[Tarantool-patches] [PATCH luajit] Fix recording of __concat metamethod.

[Sergey Kaplun]

Hi, Maxim!
Thanks for the patch!
LGTM, after adding a comment about the magic sequence of values for the
concatenation (see the comment below).

On 15.01.24, Maxim Kokryashkin wrote:
> From: Mike Pall <mike>
> 
> Reported by Elias Oelschner. Analyzed by XmiliaH.
> 
> (cherry-picked from commit 3ee3c9cfa988339f1bf3068530515e2a6fb179d2)
> 
> During the recording of `__concat` methametod, the `rec_mm_arith`
> function overrides stack slots that are not restored for GC64
> mode later after the call. This leads to a segmentation fault
> later on. This patch fixes the issue by accounting for those
> additional slots in the array that is used to restore stack

Minor: "slots in the array that is used to restore" sounds a little
cumbersome.
I suggest to say  "slots in the array used for restoring"
Feel free to ignore.

> values.
> 
> Maxim Kokryashkin:
> * added the description and the test for the problem
> 
> Part of tarantool/tarantool#9145
> ---
> Branch: https://github.com/tarantool/luajit/tree/fckxorg/lj-839-concat-recording
> PR: https://github.com/tarantool/tarantool/pull/9597
> Issues: https://github.com/tarantool/tarantool/issues/9145
> https://github.com/luajit/luajit/issues/839
> 
>  src/lj_record.c                               |  2 +-
>  .../lj-839-concat-recording.test.lua          | 27 +++++++++++++++++++
>  2 files changed, 28 insertions(+), 1 deletion(-)
>  create mode 100644 test/tarantool-tests/lj-839-concat-recording.test.lua
> 
> diff --git a/src/lj_record.c b/src/lj_record.c
> index a929b8aa..59549b03 100644
> --- a/src/lj_record.c
> +++ b/src/lj_record.c
> @@ -1932,7 +1932,7 @@ static TRef rec_tnew(jit_State *J, uint32_t ah)
>  static TRef rec_cat(jit_State *J, BCReg baseslot, BCReg topslot)
>  {
>    TRef *top = &J->base[topslot];
> -  TValue savetv[5];
> +  TValue savetv[5+LJ_FR2];
>    BCReg s;
>    RecordIndex ix;
>    lj_assertJ(baseslot < topslot, "bad CAT arg");
> diff --git a/test/tarantool-tests/lj-839-concat-recording.test.lua b/test/tarantool-tests/lj-839-concat-recording.test.lua
> new file mode 100644
> index 00000000..9ec0ed96
> --- /dev/null
> +++ b/test/tarantool-tests/lj-839-concat-recording.test.lua
> @@ -0,0 +1,27 @@
> +local tap = require('tap')
> +local test = tap.test('lj-839-concat-recording'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +test:plan(1)
> +
> +-- Test file to demonstrate LuaJIT overriding stack slots without
> +-- restoration during the recording of the concat metamethod.
> +-- See also: https://github.com/LuaJIT/LuaJIT/issues/839.
> +
> +-- Setup value with the `__concat` metamethod.
> +local v1 = setmetatable({}, {
> +    __concat = function() return "" end,

Typo: s/""/''/, according to the codestyle.

Nit: 2 extra whitespaces at the beginning.

> +});
> +
> +jit.opt.start('hotloop=1')
> +local result
> +for _ = 1, 4 do
> +  result = v1 .. ""  .. v1 ..  ""

Typo: s/""/''/g, according to the codestyle.

Why do we need exactly this sequence? Please add a comment.

> +end
> +
> +-- There should be an empty string in case of success.

Typo: s/case/the case/

> +-- Failure results in a segmentation fault.
> +-- The issue is GC64-specific, yet it is still being tested for
> +-- other builds.
> +test:is(result, '', 'correct stack restoration')
> +test:done(true)
> --
> 2.43.0
> 

-- 
Best regards,
Sergey Kaplun