Re: [PATCH] fxprintf: Get rid of alloca

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 07/07/23 14:53, Joe Simmons-Talbott via Libc-alpha wrote:
> Use a scratch_buffer rather than alloca/malloc to avoid potential stack
> overflow.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  stdio-common/fxprintf.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/stdio-common/fxprintf.c b/stdio-common/fxprintf.c
> index f0ac9654ab..88501ab61f 100644
> --- a/stdio-common/fxprintf.c
> +++ b/stdio-common/fxprintf.c
> @@ -15,6 +15,7 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> +#include <scratch_buffer.h>
>  #include <stdarg.h>
>  #include <stdio.h>
>  #include <stdlib.h>
> @@ -34,20 +35,18 @@ locked_vfxprintf (FILE *fp, const char *fmt, va_list ap,
>    wchar_t *wfmt;
>    mbstate_t mbstate;
>    int res;
> -  int used_malloc = 0;
>    size_t len = strlen (fmt) + 1;
> +  struct scratch_buffer buf;
> +  scratch_buffer_init (&buf);
>  
>    if (__glibc_unlikely (len > SIZE_MAX / sizeof (wchar_t)))
>      {
>        __set_errno (EOVERFLOW);
>        return -1;
>      }

This check is redundant, but scratch_buffer_set_array_size would return
ENOMEM in this case.  I guess it should not change this for now.

> -  if (__libc_use_alloca (len * sizeof (wchar_t)))
> -    wfmt = alloca (len * sizeof (wchar_t));
> -  else if ((wfmt = malloc (len * sizeof (wchar_t))) == NULL)
> +  if (!scratch_buffer_set_array_size (&buf, sizeof (wchar_t), len))
>      return -1;
> -  else
> -    used_malloc = 1;
> +  wfmt = buf.data;
>  
>    memset (&mbstate, 0, sizeof mbstate);
>    res = __mbsrtowcs (wfmt, &fmt, len, &mbstate);
> @@ -55,8 +54,7 @@ locked_vfxprintf (FILE *fp, const char *fmt, va_list ap,
>    if (res != -1)
>      res = __vfwprintf_internal (fp, wfmt, ap, mode_flags);
>  
> -  if (used_malloc)
> -    free (wfmt);
> +  scratch_buffer_free (&buf);
>  
>    return res;
>  }