Re: [Tarantool-patches] [PATCH luajit] Fix predict_next() in parser.

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

Hi, Sergey!
Thanks for the review!
Fixed your comments inline.

On 21.08.23, Sergey Bronnikov wrote:
> Hi, Sergey
> 
> 
> thanks for the patch! See comments inline.
> 
> 
> On 8/15/23 17:25, Sergey Kaplun wrote:
> > From: Mike Pall <mike>
> >
> > Reported by Sergey Kaplun.
> >
> > (cherry-picked from commit caf7cbc57c945f7b68871ad72abafb2b6e6fb7f5)
> >
> > Assume, we have the following Lua code:
> > | local _
> > | for _ in (nil):foo() do end
> >
> > The first part of the bytecode emitted for it is the following:
> > | 0001    KNIL     0   1
> > | 0002    MOV      2   1
> > | 0003    TGETS    1   1   0  ; "foo"
> > | 0004    CALL     1   4   2
> >
> > The `0001 KNIL` is a result of merging two `KPRI` instructions: one for
> > the local variable, one for the slot with `nil` object. During parsing in
> > `predict_next()` the second `MOV` bytecode is examined to set `pairs` or
> > `next` local variable. But, as far as it moves `nil` value, that isn't
> > an actual variable, so it has no the name this leads to the crash.
> >
> > This patch adds the check to be sure that `RD` in the `MOV` bytecode is
> > an actual variable.
> >
> > Sergey Kaplun:
> > * added the description and the test for the problem
> >
> > Part of tarantool/tarantool#8825
> > ---
> >
> > Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-1033-fix-parsing-predict-next
> > PR: https://github.com/tarantool/tarantool/pull/8987
> > Related issues:
> > * https://github.com/LuaJIT/LuaJIT/issues/1033
> > * https://github.com/tarantool/tarantool/issues/8825
> >
> >   src/lj_parse.c                                |  1 +
> >   .../lj-1033-fix-parsing-predict-next.test.lua | 30 +++++++++++++++++++
> >   2 files changed, 31 insertions(+)
> >   create mode 100644 test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua
> >
> > diff --git a/src/lj_parse.c b/src/lj_parse.c
> > index 3f6caaec..420b95cb 100644
> > --- a/src/lj_parse.c
> > +++ b/src/lj_parse.c
> > @@ -2532,6 +2532,7 @@ static int predict_next(LexState *ls, FuncState *fs, BCPos pc)
> >     cTValue *o;
> >     switch (bc_op(ins)) {
> >     case BC_MOV:
> > +    if (bc_d(ins) >= fs->nactvar) return 0;
> >       name = gco2str(gcref(var_get(ls, fs, bc_d(ins)).name));
> >       break;
> >     case BC_UGET:
> > diff --git a/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua b/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua
> > new file mode 100644
> > index 00000000..624344eb
> > --- /dev/null
> > +++ b/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua
> > @@ -0,0 +1,30 @@
> > +local tap = require('tap')
> > +local test = tap.test('lj-1033-fix-parsing-predict-next')
> > +
> > +test:plan(3)
> > +
> > +local res_f = loadstring([[
> > +-- This local variable is necessary, because it emits `KPRI`
> > +-- bytecode, with which the next `KPRI` bytecode will be merged.
> > +--
> > +-- The resulting bytecode is the following:
> > +--
> > +-- 0001    KNIL     0   1
> > +-- 0002    MOV      2   1
> > +-- 0003    TGETS    1   1   0  ; "foo"
> > +-- 0004    CALL     1   4   2
> > +--
> > +-- This MOV don't use any variable value from the stack, so the
> > +-- attempt to get the name in `predict_next() leads to the crash.
> What is a point to put a comment inside loadstring, not before it?

Moved the part about bytecode before. Still assume, that comment about
the variable is needed inside the code (i.e. this variable).

> > +local _
> > +for _ in (nil):foo() do end
> > +]])
> > +
> > +test:ok(res_f, 'chunk loaded sucsessfully')
> typo: sucsessfully -> successfully

Fixed, thanks!

> > +
> > +local res, err = pcall(res_f)
> > +
> > +test:ok(not res, 'loaded function not executed')
> 
> it is not clear for me what for do you need checking result code. I 
> would omit it.
> 
> Feel free to ignore.

Added the following comment to avoid confusing (see the whole iterative
patch below):

===================================================================
diff --git a/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua b/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua
index 63998d8c..fed3ff6c 100644
--- a/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua
+++ b/test/tarantool-tests/lj-1033-fix-parsing-predict-next.test.lua
@@ -3,10 +3,6 @@ local test = tap.test('lj-1033-fix-parsing-predict-next')
 
 test:plan(3)
 
-local res_f = loadstring([[
--- This local variable is necessary, because it emits `KPRI`
--- bytecode, with which the next `KPRI` bytecode will be merged.
---
 -- The resulting bytecode is the following:
 --
 -- 0001    KNIL     0   1
@@ -16,14 +12,18 @@ local res_f = loadstring([[
 --
 -- This MOV doesn't use any variable value from the stack, so the
 -- attempt to get the name in `predict_next() leads to the crash.
+local res_f = loadstring([[
+-- This local variable is necessary, because it emits `KPRI`
+-- bytecode, with which the next `KPRI` bytecode will be merged.
 local _
 for _ in (nil):foo() do end
 ]])
 
-test:ok(res_f, 'chunk loaded sucsessfully')
+test:ok(res_f, 'chunk loaded successfully')
 
 local res, err = pcall(res_f)
 
+-- Check consistency with PUC Rio Lua 5.1 behaviour.
 test:ok(not res, 'loaded function not executed')
 test:like(err, 'attempt to index a nil value', 'correct error message')
 
===================================================================

Branch is force-pushed.

> 
> 
> > +test:like(err, 'attempt to index a nil value', 'correct error message')
> > +
> > +test:done(true)

-- 
Best regards,
Sergey Kaplun