Bug in memccpy

[Jeff Johnston]

Hello,

Thanks for the patch.  It has been pushed to the master repo.

-- Jeff J.

On Wed, Dec 28, 2022 at 6:32 AM Bugs Reporter <bugs@compiler.ai> wrote:

> Hi,
>
> I am writing to report a bug in the memccpy function of newlib.
> The bug is in the C implementation of memccpy as located in the
> newlib/libc/string/ directory of the newlib repository. The newlib version
> was `4.2.0` and the source code was downloaded from the latest release
> <https://sourceware.org/pub/newlib/newlib-4.2.0.20211231.tar.gz> on the
> official website.
> Please find a detailed report below.
>
> memccpy():
> As specified in the OpenBSD manpage[0], memccpy should convert the input
> argument `c` to `unsigned char` before performing the check. Newlib's fast
> implementation of `memccpy` (when the macros `PREFER_SIZE_OVER_SPEED` and
> `__OPTIMIZE_SIZE__` are not defined) converts `c` to `signed char` (stored
> in endchar) and computes a mask as follows:
> ```
> for (i = 0; i < LITTLEBLOCKSIZE; i++)
>     mask = (mask << 8) + endchar;
> ```
> This is used to detect `endchar` in one long word and is supposed to
> represent a word whose each byte has the same value as `endchar`. But if
> the input character lies in the extended ASCII set, then `endchar` is
> negative, which leads to an incorrect mask computation.
>
> An example input is:
>     const char src[] = {1, 2, 3, 4, 5, 192, 6, 7};
>     char dst[8] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H'};
>     memccpy(dst, src, 192, 8);
>     if (dst[7] != 'H') {
>         printf("BUG!\n");
>     }
>
> The file that demonstrates the bug can be found here
> <
> https://github.com/compilerai/bug-reports/blob/master/bug_files/newlib_memccpy_bug.c
> >
> .
> A patch that applies the necessary fix is available here
> <
> https://github.com/compilerai/bug-reports/blob/master/patch/newlib_memccpy.patch
> >
> .
>
> 0: https://man.openbsd.org/memccpy
>
> Thanks,
>
> Jai Arora, Abhishek Rose, Shubhani Gupta, Sorav Bansal
> CompilerAI Research Group
> IIT Delhi, India
>
>