after frame unwind"
-    # Skip nil slot for the last frame for 2-slot frames.
+    # Skip a nil slot for the last frame for 2-slot frames.
     slot -= LJ_FR2
 
     dump += '{fr}{padding} [S   ] FRAME: dummy L'.format(
===================================================================

> 
> >      while framelink > mref('TValue *', L['stack']):
> > -        while slot > framelink + LJ_FR2:
> > -            dump += dump_stack_slot(L, slot, base, top)
> > -            slot -= 1
> > +        assert slot == framelink + LJ_FR2, "Invalid slot during frame unwind"
> 
> Please, use parentheses for assert call.

It's done by design to avoid always true assertions [1]:

| $ python3
| Python 3.7.10 (default, Mar 18 2021, 04:43:06)
| [GCC 9.3.0] on linux
| Type "help", "copyright", "credits" or "license" for more information.
| >>> assert(1!=1, "stupid assert")
| <stdin>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?

> 
> >          dump += dump_framelink(L, framelink)
> >          framelink = frame_prev(framelink + LJ_FR2) - LJ_FR2
> >          slot -= 1 + LJ_FR2
> > +        while slot > framelink + LJ_FR2:
> > +            dump += dump_stack_slot(L, slot, base, top)
> > +            slot -= 1
> > +
> > +    assert slot == framelink + LJ_FR2, "Invalid slot after frame unwind"
> 
> Ditto.

Ditto.

> 
> > +    # Skip nil slot for the last frame for 2-slot frames.
> > +    slot -= LJ_FR2
> >  
> >      dump += '{fr}{padding} [S   ] FRAME: dummy L'.format(
> >          fr = slot,
> > -- 
> > 2.31.0
> > 
> 
> -- 
> Best regards,
> IM

[1]: https://docs.python.org/3/reference/simple_stmts.html#grammar-token-assert-stmt

-- 
Best regards,
Sergey Kaplun

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Igor Munkin via Tarantool-patches]

Sergey,

Thanks for the fixes! Everything is perfect now, I'll ask Sergos to
proceed with the review.

On 21.07.21, Sergey Kaplun wrote:
> Igor,
> 
> On 20.07.21, Igor Munkin wrote:
> > Sergey,
> > 
> > Thanks for the patch! Nice catch, actually! I wonder, how much impact
> > this bug made while digging the crash reports. Anyway, LGTM, but
> > consider several nits below.
> 
> Actually, not too much. It is the unique situation, when we have naked
> guest Lua stack without any function call.

Hm, I guess this bug affects all the cases when dummy frame was used,
but you know it better anyway :)

> 
> Branch is force-pushed with the changes below.
> 
> > 
> > On 18.06.21, Sergey Kaplun wrote:

<snipped>

> 
> Thanks for your comments!
> The new commit message version:
> 
> | Dummy frame is the "initial" coroutine state, when the framelink slot (i.e.
> | L->base - (1 + LJ_FR2)) is the bottom slot of the guest stack
> | (i.e. L->stack).  Since coroutine stack unwinding is implemented via
> | precondition loop, lj-stack doesn't dump the slots for the dummy frame, since
> | the framelink points to the stack bottom.
> |
> | The output looks like the following:
> |
> | | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | | 0x7fb512ac38              [   M]
> | | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | | 0x7fb512ab20              [  T ]
> | | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L
> |
> | Python doesn't provide post-condition (do-while) syntax construction,
> | that fits better for this case, so the unwinding of the topmost frame is just manually
> | unrolled.
> |
> | As a result of the patch the output looks like the following:
> |
> | | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | | 0x7fb512ac38              [   M]
> | | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | | 0x7fb512ab20              [  T ]
> | | 0x7fb512ab18              [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> | | 0x7fb512ab10              [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> | | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L

Side note: I guess this is a previous version, since everything is fine
on the branch. BTW, wording is neat!

> 

<snipped>

> > > diff --git a/src/luajit-gdb.py b/src/luajit-gdb.py
> > > index f1fd6230..a3dad585 100644
> > > --- a/src/luajit-gdb.py
> > > +++ b/src/luajit-gdb.py
> > > @@ -424,16 +424,26 @@ def dump_stack(L, base=None, top=None):

<snipped>

> 
> > 
> > >      while framelink > mref('TValue *', L['stack']):
> > > -        while slot > framelink + LJ_FR2:
> > > -            dump += dump_stack_slot(L, slot, base, top)
> > > -            slot -= 1
> > > +        assert slot == framelink + LJ_FR2, "Invalid slot during frame unwind"
> > 
> > Please, use parentheses for assert call.
> 
> It's done by design to avoid always true assertions [1]:
> 
> | $ python3
> | Python 3.7.10 (default, Mar 18 2021, 04:43:06)
> | [GCC 9.3.0] on linux
> | Type "help", "copyright", "credits" or "license" for more information.
> | >>> assert(1!=1, "stupid assert")
> | <stdin>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?

PEBKAC, didn't expect such a surprise here. I looked on the <assert>
usage below, but those parentheses are just excess. Everything is fine
here then, and thanks for clarification!

> 

<snipped>

> 
> [1]: https://docs.python.org/3/reference/simple_stmts.html#grammar-token-assert-stmt
> 
> -- 
> Best regards,
> Sergey Kaplun

-- 
Best regards,
IM

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Sergey Kaplun via Tarantool-patches]

Sorry, one more change:
Fix allignment in the commit message:

| gdb: fix dump stack without function frame
|
| Dummy frame is the "initial" coroutine state, when the framelink slot
| (i.e. L->base - (1 + LJ_FR2)) is the bottom slot of the guest stack
| (i.e. L->stack). Since coroutine stack unwinding is implemented via
| precondition loop, lj-stack doesn't dump the slots for the dummy frame,
| since the framelink points to the stack bottom.
|
| The output looks like the following:
|
| | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| | 0x7fb512ac38              [   M]
| | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| | 0x7fb512ab20              [  T ]
| | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L
|
| Python doesn't provide post-condition (do-while) syntax construction,
| that fits better for this case, so the unwinding of the topmost frame is
| just manually unrolled.
|
| As a result of the patch the output looks like the following:
|
| | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
| | 0x7fb512ac38              [   M]
| | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
| | 0x7fb512ab20              [  T ]
| | 0x7fb512ab18              [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
| | 0x7fb512ab10              [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
| | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L


-- 
Best regards,
Sergey Kaplun

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Sergey Ostanevich via Tarantool-patches]

Hi!

Thanks for the patch!

I would rather remove all mentions of ‘bottom loop’ as we named it at
Intel. I'm not a big pro in python, still the ‘for’ loop works fine 
for me:

If we can unwind to the next frame - dump it (and further)

>>> s=[1]
>>> for i in s:
...   if (i==1):
...     s.append(2)
...     continue
...   print(i)
... 
2

If it is the only one (dummy):

>>> s=[3]
>>> for i in s:
...   if (i==1):
...     s.append(2)
...     continue
...   print(i)
... 
3

Does it make sense in this case?

Anyways, it’s too much of nitpicking - LGTM in its current version.

Regards,
Sergos


> On 21 Jul 2021, at 11:54, Igor Munkin <imun@tarantool.org> wrote:
> 
> Sergey,
> 
> Thanks for the fixes! Everything is perfect now, I'll ask Sergos to
> proceed with the review.
> 
> On 21.07.21, Sergey Kaplun wrote:
>> Igor,
>> 
>> On 20.07.21, Igor Munkin wrote:
>>> Sergey,
>>> 
>>> Thanks for the patch! Nice catch, actually! I wonder, how much impact
>>> this bug made while digging the crash reports. Anyway, LGTM, but
>>> consider several nits below.
>> 
>> Actually, not too much. It is the unique situation, when we have naked
>> guest Lua stack without any function call.
> 
> Hm, I guess this bug affects all the cases when dummy frame was used,
> but you know it better anyway :)
> 
>> 
>> Branch is force-pushed with the changes below.
>> 
>>> 
>>> On 18.06.21, Sergey Kaplun wrote:
> 
> <snipped>
> 
>> 
>> Thanks for your comments!
>> The new commit message version:
>> 
>> | Dummy frame is the "initial" coroutine state, when the framelink slot (i.e.
>> | L->base - (1 + LJ_FR2)) is the bottom slot of the guest stack
>> | (i.e. L->stack).  Since coroutine stack unwinding is implemented via
>> | precondition loop, lj-stack doesn't dump the slots for the dummy frame, since
>> | the framelink points to the stack bottom.
>> |
>> | The output looks like the following:
>> |
>> | | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
>> | | 0x7fb512ac38              [   M]
>> | | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
>> | | 0x7fb512ab20              [  T ]
>> | | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L
>> |
>> | Python doesn't provide post-condition (do-while) syntax construction,
>> | that fits better for this case, so the unwinding of the topmost frame is just manually
>> | unrolled.
>> |
>> | As a result of the patch the output looks like the following:
>> |
>> | | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
>> | | 0x7fb512ac38              [   M]
>> | | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
>> | | 0x7fb512ab20              [  T ]
>> | | 0x7fb512ab18              [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
>> | | 0x7fb512ab10              [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
>> | | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
> 
> Side note: I guess this is a previous version, since everything is fine
> on the branch. BTW, wording is neat!
> 
>> 
> 
> <snipped>
> 
>>>> diff --git a/src/luajit-gdb.py b/src/luajit-gdb.py
>>>> index f1fd6230..a3dad585 100644
>>>> --- a/src/luajit-gdb.py
>>>> +++ b/src/luajit-gdb.py
>>>> @@ -424,16 +424,26 @@ def dump_stack(L, base=None, top=None):
> 
> <snipped>
> 
>> 
>>> 
>>>>     while framelink > mref('TValue *', L['stack']):
>>>> -        while slot > framelink + LJ_FR2:
>>>> -            dump += dump_stack_slot(L, slot, base, top)
>>>> -            slot -= 1
>>>> +        assert slot == framelink + LJ_FR2, "Invalid slot during frame unwind"
>>> 
>>> Please, use parentheses for assert call.
>> 
>> It's done by design to avoid always true assertions [1]:
>> 
>> | $ python3
>> | Python 3.7.10 (default, Mar 18 2021, 04:43:06)
>> | [GCC 9.3.0] on linux
>> | Type "help", "copyright", "credits" or "license" for more information.
>> | >>> assert(1!=1, "stupid assert")
>> | <stdin>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
> 
> PEBKAC, didn't expect such a surprise here. I looked on the <assert>
> usage below, but those parentheses are just excess. Everything is fine
> here then, and thanks for clarification!
> 
>> 
> 
> <snipped>
> 
>> 
>> [1]: https://docs.python.org/3/reference/simple_stmts.html#grammar-token-assert-stmt
>> 
>> -- 
>> Best regards,
>> Sergey Kaplun
> 
> -- 
> Best regards,
> IM

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Igor Munkin via Tarantool-patches]

Sergos,

Thanks for your review! TL;DR: I would rather leave the patch with no
changes, but please consider my comments regarding your alternative
approach for iterating through the guest stack.

On 22.07.21, Sergey Ostanevich wrote:
> Hi!
> 
> Thanks for the patch!
> 
> I would rather remove all mentions of ‘bottom loop’ as we named it at
> Intel. I'm not a big pro in python, still the ‘for’ loop works fine 
> for me:
> 
> If we can unwind to the next frame - dump it (and further)
> 
> >>> s=[1]
> >>> for i in s:
> ...   if (i==1):
> ...     s.append(2)
> ...     continue
> ...   print(i)
> ... 
> 2
> 
> If it is the only one (dummy):
> 
> >>> s=[3]
> >>> for i in s:
> ...   if (i==1):
> ...     s.append(2)
> ...     continue
> ...   print(i)
> ... 
> 3
> 
> Does it make sense in this case?

Let's see what would we get for our case with guest stack unwinding. The
algorithm is described by Sergey in the comment near the loop. But
imagine, we are iterating (backwards) through [L->stack, L-top] segment
via for loop (as you suggest above). There is one condition to check
whether we have reached the bottom of the frame (dump framelink slot and
continue the same way you showed above). However, we need one more
condition to skip the second slot for the framelink in LJ_FR2 mode.  As
a result we have a loop that iterates through the stack with at least
two conditions to skip some slots or dump them in other way.
Furthermore, your proposal leads to much more complex change in the
extension, comparing to those made by Sergey.

As I've already said, postcondition loop is the classic way for this
case. Moreover, I googled "how to emulate a do-while loop in Python?"
and the second option is unrolling the first iteration[1]. If one
wonders what is the first option -- use "while-true" loop with the
conditional break at the end. I equally hate both, so I'm OK with the
one chosen by Sergey. If you prefer "while-true", I believe Sergey can
rewrite this part, but considering your LGTM below, I guess you're OK
also with the current solution. So, I'll push the patch in a jiffy.

> 
> Anyways, it’s too much of nitpicking - LGTM in its current version.
> 
> Regards,
> Sergos
> 
> 

<snipped>

> 

[1]: https://stackoverflow.com/a/743186

-- 
Best regards,
IM

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Sergey Ostanevich via Tarantool-patches]

[-- Attachment #1: Type: text/plain, Size: 2644 bytes --]

Igor,

Sure you can proceed with the push.

Sergos.

> On 22 Jul 2021, at 14:48, Igor Munkin <imun@tarantool.org> wrote:
> 
> Sergos,
> 
> Thanks for your review! TL;DR: I would rather leave the patch with no
> changes, but please consider my comments regarding your alternative
> approach for iterating through the guest stack.
> 
> On 22.07.21, Sergey Ostanevich wrote:
>> Hi!
>> 
>> Thanks for the patch!
>> 
>> I would rather remove all mentions of ‘bottom loop’ as we named it at
>> Intel. I'm not a big pro in python, still the ‘for’ loop works fine 
>> for me:
>> 
>> If we can unwind to the next frame - dump it (and further)
>> 
>>>>> s=[1]
>>>>> for i in s:
>> ...   if (i==1):
>> ...     s.append(2)
>> ...     continue
>> ...   print(i)
>> ... 
>> 2
>> 
>> If it is the only one (dummy):
>> 
>>>>> s=[3]
>>>>> for i in s:
>> ...   if (i==1):
>> ...     s.append(2)
>> ...     continue
>> ...   print(i)
>> ... 
>> 3
>> 
>> Does it make sense in this case?
> 
> Let's see what would we get for our case with guest stack unwinding. The
> algorithm is described by Sergey in the comment near the loop. But
> imagine, we are iterating (backwards) through [L->stack, L-top] segment
> via for loop (as you suggest above). There is one condition to check
> whether we have reached the bottom of the frame (dump framelink slot and
> continue the same way you showed above). However, we need one more
> condition to skip the second slot for the framelink in LJ_FR2 mode.  As
> a result we have a loop that iterates through the stack with at least
> two conditions to skip some slots or dump them in other way.
> Furthermore, your proposal leads to much more complex change in the
> extension, comparing to those made by Sergey.
> 
> As I've already said, postcondition loop is the classic way for this
> case. Moreover, I googled "how to emulate a do-while loop in Python?"
> and the second option is unrolling the first iteration[1]. If one
> wonders what is the first option -- use "while-true" loop with the
> conditional break at the end. I equally hate both, so I'm OK with the
> one chosen by Sergey. If you prefer "while-true", I believe Sergey can
> rewrite this part, but considering your LGTM below, I guess you're OK
> also with the current solution. So, I'll push the patch in a jiffy.
> 
>> 
>> Anyways, it’s too much of nitpicking - LGTM in its current version.
>> 
>> Regards,
>> Sergos
>> 
>> 
> 
> <snipped>
> 
>> 
> 
> [1]: https://stackoverflow.com/a/743186 <https://stackoverflow.com/a/743186>
> 
> -- 
> Best regards,
> IM


[-- Attachment #2: Type: text/html, Size: 29396 bytes --]

Re: [Tarantool-patches] [PATCH luajit v1] tools: fix luajit-gdb stack dump

[Igor Munkin via Tarantool-patches]

Sergey,

I've checked the patch into all long-term branches in tarantool/luajit
and bumped a new version in 1.10, 2.7, 2.8 and master.

On 18.06.21, Sergey Kaplun wrote:
> When there is only one frame (so-called dummy frame) with a dummy thread
> on stack bottom at L->base - 1 - LJ_FR2, stack dump does not show stack
> slots from bottom to top, because the frame link is already pointed to
> stack bottom (dummy thread mentioned above). Its output looks like the
> following:
> 
> | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | 0x7fb512ac38            [   M]
> | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | 0x7fb512ab20            [  T ]
> | 0x7fb512ab08:0x7fb512ab10 [S   ] FRAME: dummy L
> 
> This patch unrolled first loop iteration. Stack dump first inspect all
> fields for the top frame and then continue if there are other frames.
> After patch the output looks like the following:
> 
> | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | 0x7fb512ac38            [   M]
> | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | 0x7fb512ab20            [  T ]
> | 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> | 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/skaplun/fix-luajit-gdb
> 
> Steps to test behaviour:
> 0) Clone branch
> 1) Build tarantool with GC64 or not (I've tested both cases)
> 2) Run the following script </tmp/net_box.lua> by Tarantool via gdb.
> 
> | $ cat /tmp/netbox.lua
> | box.cfg{log_level = 50, listen = "127.0.0.1:3802"}
> | box.schema.user.grant('guest','read,write,execute','universe', nil, {if_not_exists=true})
> | local net_box = require"net.box"
> | local cn = net_box:connect("127.0.0.1:3802")
> | local function f()
> |   return setmetatable({}, {__serialize = function() error[[err in ser]] end})
> | end
> | _G.f = f
> | local r, st = cn:call("f")
> | print(r,st)
> 
> | $ gdb --args ./tarantool /tmp/netbox.lua
> | ...
> | (gdb) source ~/builds_workspace/luajit/fix-luajit-gdb/src/luajit-gdb.py
> | ...
> 
> 3) Set breakpoint to ./src/lua/msgpack.c:234, and run
> 
> | (gdb) b ./src/lua/msgpack.c:234
> 
> I'm not sure about your sources version, so you can just set breakpoin to
> mp_encode(), run and continue (need two entries). We are interested in
> the line #234:
> 
> | 233|.......if (luaL_tofield(L, cfg, opts, lua_gettop(L), &field) < 0)
> | 234|.......|.......return luaT_error(L);
> 
> And dump the stack:
> 
> | (gdb) lj-stack L
> | 0x7fb512ac40:0x7fb512ac70 [    ] 7 slots: Red zone
> | 0x7fb512ac38            [   M]
> | 0x7fb512ab28:0x7fb512ac30 [    ] 34 slots: Free stack slots
> | 0x7fb512ab20            [  T ]
> | 0x7fb512ab18            [    ] VALUE: string 0 "/tmp/net_box.lua:6: err in ser" @ 0x7fb512ade8
> | 0x7fb512ab10            [ B  ] VALUE: table @ 0x7fb512ac80 (asize: 0, hmask: 0x0)
> | 0x7fb512ab00:0x7fb512ab08 [S   ] FRAME: dummy L
> 
>  src/luajit-gdb.py | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 

<snipped>

> -- 
> 2.31.0
> 

-- 
Best regards,
IM