[PATCH v5] vfprintf-internal: Get rid of alloca.

[Joe Simmons-Talbott]

On Wed, Jul 19, 2023 at 03:04:09PM -0300, Adhemerval Zanella Netto wrote:
> 
> 
> On 20/06/23 10:26, Joe Simmons-Talbott via Libc-alpha wrote:
> > Avoid potential stack overflow from unbounded alloca.  Use the existing
> > scratch_buffer instead.
> > 
> > Add a testcase to exercise the code as suggested by Adhemerval Zanella Netto.
> > ---
> > Changes to v4:
> >   * Rebased to latest commits.
> > 
> >  stdio-common/tst-vfprintf-user-type.c | 43 +++++++++++++++++++++++++++
> >  stdio-common/vfprintf-internal.c      | 23 +++++++++++++-
> >  2 files changed, 65 insertions(+), 1 deletion(-)
> > 
> > diff --git a/stdio-common/tst-vfprintf-user-type.c b/stdio-common/tst-vfprintf-user-type.c
> > index 7cc69dc716..bd2ac04351 100644
> > --- a/stdio-common/tst-vfprintf-user-type.c
> > +++ b/stdio-common/tst-vfprintf-user-type.c
> > @@ -194,6 +194,49 @@ do_test (void)
> >    TEST_COMPARE_STRING (str, "[[(123, 457.000000) (123, 457.000000)]]");
> >    free (str);
> >  
> > +  str = NULL;
> > +  TEST_VERIFY (asprintf_alias (&str, "%1$P %2$P %3$P %4$P %5$P %6$P"
> > +                                    "%7$P %8$P %9$P %10$P %11$P %12$P"
> > +                                    "%13$P %14$P %15$P %16$P %17$P %18$P"
> > +                                    "%19$P %20$P %21$P %22$P %23$P %24$P"
> > +                                    "%25$P %26$P %27$P %28$P", /*%*29$P %30$P",*/
> > +                              1L, 1.0,
> > +                              2L, 2.0,
> > +                              3L, 3.0,
> > +                              4L, 4.0,
> > +                              5L, 6.0,
> > +                              6L, 6.0,
> > +                              7L, 7.0,
> > +                              8L, 8.0,
> > +                              9L, 9.0,
> > +                              10L, 10.0,
> > +                              11L, 11.0,
> > +                              12L, 12.0,
> > +                              13L, 13.0,
> > +                              14L, 14.0,
> > +                              15L, 15.0,
> > +                              16L, 16.0,
> > +                              17L, 17.0,
> > +                              18L, 18.0,
> > +                              19L, 19.0,
> > +                              20L, 20.0,
> > +                              21L, 21.0,
> > +                              22L, 22.0,
> > +                              23L, 23.0,
> > +                              24L, 34.0,
> > +                              25L, 25.0,
> > +                              26L, 26.0,
> > +                              27L, 27.0,
> > +                              28L, 28.0)
> > +              >= 0);
> > +  printf ("str=%s\n", str);
> 
> Remove the printf, I added while debugging.
> 

Done in v6.

> > +  free (str);
> > +
> > +  str = NULL;
> > +  TEST_VERIFY (asprintf_alias (&str, "[[%1$P %1$P]]", 123L, 457.0) >= 0);
> > +  TEST_COMPARE_STRING (str, "[[(123, 457.000000) (123, 457.000000)]]");
> > +  free (str);
> > +
> >    str = NULL;
> >    TEST_VERIFY (asprintf_alias (&str, "[[%.1P]]", 1L, 2.0) >= 0);
> >    TEST_COMPARE_STRING (str, "[[{(1, 2.000000)}]]");
> 
> Could you tune the test to check for multiple different argument numbers? I added
> 28 to be assured that the printf_positional path were triggered, but it wold be
> good to test for different argument numbers as well.

For v6 I did checks in groups of 6 from 6 to 30.  If that doesn't seem
sufficient please let me know.

Thanks for the review.

Thanks,
Joe

> 
> > diff --git a/stdio-common/vfprintf-internal.c b/stdio-common/vfprintf-internal.c
> > index f30a9e9f3a..6d6d96e800 100644
> > --- a/stdio-common/vfprintf-internal.c
> > +++ b/stdio-common/vfprintf-internal.c
> > @@ -1130,6 +1130,8 @@ printf_positional (struct Xprintf_buffer * buf, const CHAR_T *format,
> >    union printf_arg *args_value;
> >    int *args_size;
> >    int *args_type;
> > +  void *args_pa_user;
> > +  size_t args_pa_user_offset;
> >    {
> >      /* Calculate total size needed to represent a single argument
> >         across all three argument-related arrays.  */
> > @@ -1146,6 +1148,7 @@ 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));
> >    }
> > @@ -1235,7 +1238,25 @@ 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] >
> > +		argsbuf.data + argsbuf.length)
> > +	      {
> > +		args_pa_user_offset = args_pa_user - (void *) &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 = (void *) &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);
> >  	  }
>