[Tarantool-patches] [PATCH luajit] Fix frame traversal for __gc handler frames.

[sergos]

Hi!
Thanks for the patch! See my 2 comments below.

Sergos

> On 5 Oct 2021, at 13:28, Sergey Kaplun <skaplun at tarantool.org> wrote:
> 
> From: Mike Pall <mike>
> 
> Reported by Changochen.
> 
> (cherry picked from 53f82e6e2e858a0a62fd1a2ff47e9866693382e6)
> 
> Additional stack traversal is needed to find an error function set for
   ^^^                          ^^^
Additional to what?            
 				
> handling runtime errors. cframe unwinding is missed for a C protected
> frame during this stack traversal.

I would rephrase - 
A cframe unwinding is miseed for a C protected frame during a serach for 
an error function to handle a runtime error. 

> It leads to undefined behaviour or
> crash, when raising a runtime error on stack with the CP frame before an
> error function handler (for example, an error in __gc handler).
> 
> This patch adds missing unwinding for CP frame.
> 
> Sergey Kaplun:
> * added the description and the test for the problem
> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-601-fix-gc-finderrfunc
> Tarantool branch: https://github.com/tarantool/tarantool/tree/skaplun/gh-noticket-fix-gc-finderrfunc
> LuaJIT issue: https://github.com/LuaJIT/LuaJIT/issues/601
> 
> src/lj_err.c                                  |  1 +
> .../lj-601-fix-gc-finderrfunc.test.lua        | 25 +++++++++++++++++++
> 2 files changed, 26 insertions(+)
> create mode 100644 test/tarantool-tests/lj-601-fix-gc-finderrfunc.test.lua
> 
> diff --git a/src/lj_err.c b/src/lj_err.c
> index b6be357e..b520b3d3 100644
> --- a/src/lj_err.c
> +++ b/src/lj_err.c
> @@ -585,6 +585,7 @@ static ptrdiff_t finderrfunc(lua_State *L)
>       if (cframe_canyield(cf)) return 0;
>       if (cframe_errfunc(cf) >= 0)
> 	return cframe_errfunc(cf);
> +      cf = cframe_prev(cf);
>       frame = frame_prevd(frame);
>       break;
>     case FRAME_PCALL:
> diff --git a/test/tarantool-tests/lj-601-fix-gc-finderrfunc.test.lua b/test/tarantool-tests/lj-601-fix-gc-finderrfunc.test.lua
> new file mode 100644
> index 00000000..d8d79100
> --- /dev/null
> +++ b/test/tarantool-tests/lj-601-fix-gc-finderrfunc.test.lua

Unfortunately the test passes on the ’tarantool’ branch 

s-ostanevich:luajit s.ostanevich$ git checkout tarantool
Switched to branch 'tarantool'
s-ostanevich:luajit s.ostanevich$ git clean -xdff
[…]
s-ostanevich:luajit s.ostanevich$ cmake .
[…]
s-ostanevich:luajit s.ostanevich$ make
[…]
[100%] Built target libluajit_shared
[100%] Built target libluajit
[100%] Built target luajit
s-ostanevich:luajit s.ostanevich$ git checkout   skaplun/lj-601-fix-gc-finderrfunc
s-ostanevich:luajit s.ostanevich$ cd test/tarantool-tests
s-ostanevich:tarantool-tests s.ostanevich$ ../../src/luajit lj-601-fix-gc-finderrfunc.test.lua 
TAP version 13
1..1
ok - successfully collectgarbage with error

> @@ -0,0 +1,25 @@
> +local tap = require('tap')
> +
> +local test = tap.test('lj-601-fix-gc-finderrfunc')
> +test:plan(1)
> +
> +-- Test file to demonstrate LuaJIT incorrect behaviour, when
> +-- throwing error in __gc finalizer.
> +-- See also, https://github.com/LuaJIT/LuaJIT/issues/601.
> +
> +collectgarbage()
> +
> +local a = newproxy(true)
> +getmetatable(a).__gc = function()
> +  -- Function to raise error via `lj_err_run()` inside __gc.
> +  local _ = load(function() collectgarbage()() end)
> +end
> +
> +-- XXX: Generate a small bunch of proxies. Need several to call
> +-- `collectgarbage()` on another proxy inside __gc. N cycles is
> +-- empirical number.
> +for _ = 1, 4 do newproxy(a) end
> +collectgarbage('collect')
> +
> +test:ok(true, 'successfully collectgarbage with error')
> +os.exit(test:check() and 0 or 1)
> -- 
> 2.31.0
>