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

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

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.

> +  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.

> 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);
>  	  }