[PATCH] Fix for bug libstdc++/111102 pointer arithmetic on nullptr

[Paul Dreik via Gcc-patches]

This fixes pointer arithmetic made on a null pointer, which I found through fuzzing.

Tested on debian/amd64.

Thanks, Paul

------------------------------------------------------------------------
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


    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>


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];

-         return {__v, __first + (__ptr - __buf)};
+         if (__ptr) [[likely]]
+           return {__v, __first + (__ptr - __buf)};
        }
       return {0, nullptr};
     }

when parsing a format string, the width is parsed into an unsigned short diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format auto [__v, __ptr] = __format::__parse_integer(__buf, __buf + __n);

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