debug: Improve '%n' fortify detection (BZ 30932)

The 7bb8045ec0 path made the '%n' fortify check ignore EMFILE errors
while trying to open /proc/self/maps, and this added a security
issue where EMFILE can be attacker-controlled thus making it
ineffective for some cases.

The EMFILE failure is reinstated but with a different error
message.  Also, to improve the false positive of the hardening for
the cases where no new files can be opened, the
_dl_readonly_area now uses  _dl_find_object to check if the
memory area is within a writable ELF segment.  The procfs method is
still used as fallback.

Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Arjun Shankar <arjun@redhat.com>

stdio-common/vfprintf-internal.c

@@ -576,7 +576,8 @@ static const uint8_t jump_table[] =
 
 /* Handle positional format specifiers.  */
 static void printf_positional (struct Xprintf_buffer *buf,
-			       const CHAR_T *format, int readonly_format,
+			       const CHAR_T *format,
+			       enum readonly_error_type readonly_format,
 			       va_list ap, va_list *ap_savep,
 			       int nspecs_done, const UCHAR_T *lead_str_end,
 			       CHAR_T *work_buffer, int save_errno,
@@ -626,9 +627,7 @@ Xprintf_buffer (struct Xprintf_buffer *buf, const CHAR_T *format,
   /* For the %m format we may need the current `errno' value.  */
   int save_errno = errno;
 
-  /* 1 if format is in read-only memory, -1 if it is in writable memory,
-     0 if unknown.  */
-  int readonly_format = 0;
+  enum readonly_error_type readonly_format = readonly_noerror;
 
   /* Initialize local variables.  */
   grouping = (const char *) -1;
@@ -1045,7 +1044,7 @@ do_positional:
 
 static void
 printf_positional (struct Xprintf_buffer * buf, const CHAR_T *format,
-		   int readonly_format,
+		   enum readonly_error_type readonly_format,
 		   va_list ap, va_list *ap_savep, int nspecs_done,
 		   const UCHAR_T *lead_str_end,
 		   CHAR_T *work_buffer, int save_errno,