Re: [PATCH v3] vfprintf-internal: Get rid of alloca.

[Joe Simmons-Talbott <josimmon@redhat.com>]

On Thu, Jun 01, 2023 at 09:34:28AM -0400, Joe Simmons-Talbott via Libc-alpha wrote:
> On Mon, May 22, 2023 at 01:06:27PM -0400, Joe Simmons-Talbott wrote:
> > Avoid potential stack overflow from unbounded alloca.  Use the existing
> > scratch_buffer instead.
> 
> Ping.

x2

Thanks,
Joe
> 
> Thanks,
> Joe
> > ---
> > Changes to v2:
> > * Don't assume the first scratch buffer resize adds enough space.
> > 
> >  stdio-common/vfprintf-internal.c | 23 ++++++++++++++++++++++-
> >  1 file changed, 22 insertions(+), 1 deletion(-)
> > 
> > diff --git a/stdio-common/vfprintf-internal.c b/stdio-common/vfprintf-internal.c
> > index c76c06e49b..20fd3c3043 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 +1174,25 @@ 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]);
> > +	    while (args_pa_user + args_size[cnt] >
> > +	        (int *) &argsbuf + argsbuf.length)
> > +	      {
> > +		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);
> >  	  }
> > -- 
> > 2.39.2
> > 
>