Re: [PATCH] vfprintf-internal: Get rid of alloca.

[Carlos O'Donell <carlos@redhat.com>]

On 5/17/23 17:28, Joe Simmons-Talbott via Libc-alpha wrote:
> Avoid potential stack overflow from unbounded alloca.  Use the existing
> scratch_buffer instead.

Fails 32-bit i686 CI:
https://patchwork.sourceware.org/project/glibc/patch/20230517212847.1821277-1-josimmon@redhat.com/

Please have a look.

> ---
>  stdio-common/vfprintf-internal.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/stdio-common/vfprintf-internal.c b/stdio-common/vfprintf-internal.c
> index c76c06e49b..baaefb763a 100644
> --- a/stdio-common/vfprintf-internal.c
> +++ b/stdio-common/vfprintf-internal.c
> @@ -1066,6 +1066,8 @@ printf_positional (struct Xprintf_buffer * buf, const CHAR_T *format,
>    union printf_arg *args_value;
>    int *args_size;
>    int *args_type;
> +  int *args_pa_user;
> +  size_t args_pa_user_offset;
>    {
>      /* Calculate total size needed to represent a single argument
>         across all three argument-related arrays.  */
> @@ -1082,6 +1084,7 @@ printf_positional (struct Xprintf_buffer * buf, const CHAR_T *format,
>         now.  */
>      args_size = &args_value[nargs].pa_int;
>      args_type = &args_size[nargs];
> +    args_pa_user = &args_type[nargs];
>      memset (args_type, (mode_flags & PRINTF_FORTIFY) != 0 ? '\xff' : '\0',
>  	    nargs * sizeof (*args_type));
>    }
> @@ -1171,7 +1174,24 @@ printf_positional (struct Xprintf_buffer * buf, const CHAR_T *format,
>  	else if (__glibc_unlikely (__printf_va_arg_table != NULL)
>  		 && __printf_va_arg_table[args_type[cnt] - PA_LAST] != NULL)
>  	  {
> -	    args_value[cnt].pa_user = alloca (args_size[cnt]);
> +	    if (args_pa_user > argsbuf + args_size)
> +	      {
> +		args_pa_user_offset = args_pa_user - &args_type[nargs];
> +	        if (!scratch_buffer_grow_preserve (&argsbuf))
> +	          {
> +	            Xprintf_buffer_mark_failed (buf);
> +	            goto all_done;
> +	          }
> +                args_value = argsbuf.data;
> +                /* Set up the remaining two arrays to each point past the end of
> +                   the prior array, since space for all three has been allocated
> +                   now.  */
> +                args_size = &args_value[nargs].pa_int;
> +                args_type = &args_size[nargs];
> +                args_pa_user = &args_type[nargs] + args_pa_user_offset;
> +	      }
> +	    args_value[cnt].pa_user = args_pa_user;
> +	    args_pa_user += args_size[cnt];
>  	    (*__printf_va_arg_table[args_type[cnt] - PA_LAST])
>  	      (args_value[cnt].pa_user, ap_savep);
>  	  }

-- 
Cheers,
Carlos.