On Wed, 23 Aug 2023 at 19:48, Paul Dreik via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> This fixes pointer arithmetic made on a null pointer, which I found
> through fuzzing.
> Tested on debian/amd64.
>
> Thanks, Paul
Thanks. Pushed to trunk, backport to gcc-13 to follow.
I also added your testcase from the bug report to the testsuite.
>
> ------------------------------------------------------------------------
> commit 78ac41590432f4f01036797fd9d661f6ed80cf37 (HEAD -> master)
> Author: Paul Dreik <gccpatc...@pauldreik.se>
> Date: Tue Aug 22 19:16:57 2023 +0200
>
> libstdc++: fix illegal pointer arithmetic in format
>
> when parsing a format string, the width is parsed into an unsigned
> short
> but the result is not checked in the case the format string is not a
> char string (such as a wide string). in case the parse fails,
> a null pointer is returned which is used for pointer arithmetic
> which is undefined behaviour.
>
> Signed-off-by: Paul Dreik <gccpatc...@pauldreik.se>
>
> diff --git a/libstdc++-v3/include/std/format
> b/libstdc++-v3/include/std/format
> index f3d9ae152f..fe2caa5868 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -285,7 +285,8 @@ namespace __format
> for (int __i = 0; __i < __n && (__first + __i) != __last; ++__i)
> __buf[__i] = __first[__i];
> auto [__v, __ptr] = __format::__parse_integer(__buf, __buf +
> __n);
> - return {__v, __first + (__ptr - __buf)};
> + if (__ptr) [[likely]]
> + return {__v, __first + (__ptr - __buf)};
> }
> return {0, nullptr};
> }
