Re: [Tarantool-patches] [PATCH luajit] Fix zero stripping in %g number formatting.

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

Hi, Sergey!
Thanks for the patch!
LGTM, except for two nits regarding the commit message,
and two nits regarding the test case comment.
On Tue, Jan 30, 2024 at 06:04:37PM +0300, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by pwnhacker0x18.
>
> (cherry picked from commit 343ce0edaf3906a62022936175b2f5410024cbfc)
>
> In the situation when the precision (`prec`) and amount of digits
> (`hilen`) for the decimal representation are the same and `ndhi` == 0,
> the `ndlo` part will become 64 (the size of the `nd` stack buffer), and
Typo: s/will become/becomes/
> the overflow occurs.
>
> This patch adds the corresponding mask (0x3f == 63) for the `ndlo`
> incrementation result.
Please mention that all of this happens in the `lj_strfmt_wfnum`
function in the commit message.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#9595
> ---
>
> Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-1149-g-number-formating
> Tarantool PR: https://github.com/tarantool/tarantool/pull/9633
>
> The test <app-tap/gh-2717-no-quit-sigint.test.lua> fails on M1 with the
> timeout (see the example [1]). This fail is patch-unrelated, since I've
> obscured this failure even for the branch without sources changes (tests
> only).
>
> Related Issues:
> * https://github.com/LuaJIT/LuaJIT/issues/1149
> * https://github.com/tarantool/tarantool/issues/9595
>
> [1]: https://github.com/tarantool/luajit/actions/runs/7712549489/job/21020513973#step:8:5522
>
> Duration of failed tests (seconds):
> * 60.54 app-tap/gh-2717-no-quit-sigint.test.lua
>
>  src/lj_strfmt_num.c                           |  3 ++-
>  .../lj-1149-g-number-formating-bufov.test.lua | 20 +++++++++++++++++++
>  2 files changed, 22 insertions(+), 1 deletion(-)
>  create mode 100644 test/tarantool-tests/lj-1149-g-number-formating-bufov.test.lua
>
> diff --git a/src/lj_strfmt_num.c b/src/lj_strfmt_num.c
> index c26204b7..c8d9febf 100644
> --- a/src/lj_strfmt_num.c
> +++ b/src/lj_strfmt_num.c
> @@ -454,7 +454,8 @@ static char *lj_strfmt_wfnum(SBuf *sb, SFormat sf, lua_Number n, char *p)
>  	    prec--;
>  	    if (!i) {
>  	      if (ndlo == ndhi) { prec = 0; break; }
> -	      lj_strfmt_wuint9(tail, nd[++ndlo]);
> +	      ndlo = (ndlo + 1) & 0x3f;
> +	      lj_strfmt_wuint9(tail, nd[ndlo]);
>  	      i = 9;
>  	    }
>  	  }
> diff --git a/test/tarantool-tests/lj-1149-g-number-formating-bufov.test.lua b/test/tarantool-tests/lj-1149-g-number-formating-bufov.test.lua
> new file mode 100644
> index 00000000..040fd5de
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1149-g-number-formating-bufov.test.lua
> @@ -0,0 +1,20 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate stack-buffer-overflow in the
> +-- `lj_strfmt_wfnum()` call.
> +-- See also: https://github.com/LuaJIT/LuaJIT/issues/1149.
> +
> +local test = tap.test('lj-1149-g-number-formating-bufov')
> +test:plan(1)
> +
> +-- XXX: The test shows stack-buffer-overflow only under ASAN.
> +-- The number value for the test is with the same precision
Typo: s/is with/has/
> +-- (`prec` = 5) and amount of digits (`hilen` = 5) for the decimal
> +-- representation. Hence, with `ndhi` == 0, the `ndlo` part will
> +-- become 64 (the size of the `nd` stack buffer), and the overflow
Typo: s/will become/becomes/
> +-- occurs.
> +-- See details in the <src/lj_strfmt_num.c>:`lj_strfmt_wfnum()`.
> +test:is(string.format('%7g', 0x1.144399609d407p+401), '5.5733e+120',
> +        'correct format %7g result')
> +
> +test:done(true)
> --
> 2.43.0
>