Re: [Tarantool-patches] [PATCH luajit][v2] Prevent loop in snap_usedef().

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Hi, Sergey!
Thanks for the patch and the fixes!
LGTM, after fixing my several suggestions about comments rephrasing and
a bunch of nits below.

On 16.01.25, Sergey Bronnikov wrote:
> Reported by XmiliaH.
> 
> (cherry picked from commit 0e66fc96377853d898390f1a02723c54ec3a42f7)
> 
> It is possible to get an infinite loop in a function `snap_usedef`
> when a `UCLO` makes a tight loop. This infinite loop could happen
> when `snap_usedef()` is called on trace exit processes UCLO

It is called during trace recording (more precisely on creation of the
snapshot for the guarded trace check).

> bytecode instruction and this instruction attempts a jump with
> negative value. The patch fixes the problem by checking a number
> of slots in a jump argument and replace this value my `maxslot` if
> a value is negative.

Please also mention that "This means that no values will be purged from
the snapshot".

> 
> Sergey Bronnikov:
> * added the description and the test for the problem
> 
> Part of tarantool/tarantool#10709
> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/ligurio/lj-736-prevent-loop-in-snap_usedef
> 
> Related issues:
> * https://github.com/tarantool/tarantool/issues/10709
> * https://github.com/LuaJIT/LuaJIT/issues/736
> 
> v2 changes:
> 
> - Updated test, now it hangs without patch with fix.
> - Added more comments to the test with explanations.
> 
>  src/lj_snap.c                                 |  7 +-
>  .../lj-736-BC_UCLO-triggers-infinite-loop.lua | 82 +++++++++++++++++++
>  2 files changed, 88 insertions(+), 1 deletion(-)
>  create mode 100644 test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
> 
> diff --git a/src/lj_snap.c b/src/lj_snap.c
> index 8a33dc22..8d7bd868 100644
> --- a/src/lj_snap.c
> +++ b/src/lj_snap.c

<snipped>

> diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
> new file mode 100644
> index 00000000..fb053e9a
> --- /dev/null
> +++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua

Filename suffix should be ".test.lua".
The ctest doesn't recognize this file.

> @@ -0,0 +1,82 @@
> +local tap = require('tap')
> +local test = tap.test('lj-736-BC_UCLO-triggers-infinite-loop'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +
> +test:plan(2)
> +
> +-- Test reproduces an issue when BC_UCLO triggers an infinite loop.
> +-- See details in https://github.com/LuaJIT/LuaJIT/issues/736.

Nit: Lets be consistent with most of our test files and put the short
description before the `tap.test()`:

===================================================================
diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
index fb053e9a..f38098a8 100644
--- a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
+++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
@@ -1,13 +1,13 @@
 local tap = require('tap')
+-- Test file to demonstrate the infinite loop in LuaJIT during the
+-- use-def analysis for upvalues.
+-- See also https://github.com/LuaJIT/LuaJIT/issues/736.
 local test = tap.test('lj-736-BC_UCLO-triggers-infinite-loop'):skipcond({
   ['Test requires JIT enabled'] = not jit.status(),
 })
 
 test:plan(2)
 
--- Test reproduces an issue when BC_UCLO triggers an infinite loop.
--- See details in https://github.com/LuaJIT/LuaJIT/issues/736.
---
 -- Listing below demonstrates a problem -
 -- the bytecode UCLO on the line 13 makes a loop at 0013-0014:
 --
===================================================================

> +--
> +-- Listing below demonstrates a problem -
> +-- the bytecode UCLO on the line 13 makes a loop at 0013-0014:
> +--
> +-- - BYTECODE -- bc_uclo.lua:0-20
> +-- 0001    KPRI     0   0
> +-- 0002    FNEW     1   0      ; bc_uclo.lua:5
> +-- 0003    KSHORT   2   1
> +-- 0004    KSHORT   3   4
> +-- 0005    KSHORT   4   1
> +-- 0006    FORI     2 => 0011
> +-- 0007 => ISNEN    5   0      ; 2
> +-- 0008    JMP      6 => 0010
> +-- 0009    UCLO     0 => 0012
> +-- 0010 => FORL     2 => 0007
> +-- 0011 => UCLO     0 => 0012
> +-- 0012 => KPRI     0   0
> +-- 0013    UCLO     0 => 0012
> +-- 0014    FNEW     1   1      ; bc_uclo.lua:18
> +-- 0015    UCLO     0 => 0016
> +-- 0016 => RET0     0   1

I suggest to list only the necessary part of the bytecodes, also
lets take the bytecode dump for this particular function:

===================================================================
diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
index f38098a8..8d0558e9 100644
--- a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
+++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
@@ -8,27 +8,18 @@ local test = tap.test('lj-736-BC_UCLO-triggers-infinite-loop'):skipcond({
 
 test:plan(2)
 
--- Listing below demonstrates a problem -
--- the bytecode UCLO on the line 13 makes a loop at 0013-0014:
---
--- - BYTECODE -- bc_uclo.lua:0-20
--- 0001    KPRI     0   0
--- 0002    FNEW     1   0      ; bc_uclo.lua:5
--- 0003    KSHORT   2   1
--- 0004    KSHORT   3   4
--- 0005    KSHORT   4   1
--- 0006    FORI     2 => 0011
--- 0007 => ISNEN    5   0      ; 2
--- 0008    JMP      6 => 0010
--- 0009    UCLO     0 => 0012
--- 0010 => FORL     2 => 0007
--- 0011 => UCLO     0 => 0012
--- 0012 => KPRI     0   0
--- 0013    UCLO     0 => 0012
--- 0014    FNEW     1   1      ; bc_uclo.lua:18
--- 0015    UCLO     0 => 0016
--- 0016 => RET0     0   1
-
+-- Before the patch, the code flow like in the `testcase()` below
+-- may cause the problem -- use-def analysis for the 0019 UCLO
+-- creates an infinite loop in 0014 - 0019:
+-- | 0008 FORI   base:    4 jump:  => 0013
+-- | 0009 ISNEN  var:     7 num:     0 ; number 2
+-- | 0010 JMP    rbase:   8 jump:  => 0012
+-- | 0011 UCLO   rbase:   2 jump:  => 0014
+-- | 0012 FORL   base:    4 jump:  => 0009
+-- | 0013 UCLO   rbase:   2 jump:  => 0014
+-- | 0014 KPRI   dst:     2 pri:     0 ; Start of `assert()` line.
+-- | ...
+-- | 0019 UCLO   rbase:   2 jump:  => 0014
 jit.opt.start('hotloop=1')
 
 local assert_msg = 'Infinite loop is not reproduced.'
===================================================================

> +
> +jit.opt.start('hotloop=1')
> +
> +local assert_msg = 'Infinite loop is not reproduced.'
> +local assert = assert
> +
> +local function testcase()
> +  -- The code in the first scope `do`/`end` is a prerequisite.
> +  -- It is needed so that we have a trace at the exit from which
> +  -- the creation of the snapshot will begin.

The needed part is not the creation of the snapshot but the use-def
analysis for the first (`uv1`) UCLO. I would rephrase it like the
following:

===================================================================
diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
index 8d0558e9..3e6508d1 100644
--- a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
+++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
@@ -27,11 +27,9 @@ local assert = assert
 
 local function testcase()
   -- The code in the first scope `do`/`end` is a prerequisite.
-  -- It is needed so that we have a trace at the exit from which
-  -- the creation of the snapshot will begin.
+  -- It contains the UCLO instruction for the `uv1`. The use-def
+  -- analysis for it escapes this `do`/`end` scope.
   do
-    -- Upvalue below is not used actually, but it is required
-    -- for calling `snap_usedef()` on trace exit.
     local uv1 -- luacheck: ignore
     local _ = function() return uv1 end
 
===================================================================

> +  do
> +    -- Upvalue below is not used actually, but it is required
> +    -- for calling `snap_usedef()` on trace exit.

Side note: The use-def analysis is called on trace recording.
I would rather merge this comment with the comment above, as you can see in
the diff.

> +    local uv1 -- luacheck: ignore

Nit: It is better to use "no unused" here to be more specific.

> +    local _ = function() return uv1 end
> +
> +    -- The loop below is required for recording a trace.
> +    -- The condition inside a loop executes `goto` to a label
> +    -- outside of the loop when the code executed by JIT and
> +    -- this triggers snapshotting.
> +    for i = 1, 2 do
> +      -- Exit to interpreter once trace is compiled.

I would rephrase this 2 comments, since they a little bit misleading:

===================================================================
diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
index 3e6508d1..a95651fd 100644
--- a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
+++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
@@ -33,12 +33,11 @@ local function testcase()
     local uv1 -- luacheck: ignore
     local _ = function() return uv1 end
 
-    -- The loop below is required for recording a trace.
-    -- The condition inside a loop executes `goto` to a label
-    -- outside of the loop when the code executed by JIT and
-    -- this triggers snapshotting.
+    -- Records the trace for which use-def analysis is applied.
     for i = 1, 2 do
-      -- Exit to interpreter once trace is compiled.
+      -- This condition triggers snapshoting and use-def analysis.
+      -- Before the patch this triggers the infinite loop in the
+      -- `snap_usedef()`, so the `goto` is never taken.
       if i == 2 then
         goto x
       end
===================================================================

> +      if i == 2 then
> +        goto x
> +      end
> +    end
> +  end
> +
> +::x::
> +  do
> +    local uv2 -- luacheck: no unused
> +
> +    -- `goto` if not executed without a patch and generates an
> +    -- UCLO bytecode that makes an infinite loop in a function
> +    -- `snap_usedef` when patch is not applied. `goto` must point
> +    -- to the label on one of the previous lines. `assert()` is
> +    -- executed when patch is applied.
> +    assert(nil, assert_msg)
> +    goto x
> +
> +    -- Line below is required, it makes `uv` upvalue, and must be
> +    -- placed after `goto`, otherwise reproducer become broken.
> +    local _ = function() return uv2 end -- luacheck: ignore

I would also rephrase this 2 comments like the following since they
are repeated what we say before:

===================================================================
diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
index a95651fd..8111d752 100644
--- a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
+++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
@@ -47,17 +47,11 @@ local function testcase()
 ::x::
   do
     local uv2 -- luacheck: no unused
-
-    -- `goto` if not executed without a patch and generates an
-    -- UCLO bytecode that makes an infinite loop in a function
-    -- `snap_usedef` when patch is not applied. `goto` must point
-    -- to the label on one of the previous lines. `assert()` is
-    -- executed when patch is applied.
     assert(nil, assert_msg)
+    -- Create a tight loop for the one more upvalue (`uv2`).
+    -- Before the patch, use-def analysis gets stuck in this code
+    -- flow.
     goto x
-
-    -- Line below is required, it makes `uv` upvalue, and must be
-    -- placed after `goto`, otherwise reproducer become broken.
     local _ = function() return uv2 end -- luacheck: ignore
   end
 end
===================================================================

Also I would add the following comment:
| -- This code is unreachable by design. Prevent luacheck warning.

> +  end
> +end
> +
> +local ok, err = pcall(testcase)
> +
> +test:is(ok, false, 'assertion is triggered in a function with testcase')
> +test:ok(err:match(assert_msg), 'BC_UCLO does not trigger an infinite loop')

I would rather say here 'correct error message'. Or, as an alternative,
we may just use 1 test check:

| pcall(testcase)
|
| test:ok(true, 'no infinite loop in the use-def analysis')

instead of these 2 checks.

> +
> +os.exit(test:check() and 0 or 1)

Minor: Please use `test:done(true)` like in all other tests.

> -- 
> 2.34.1
> 

-- 
Best regards,
Sergey Kaplun