Re: Warning in svg_load_image

[Eli Zaretskii]

> From: Michael Welsh Duggan <mwd@md5i.com>
> Cc: Po Lu <luangruo@yahoo.com>,  emacs-devel@gnu.org
> Date: Wed, 23 Feb 2022 12:11:24 -0500
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >        "xmlns:xi=\"http://www.w3.org/2001/XInclude\"; "
> >        "style=\"color: #%06X; fill: currentColor;\" "
> >        "width=\"%d\" height=\"%d\" preserveAspectRatio=\"none\" "
> > -      "viewBox=\"0 0 %f %f\">"
> > +      "viewBox=\"0 0 %5.0f %5.0f\">"
> >        "<rect width=\"100%%\" height=\"100%%\" fill=\"#%06X\"/>"
> >        "<xi:include href=\"data:image/svg+xml;base64,%s\"></xi:include>"
> >        "</svg>";
> > @@ -10801,7 +10801,9 @@ svg_load_image (struct frame *f, struct image *img, 
> > char *contents,
> >  
> >      if (buffer_size <= snprintf (wrapped_contents, buffer_size, wrapper,
> >                              foreground & 0xFFFFFF, width, height,
> > -                            viewbox_width, viewbox_height,
> > +                            /* Sanitize the viewBox dimensions.  */
> > +                            min (viewbox_width, 10000.),
> > +                            min (viewbox_height, 10000.),
> >                              background & 0xFFFFFF,
> >                              SSDATA (encoded_contents)))
> >        goto rsvg_error;
> >
> 
> So, a couple of questions and comments...  
> 
> As mentioned in other messages, % sizes affect only the minimum sizes of
> results, so changing those values shouldn't help.  (It's possible that
> using * might, but only as a possible subversion of the heuristics that
> this warning uses.)

If the compiler doesn't understand that the value is being limited to
a maximum of 5 digits, then it shouldn't attempt to emit such
"helpful" warnings.

> The principled way to solve this would be to call the snprintf twice,
> the first time with a zero-sized buffer, and then to use the return
> value to allocate the actual buffer.  This is a pessimisation, but I
> don't know if it's a bad one (it depends on how frequently this code
> would be called.

This is madness.  I'd rather we used a pragma to disable that
particular warning around this part of the code than jump through
hoops because the compiler is too stupid to understand the code it
warns about.