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

[Jan Beulich]

On 25.09.2025 08:32, H.J. Lu wrote:
> 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.
> 
> 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.

That's not how I look at things, sorry. If Alan or Nick want to approve this
patch, I'm not going to stand in the way, but I won't (and I object to you
putting it in without approval, ftaod).

Jan