[PATCH] Avoid -Wstringop-overflow= warning in iconv module.

[Siddhesh Poyarekar]

On 03/06/2022 18:22, Stefan Liebler via Libc-alpha wrote:
> On s390x when compiling with GCC 12, I get this warning:
> utf8-utf16-z9.c:
> ../iconv/loop.c: In function ‘__from_utf8_loop_etf3eh_single’:
> ../iconv/loop.c:445:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
>    445 |     bytebuf[inlen++] = *inptr++;
>        |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
> ../iconv/loop.c:381:17: note: at offset 4 into destination object ‘bytebuf’ of size 4
>    381 |   unsigned char bytebuf[MAX_NEEDED_INPUT];
>        |                 ^~~~~~~
> ../iconv/loop.c:445:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
>    445 |     bytebuf[inlen++] = *inptr++;
>        |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
> ../iconv/loop.c:381:17: note: at offset 5 into destination object ‘bytebuf’ of size 4
>    381 |   unsigned char bytebuf[MAX_NEEDED_INPUT];
>        |                 ^~~~~~~
> 
> This patch tells the compiler that inend is always behind inptr which
> avoids the warning.  Note that the SINGLE function is only used to
> implement the mb*towc*() or wc*tomb*() functions.  Those functions use
> inptr and inend pointing to a variable on stack, compute the inend pointer
> or explicitly check the arguments which always leads to inptr < inend.

LGTM; there was a time that this wasn't true for applications that 
assumed POSIX behaviour in wcrtomb but it is true now.  See also:

commit 9bcd12d223a8990254b65e2dada54faa5d2742f3
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Fri May 13 19:10:15 2022 +0530

     wcrtomb: Make behavior POSIX compliant

Please add a note in the git commit log that if someone wants to 
backport this patch to release branches, they should also backport this 
wcrtomb change.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

> ---
>   iconv/loop.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/iconv/loop.c b/iconv/loop.c
> index f8727a637a..09ade3b765 100644
> --- a/iconv/loop.c
> +++ b/iconv/loop.c
> @@ -435,11 +435,17 @@ SINGLE(LOOPFCT) (struct __gconv_step *step,
>       return __GCONV_FULL_OUTPUT;
>   
>     /*  Now add characters from the normal input buffer.  */
> -  if (inlen >= MAX_NEEDED_INPUT)
> +  if (inlen >= MAX_NEEDED_INPUT || inptr >= inend)
>       /* Avoid a -Wstringop-overflow= warning when this loop is
>          unrolled.  The compiler cannot otherwise see that this is
>          unreachable because it depends on (state->__count & 7) not
> -       being too large after a previous conversion step.  */
> +       being too large after a previous conversion step.
> +       Starting with GCC 12, we also have mark the inptr >= inend
> +       case as unreachable to omit the warning.  Note that this SINGLE
> +       function is only used to implement the mb*towc*() or wc*tomb*()
> +       functions.  Those functions use inptr and inend pointing to a
> +       variable on stack, compute the inend pointer or explicitly check
> +       the arguments which always leads to inptr < inend.  */
>       __builtin_unreachable ();
>     do
>       bytebuf[inlen++] = *inptr++;