Re: potential bug in display_mode_element?

["Richard M. Stallman" <rms@gnu.org>]

I think you are right about the bug, but I think your patch does not
completely fix it.  This patch, which seems to work, should fix the bug
completely by accessing the string only through its Lisp object pointer.

*** xdisp.c	08 Sep 2005 20:35:28 -0400	1.1050
--- xdisp.c	12 Sep 2005 06:18:48 -0400	
***************
*** 15949,15955 ****
        {
  	/* A string: output it and check for %-constructs within it.  */
  	unsigned char c;
! 	const unsigned char *this, *lisp_string;
  
  	if (!NILP (props) || risky)
  	  {
--- 15949,15955 ----
        {
  	/* A string: output it and check for %-constructs within it.  */
  	unsigned char c;
! 	int offset = 0;
  
  	if (!NILP (props) || risky)
  	  {
***************
*** 16007,16014 ****
  	      }
  	  }
  
! 	this = SDATA (elt);
! 	lisp_string = this;
  
  	if (literal)
  	  {
--- 16007,16013 ----
  	      }
  	  }
  
! 	offset = 0;
  
  	if (literal)
  	  {
***************
*** 16031,16072 ****
  	    break;
  	  }
  
  	while ((precision <= 0 || n < precision)
! 	       && *this
  	       && (mode_line_target != MODE_LINE_DISPLAY
  		   || it->current_x < it->last_visible_x))
  	  {
! 	    const unsigned char *last = this;
  
  	    /* Advance to end of string or next format specifier.  */
! 	    while ((c = *this++) != '\0' && c != '%')
  	      ;
  
! 	    if (this - 1 != last)
  	      {
  		int nchars, nbytes;
  
  		/* Output to end of string or up to '%'.  Field width
  		   is length of string.  Don't output more than
  		   PRECISION allows us.  */
! 		--this;
  
! 		prec = c_string_width (last, this - last, precision - n,
  				       &nchars, &nbytes);
  
  		switch (mode_line_target)
  		  {
  		  case MODE_LINE_NOPROP:
  		  case MODE_LINE_TITLE:
! 		    n += store_mode_line_noprop (last, 0, prec);
  		    break;
  		  case MODE_LINE_STRING:
  		    {
! 		      int bytepos = last - lisp_string;
  		      int charpos = string_byte_to_char (elt, bytepos);
  		      int endpos = (precision <= 0
! 				    ? string_byte_to_char (elt,
! 							   this - lisp_string)
  				    : charpos + nchars);
  
  		      n += store_mode_line_string (NULL,
--- 16030,16073 ----
  	    break;
  	  }
  
+ 	/* Handle the non-literal case.  */
+ 
  	while ((precision <= 0 || n < precision)
! 	       && SREF (elt, offset) != 0
  	       && (mode_line_target != MODE_LINE_DISPLAY
  		   || it->current_x < it->last_visible_x))
  	  {
! 	    int last_offset = offset;
  
  	    /* Advance to end of string or next format specifier.  */
! 	    while ((c = SREF (elt, offset++)) != '\0' && c != '%')
  	      ;
  
! 	    if (offset - 1 != last_offset)
  	      {
  		int nchars, nbytes;
  
  		/* Output to end of string or up to '%'.  Field width
  		   is length of string.  Don't output more than
  		   PRECISION allows us.  */
! 		offset--;
  
! 		prec = c_string_width (SDATA (elt) + last_offset,
! 				       offset - last_offset, precision - n,
  				       &nchars, &nbytes);
  
  		switch (mode_line_target)
  		  {
  		  case MODE_LINE_NOPROP:
  		  case MODE_LINE_TITLE:
! 		    n += store_mode_line_noprop (SDATA (elt) + last_offset, 0, prec);
  		    break;
  		  case MODE_LINE_STRING:
  		    {
! 		      int bytepos = last_offset;
  		      int charpos = string_byte_to_char (elt, bytepos);
  		      int endpos = (precision <= 0
! 				    ? string_byte_to_char (elt, offset)
  				    : charpos + nchars);
  
  		      n += store_mode_line_string (NULL,
***************
*** 16077,16083 ****
  		    break;
  		  case MODE_LINE_DISPLAY:
  		    {
! 		      int bytepos = last - lisp_string;
  		      int charpos = string_byte_to_char (elt, bytepos);
  		      n += display_string (NULL, elt, Qnil, 0, charpos,
  					   it, 0, prec, 0,
--- 16078,16084 ----
  		    break;
  		  case MODE_LINE_DISPLAY:
  		    {
! 		      int bytepos = last_offset;
  		      int charpos = string_byte_to_char (elt, bytepos);
  		      n += display_string (NULL, elt, Qnil, 0, charpos,
  					   it, 0, prec, 0,
***************
*** 16088,16099 ****
  	      }
  	    else /* c == '%' */
  	      {
! 		const unsigned char *percent_position = this;
  
  		/* Get the specified minimum width.  Zero means
  		   don't pad.  */
  		field = 0;
! 		while ((c = *this++) >= '0' && c <= '9')
  		  field = field * 10 + c - '0';
  
  		/* Don't pad beyond the total padding allowed.  */
--- 16089,16100 ----
  	      }
  	    else /* c == '%' */
  	      {
! 		int percent_position = offset;
  
  		/* Get the specified minimum width.  Zero means
  		   don't pad.  */
  		field = 0;
! 		while ((c = SREF (elt, offset++)) >= '0' && c <= '9')
  		  field = field * 10 + c - '0';
  
  		/* Don't pad beyond the total padding allowed.  */
***************
*** 16113,16119 ****
  		    int bytepos, charpos;
  		    unsigned char *spec;
  
! 		    bytepos = percent_position - lisp_string;
  		    charpos = (STRING_MULTIBYTE (elt)
  			       ? string_byte_to_char (elt, bytepos)
  			       : bytepos);
--- 16114,16120 ----
  		    int bytepos, charpos;
  		    unsigned char *spec;
  
! 		    bytepos = percent_position;
  		    charpos = (STRING_MULTIBYTE (elt)
  			       ? string_byte_to_char (elt, bytepos)
  			       : bytepos);