[PATCH 3/4] elf: Avoid linker crash on corrupt .eh_frame section

[H.J. Lu]

On Thu, Sep 25, 2025 at 2:06 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 23.09.2025 22:46, H.J. Lu wrote:
> > On Tue, Sep 23, 2025 at 10:50 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 23.09.2025 04:23, H.J. Lu wrote:
> >>> Return false on corrupt .eh_frame section.  Cap the .eh_frame section
> >>> alignment to the larger of the section size and the pointer size.
> >>
> >> Like for patch 2, I'm unconvinced of it being a good idea to build in
> >> heuristics like this. What if someone feels a need to align .eh_frame
> >> to, say, a page boundary?
> >
> > .eh_frame section is special:
> >
> >      eh_alignment = ((1 << o->alignment_power)
> >                       * bfd_octets_per_byte (output_bfd, o));
> >       /* Skip over zero terminator, and prevent empty sections from
> >          adding alignment padding at the end.  */
> >       for (i = o->map_tail.s; i != NULL; i = i->map_tail.s)
> >         if (i->size == 0)
> >           i->flags |= SEC_EXCLUDE;
> >         else if (i->size > 4)
> >           break;
> >       /* The last non-empty eh_frame section doesn't need padding.  */
> >       if (i != NULL)
> >         i = i->map_tail.s;
> >       /* Any prior sections must pad the last FDE out to the output
> >          section alignment.  Otherwise we might have zero padding
> >          between sections, which would be seen as a terminator.  */
> >       for (; i != NULL; i = i->map_tail.s)
> >         if (i->size == 4)
> >           /* All but the last zero terminator should have been removed.  */
> >           BFD_FAIL ();
> >         else
> >           {
> >             bfd_size_type size
> >               = (i->size + eh_alignment - 1) & -eh_alignment;
> >
> > If .eh_frame section alignment is larger than the section size,
> > we will get buffer overflow.
> >
> >             if (i->size != size)
> >               {
> >                 i->size = size;
> >                 changed = 1;
> >                 eh_changed = 1;
> >               }
> >           }
> >
> >> Also, please format unsigned long-s using %lu or (here) %lx. Especially
> >> when values get large, imo printing them as decimal is unhelpful.
> >
> > Will do.
> >
> >> Finally - why would .eh_frame be special in this regard? What about,
> >> say, .sframe?
> >>
> >
> > .sframe section doesn't do the code above.
>
> I understand that, but I hoped you would also understand that I'm merely
> trying to give examples. In all of your reply you didn't address my question
> "What if someone feels a need to align .eh_frame to, say, a page boundary?"
> If someone doing so resulted in a buffer overflow, that would indeed want
> fixing. But not by preventing them to use that kind of alignment.
>
> Jan

So far, we don't have any request for large .eh_frame section alignment.
My patch simply changes the linker crash to a linker error.  If people
do need large .eh_frame section alignments in the future, we can work
with them to resolve their issues.


-- 
H.J.