Re: [Tarantool-patches] [PATCH luajit v3] sysprof: fix crash during FFUNC stream

[Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi!
Thanks for the review!
 
  
>Четверг, 13 июля 2023, 11:12 +03:00 от Sergey Bronnikov <sergeyb@tarantool.org>:
> 
>Hi, Max!
>
>Thanks for the patch!
>
>
>Test is passed with reverted patch, so it is useless.
It is not useless. The issue is really hard to reproduce in a stable
way, because it is vital to land in the very small region of the
VM assembly code. I’ve tried to make it happen more often
by increasing the sampling frequency, however our CI is not
capable of handling intense cases like that. I’ve reduced
the frequency, so CI is ok now, but the issue is statistically less
likely to reproduce.
 
I’ve added the comment about that to the test,
so there won’t be any questions regarding that in the future.
>
>
>As I get it right there is a non-zero possibility to catch inconsistency
>in VM state in a future,
>
>so I propose to enable sysprof in our fuzzing test [1] with a high
>sampling interval and check that profiler
>
>will work on generated Lua programs . What do you think?
Sure, I welcome that kind of testing.
>
>
>Also see my comments inline.
>
>
>1.
>https://github.com/ligurio/tarantool/commit/b8bc3293da66ded41383faf5ea913a2554d987b8
>
>
>Sergey
>
>On 7/10/23 15:28, Maxim Kokryashkin wrote:
>> Sometimes, the Lua stack can be inconsistent during
>> the FFUNC execution, which may lead to a sysprof
>> crash during the stack unwinding.
>>
>> This patch replaces the `top_frame` property of `global_State`
>> with `lj_sysprof_topframe` structure, which contains `top_frame`
>> and `ffid` properties. `ffid` property makes sense only when the
>> LuaJIT VM state is set to `FFUNC`. That property is set to the
>> ffid of the fast function that VM is about to execute.
>> In the same time, `top_frame` property is not updated now, so
>> the top frame of the Lua stack can be streamed based on the ffid,
>> and the rest of the Lua stack can be streamed as usual.
>>
>> Also, this patch fixes build with plain makefile, by adding
>> the `LJ_HASSYSPROF` flag support to it.
>>
>> Resolves tarantool/tarantool#8594
>> ---
>> Changes in v3:
>> - Fixed comments as per review by Sergey
>>
>> Branch:  https://github.com/tarantool/luajit/tree/fckxorg/gh-8594-sysprof-ffunc-crash
>> PR:  https://github.com/tarantool/tarantool/pull/8737
>> src/Makefile.original | 3 ++
>> src/lj_obj.h | 7 +++-
>> src/lj_sysprof.c | 26 ++++++++++++---
>> src/vm_x64.dasc | 22 +++++++++++--
>> src/vm_x86.dasc | 31 ++++++++++++++---
>> .../gh-8594-sysprof-ffunc-crash.test.lua | 33 +++++++++++++++++++
>> 6 files changed, 109 insertions(+), 13 deletions(-)
>> create mode 100644 test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua
>>
>> diff --git a/src/Makefile.original b/src/Makefile.original
>> index aedaaa73..e21a0e56 100644
>> --- a/src/Makefile.original
>> +++ b/src/Makefile.original
>> @@ -441,6 +441,9 @@ ifneq (,$(findstring LJ_NO_UNWIND 1,$(TARGET_TESTARCH)))
>> DASM_AFLAGS+= -D NO_UNWIND
>> TARGET_ARCH+= -DLUAJIT_NO_UNWIND
>> endif
>> +ifneq (,$(findstring LJ_HASSYSPROF 1,$(TARGET_TESTARCH)))
>> + DASM_AFLAGS+= -D LJ_HASSYSPROF
>> +endif
>> DASM_AFLAGS+= -D VER=$(subst LJ_ARCH_VERSION_,,$(filter LJ_ARCH_VERSION_%,$(subst LJ_ARCH_VERSION ,LJ_ARCH_VERSION_,$(TARGET_TESTARCH))))
>> ifeq (Windows,$(TARGET_SYS))
>> DASM_AFLAGS+= -D WIN
>> diff --git a/src/lj_obj.h b/src/lj_obj.h
>> index 45507e0d..e17316df 100644
>> --- a/src/lj_obj.h
>> +++ b/src/lj_obj.h
>> @@ -598,6 +598,11 @@ enum {
>> GCSmax
>> };
>>
>> +struct lj_sysprof_topframe {
>> + uint8_t ffid; /* FFID of the fast function VM is about to execute. */
>> + TValue *top_frame; /* Top frame for sysprof. */
>> +};
>> +
>> typedef struct GCState {
>> GCSize total; /* Memory currently allocated. */
>> GCSize threshold; /* Memory threshold. */
>> @@ -675,7 +680,7 @@ typedef struct global_State {
>> MRef ctype_state; /* Pointer to C type state. */
>> GCRef gcroot[GCROOT_MAX]; /* GC roots. */
>> #ifdef LJ_HASSYSPROF
>> - TValue *top_frame; /* Top frame for sysprof. */
>> + struct lj_sysprof_topframe top_frame_info; /* Top frame info for sysprof. */
>> #endif
>> } global_State;
>>
>> diff --git a/src/lj_sysprof.c b/src/lj_sysprof.c
>> index 2e9ed9b3..0a341e16 100644
>> --- a/src/lj_sysprof.c
>> +++ b/src/lj_sysprof.c
>> @@ -109,6 +109,12 @@ static void stream_epilogue(struct sysprof *sp)
>> lj_wbuf_addbyte(&sp->out, LJP_EPILOGUE_BYTE);
>> }
>>
>> +static void stream_ffunc_impl(struct lj_wbuf *buf, uint8_t ffid)
>> +{
>> + lj_wbuf_addbyte(buf, LJP_FRAME_FFUNC);
>> + lj_wbuf_addu64(buf, ffid);
>> +}
>> +
>> static void stream_lfunc(struct lj_wbuf *buf, const GCfunc *func)
>> {
>> lua_assert(isluafunc(func));
>> @@ -129,8 +135,7 @@ static void stream_cfunc(struct lj_wbuf *buf, const GCfunc *func)
>> static void stream_ffunc(struct lj_wbuf *buf, const GCfunc *func)
>> {
>> lua_assert(isffunc(func));
>> - lj_wbuf_addbyte(buf, LJP_FRAME_FFUNC);
>> - lj_wbuf_addu64(buf, func->c.ffid);
>> + stream_ffunc_impl(buf, func->c.ffid);
>> }
>>
>> static void stream_frame_lua(struct lj_wbuf *buf, const cTValue *frame)
>> @@ -148,7 +153,7 @@ static void stream_frame_lua(struct lj_wbuf *buf, const cTValue *frame)
>> lua_assert(0);
>> }
>>
>> -static void stream_backtrace_lua(struct sysprof *sp)
>> +static void stream_backtrace_lua(struct sysprof *sp, uint32_t vmstate)
>> {
>> global_State *g = sp->g;
>> struct lj_wbuf *buf = &sp->out;
>> @@ -158,8 +163,19 @@ static void stream_backtrace_lua(struct sysprof *sp)
>> lua_assert(g != NULL);
>> L = gco2th(gcref(g->cur_L));
>> lua_assert(L != NULL);
>> + /*
>> + ** Lua stack may be inconsistent during the execution of a
>> + ** fast-function, so instead of updating the `top_frame` for
>> + ** it, its `ffid` is set instead. The first frame on the
>> + ** result stack is streamed manually, and the rest of the
>> + ** stack is streamed based on the previous `top_frame` value.
>> + */
>> + if (vmstate == LJ_VMST_FFUNC) {
>> + uint8_t ffid = g->top_frame_info.ffid;
>> + stream_ffunc_impl(buf, ffid);
>> + }
>>
>> - top_frame = g->top_frame - 1; //(1 + LJ_FR2)
>> + top_frame = g->top_frame_info.top_frame - 1;
>>
>> bot = tvref(L->stack) + LJ_FR2;
>> /* Traverse frames backwards */
>> @@ -234,7 +250,7 @@ static void stream_trace(struct sysprof *sp, uint32_t vmstate)
>> static void stream_guest(struct sysprof *sp, uint32_t vmstate)
>> {
>> lj_wbuf_addbyte(&sp->out, (uint8_t)vmstate);
>> - stream_backtrace_lua(sp);
>> + stream_backtrace_lua(sp, vmstate);
>> stream_backtrace_host(sp);
>> }
>>
>> diff --git a/src/vm_x64.dasc b/src/vm_x64.dasc
>> index 7b04b928..2a0c3f03 100644
>> --- a/src/vm_x64.dasc
>> +++ b/src/vm_x64.dasc
>> @@ -53,6 +53,7 @@
>> |.define RDL, RCL
>> |.define TMPR, r10
>> |.define TMPRd, r10d
>> +|.define TMPRb, r10b
>> |.define ITYPE, r11
>> |.define ITYPEd, r11d
>> |
>> @@ -353,14 +354,29 @@
>> |// it syncs with the BASE register only when the control is passed to
>> |// user code. So we need to sync the BASE on each vmstate change to
>> |// keep it consistent.
>> +|// The only exception are FFUNCs because sometimes even internal BASE
>> +|// stash is inconsistent for them. To address that issue, their ffid
>> +|// is stashed instead, so the corresponding frame can be streamed
>> +|// manually.
>> |.macro set_vmstate_sync_base, st
>> |.if LJ_HASSYSPROF
>> | set_vmstate INTERP // Guard for non-atomic VM context restoration
>> -| mov qword [DISPATCH+DISPATCH_GL(top_frame)], BASE
>> +| mov qword [DISPATCH+DISPATCH_GL(top_frame_info.top_frame)], BASE
>> |.endif
>> | set_vmstate st
>> |.endmacro
>> |
>> +|.macro set_vmstate_ffunc
>> +|.if LJ_HASSYSPROF
>> +| set_vmstate INTERP
>> +| mov TMPR, [BASE - 16]
>> +| cleartp LFUNC:TMPR // Obtain plain address value. Equivalent of `gcval`.
>> +| mov TMPRb, LFUNC:TMPR->ffid
>> +| mov byte [DISPATCH+DISPATCH_GL(top_frame_info.ffid)], TMPRb
>> +|.endif
>> +| set_vmstate FFUNC
>> +|.endmacro
>> +|
>> |// Uses TMPRd (r10d).
>> |.macro save_vmstate
>> |.if not WIN
>> @@ -376,7 +392,7 @@
>> | set_vmstate INTERP
>> | mov TMPR, SAVE_L
>> | mov TMPR, L:TMPR->base
>> -| mov qword [DISPATCH+DISPATCH_GL(top_frame)], TMPR
>> +| mov qword [DISPATCH+DISPATCH_GL(top_frame_info.top_frame)], TMPR
>> |.endif
>> | mov TMPRd, SAVE_VMSTATE
>> | mov dword [DISPATCH+DISPATCH_GL(vmstate)], TMPRd
>> @@ -1188,7 +1204,7 @@ static void build_subroutines(BuildCtx *ctx)
>> |
>> |.macro .ffunc, name
>> |->ff_ .. name:
>> - | set_vmstate_sync_base FFUNC
>> + | set_vmstate_ffunc
>> |.endmacro
>> |
>> |.macro .ffunc_1, name
>> diff --git a/src/vm_x86.dasc b/src/vm_x86.dasc
>> index bd1e940e..ff388d58 100644
>> --- a/src/vm_x86.dasc
>> +++ b/src/vm_x86.dasc
>> @@ -101,6 +101,7 @@
>> |.define FCARG2, CARG2d
>> |
>> |.define XCHGd, r11d // TMP on x64, used for exchange.
>> +|.define XCHGb, r11b // TMPRb on x64, used for exchange.
>> |.endif
>> |
>> |// Type definitions. Some of these are only used for documentation.
>> @@ -451,14 +452,36 @@
>> |// it syncs with the BASE register only when the control is passed to
>> |// user code. So we need to sync the BASE on each vmstate change to
>> |// keep it consistent.
>> +|// The only exception are FFUNCs because sometimes even internal BASE
>> +|// stash is inconsistent for them. To address that issue, their ffid
>> +|// is stashed instead, so the corresponding frame can be streamed
>> +|// manually.
>> |.macro set_vmstate_sync_base, st
>> |.if LJ_HASSYSPROF
>> | set_vmstate INTERP // Guard for non-atomic VM context restoration
>> -| mov dword [DISPATCH+DISPATCH_GL(top_frame)], BASE
>> +| mov dword [DISPATCH+DISPATCH_GL(top_frame_info.top_frame)], BASE
>> |.endif
>> | set_vmstate st
>> |.endmacro
>> |
>> +|.macro set_vmstate_ffunc
>> +|.if LJ_HASSYSPROF
>> +| set_vmstate INTERP
>> +|.if not X64
>> +| mov SPILLECX, ecx
>> +| mov LFUNC:ecx, [BASE - 4]
>> +| mov cl, LFUNC:ecx->ffid
>> +| mov byte [DISPATCH+DISPATCH_GL(top_frame_info.ffid)], cl
>> +| mov ecx, SPILLECX
>> +|.else // X64
>> +| mov LFUNC:XCHGd, [BASE - 8]
>> +| mov XCHGb, LFUNC:XCHGd->ffid
>> +| mov byte [DISPATCH+DISPATCH_GL(top_frame_info.ffid)], XCHGb
>> +|.endif // X64
>> +|.endif // LJ_HASSYSPROF
>> +| set_vmstate FFUNC
>> +|.endmacro
>> +|
>> |// Uses spilled ecx on x86 or XCHGd (r11d) on x64.
>> |.macro save_vmstate
>> |.if not WIN
>> @@ -485,7 +508,7 @@
>> |.if LJ_HASSYSPROF
>> | mov ecx, SAVE_L
>> | mov ecx, L:ecx->base
>> -| mov dword [DISPATCH+DISPATCH_GL(top_frame)], ecx
>> +| mov dword [DISPATCH+DISPATCH_GL(top_frame_info.top_frame)], ecx
>> |.endif
>> | mov ecx, SAVE_VMSTATE
>> | mov dword [DISPATCH+DISPATCH_GL(vmstate)], ecx
>> @@ -494,7 +517,7 @@
>> |.if LJ_HASSYSPROF
>> | mov XCHGd, SAVE_L
>> | mov XCHGd, L:XCHGd->base
>> -| mov dword [DISPATCH+DISPATCH_GL(top_frame)], XCHGd
>> +| mov dword [DISPATCH+DISPATCH_GL(top_frame_info.top_frame)], XCHGd
>> |.endif
>> | mov XCHGd, SAVE_VMSTATE
>> | mov dword [DISPATCH+DISPATCH_GL(vmstate)], XCHGd
>> @@ -1488,7 +1511,7 @@ static void build_subroutines(BuildCtx *ctx)
>> |
>> |.macro .ffunc, name
>> |->ff_ .. name:
>> - | set_vmstate_sync_base FFUNC
>> + | set_vmstate_ffunc
>> |.endmacro
>> |
>> |.macro .ffunc_1, name
>> diff --git a/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua b/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua
>> new file mode 100644
>> index 00000000..e5cdeb07
>> --- /dev/null
>> +++ b/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua
>> @@ -0,0 +1,33 @@
>> +local tap = require('tap')
>> +local test = tap.test('gh-8594-sysprof-ffunc-crash'):skipcond({
>> + ['Sysprof is implemented for x86_64 only'] = jit.arch ~= 'x86' and
>> + jit.arch ~= 'x64',
>> + ['Sysprof is implemented for Linux only'] = jit.os ~= 'Linux',
>> +})
>> +
>> +test:plan(1)
>> +
>> +jit.off()
>> +-- XXX: Run JIT tuning functions in a safe frame to avoid errors
>> +-- thrown when LuaJIT is compiled with JIT engine disabled.
>> +pcall(jit.flush)
>> +
>> +local TMP_BINFILE = '/dev/null'
>> +
>> +local res, err = misc.sysprof.start{
>> + mode = 'C',
>
>In the reproducer from bug description "L" mode is used.
>
>Why do you use "C" mode here?
>
>> + interval = 3,
>> + path = TMP_BINFILE,
>> +}
>> +assert(res, err)
>> +
>> +for i = 1, 1e5 do
>> + tostring(i)
>
>Please add a comment, that we use "tostring" here, because "tostring" is
>a fast function.
>
>(Sure it's may be obvious for you, but it will be more clear for those
>who will read it.)
Added.
 
Here is the diff:
================================================
diff --git a/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua b/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua
index e5cdeb07..a053e41c 100644
--- a/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua
+++ b/test/tarantool-tests/gh-8594-sysprof-ffunc-crash.test.lua
@@ -14,6 +14,12 @@ pcall(jit.flush)
 
 local TMP_BINFILE = '/dev/null'
 
+-- XXX: The best way to test the issue is to set the profile
+-- interval to be as short as possible. However, our CI is
+-- not capable of handling such intense testing, so it was a
+-- forced decision to reduce the sampling frequency. As a
+-- result, it is now less likely to reproduce the issue
+-- statistically, but the test case is still valid.
 local res, err = misc.sysprof.start{
   mode = 'C',
   interval = 3,
@@ -22,6 +28,7 @@ local res, err = misc.sysprof.start{
 assert(res, err)
 
 for i = 1, 1e5 do
+  -- XXX: `tostring` is FFUNC.
   tostring(i)
 end
================================================
Branch is force-pushed.
 
>
>> +end
>> +
>> +res, err = misc.sysprof.stop()
>> +assert(res, err)
>> +
>> +test:ok(true, 'sysprof finished successfully')
>> +
>> +os.exit(test:check() and 0 or 1)
--
Best regards,
Maxim Kokryashkin
 

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