[PATCH] x86: avoid i386_dis_printf()'s staging area for potentially long strings

[PATCH] x86: avoid i386_dis_printf()'s staging area for potentially long strings

[Jan Beulich]

PR binutils/29483

When print_insn() processes op_txt[], it may pass strings into
i386_dis_printf() which staging_area[] cannot fit; this was observed for
an invalid form of VPSCATTERDD (both broadcast and zeroing-masking bits
set). Rather than arbitrarily enlarging that local array, avoid its use
altogether when the format string is simply "%s". This merely requires
two local variables to have their type constified.

While limiting the scope of "res" it became apparent that
- no caller cares about the function's return value,
- the comment about the return value was wrong,
- a particular positive return value would have been meaningless to the
  caller.
Therefore convert the function to return "void" at the same time.
---
An alternative to the special casing would be to introduce something
like i386_dis_puts(), then to be used by all call sites which currently
pass "%s" or format strings without any format characters at all (plus,
of course, i386_dis_printf() itself).

--- a/opcodes/i386-dis.c
+++ b/opcodes/i386-dis.c
@@ -9264,31 +9264,40 @@ oappend_register (instr_info *ins, const
    STYLE is the default style to use in the fprintf_styled_func calls,
    however, FMT might include embedded style markers (see oappend_style),
    these embedded markers are not printed, but instead change the style
-   used in the next fprintf_styled_func call.
+   used in the next fprintf_styled_func call.  */
 
-   Return non-zero to indicate the print call was a success.  */
-
-static int ATTRIBUTE_PRINTF_3
+static void ATTRIBUTE_PRINTF_3
 i386_dis_printf (instr_info *ins, enum disassembler_style style,
 		 const char *fmt, ...)
 {
   va_list ap;
   enum disassembler_style curr_style = style;
-  char *start, *curr;
+  const char *start, *curr;
   char staging_area[100];
-  int res;
 
   va_start (ap, fmt);
-  res = vsnprintf (staging_area, sizeof (staging_area), fmt, ap);
-  va_end (ap);
+  /* In particular print_insn()'s processing of op_txt[] can hand rather long
+     strings here.  Bypass vsnprintf() in such cases to avoid capacity issues
+     with the staging area.  */
+  if (strcmp (fmt, "%s"))
+    {
+      int res = vsnprintf (staging_area, sizeof (staging_area), fmt, ap);
 
-  if (res < 0)
-    return res;
+      va_end (ap);
 
-  if ((size_t) res >= sizeof (staging_area))
-    abort ();
+      if (res < 0)
+	return;
 
-  start = curr = staging_area;
+      if ((size_t) res >= sizeof (staging_area))
+	abort ();
+
+      start = curr = staging_area;
+    }
+  else
+    {
+      start = curr = va_arg (ap, const char *);
+      va_end (ap);
+    }
 
   do
     {
@@ -9303,10 +9312,7 @@ i386_dis_printf (instr_info *ins, enum d
 						     curr_style,
 						     "%.*s", len, start);
 	  if (n < 0)
-	    {
-	      res = n;
-	      break;
-	    }
+	    break;
 
 	  if (*curr == '\0')
 	    break;
@@ -9340,8 +9346,6 @@ i386_dis_printf (instr_info *ins, enum d
 	++curr;
     }
   while (true);
-
-  return res;
 }
 
 static int

Re: [PATCH] x86: avoid i386_dis_printf()'s staging area for potentially long strings

[H.J. Lu]

On Mon, Aug 15, 2022 at 4:17 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> PR binutils/29483
>
> When print_insn() processes op_txt[], it may pass strings into
> i386_dis_printf() which staging_area[] cannot fit; this was observed for
> an invalid form of VPSCATTERDD (both broadcast and zeroing-masking bits
> set). Rather than arbitrarily enlarging that local array, avoid its use
> altogether when the format string is simply "%s". This merely requires
> two local variables to have their type constified.
>
> While limiting the scope of "res" it became apparent that
> - no caller cares about the function's return value,
> - the comment about the return value was wrong,
> - a particular positive return value would have been meaningless to the
>   caller.
> Therefore convert the function to return "void" at the same time.
> ---
> An alternative to the special casing would be to introduce something
> like i386_dis_puts(), then to be used by all call sites which currently
> pass "%s" or format strings without any format characters at all (plus,
> of course, i386_dis_printf() itself).
>
> --- a/opcodes/i386-dis.c
> +++ b/opcodes/i386-dis.c
> @@ -9264,31 +9264,40 @@ oappend_register (instr_info *ins, const
>     STYLE is the default style to use in the fprintf_styled_func calls,
>     however, FMT might include embedded style markers (see oappend_style),
>     these embedded markers are not printed, but instead change the style
> -   used in the next fprintf_styled_func call.
> +   used in the next fprintf_styled_func call.  */
>
> -   Return non-zero to indicate the print call was a success.  */
> -
> -static int ATTRIBUTE_PRINTF_3
> +static void ATTRIBUTE_PRINTF_3
>  i386_dis_printf (instr_info *ins, enum disassembler_style style,
>                  const char *fmt, ...)
>  {
>    va_list ap;
>    enum disassembler_style curr_style = style;
> -  char *start, *curr;
> +  const char *start, *curr;
>    char staging_area[100];
> -  int res;
>
>    va_start (ap, fmt);
> -  res = vsnprintf (staging_area, sizeof (staging_area), fmt, ap);
> -  va_end (ap);
> +  /* In particular print_insn()'s processing of op_txt[] can hand rather long
> +     strings here.  Bypass vsnprintf() in such cases to avoid capacity issues
> +     with the staging area.  */
> +  if (strcmp (fmt, "%s"))
> +    {
> +      int res = vsnprintf (staging_area, sizeof (staging_area), fmt, ap);
>
> -  if (res < 0)
> -    return res;
> +      va_end (ap);
>
> -  if ((size_t) res >= sizeof (staging_area))
> -    abort ();
> +      if (res < 0)
> +       return;
>
> -  start = curr = staging_area;
> +      if ((size_t) res >= sizeof (staging_area))
> +       abort ();
> +
> +      start = curr = staging_area;
> +    }
> +  else
> +    {
> +      start = curr = va_arg (ap, const char *);
> +      va_end (ap);
> +    }
>
>    do
>      {
> @@ -9303,10 +9312,7 @@ i386_dis_printf (instr_info *ins, enum d
>                                                      curr_style,
>                                                      "%.*s", len, start);
>           if (n < 0)
> -           {
> -             res = n;
> -             break;
> -           }
> +           break;
>
>           if (*curr == '\0')
>             break;
> @@ -9340,8 +9346,6 @@ i386_dis_printf (instr_info *ins, enum d
>         ++curr;
>      }
>    while (true);
> -
> -  return res;
>  }
>
>  static int

Please include a testcase to verify that the issue is fixed.

Thanks.

-- 
H.J.

Re: [PATCH] x86: avoid i386_dis_printf()'s staging area for potentially long strings

[Jan Beulich]

On 15.08.2022 16:25, H.J. Lu wrote:
> Please include a testcase to verify that the issue is fixed.

I was fearing I would get back such a reply - I'm not convinced of the
usefulness of testcases like what you ask for. Such a test succeeding
doesn't mean at all that a similar issue hasn't been (re)introduced.

Jan