Re: [PATCH 1/2] livepatch: do not ignore sections with 0 size

[Andrew Cooper <amc96@xxxxxxxx>]

On 17/03/2022 14:16, Roger Pau Monné wrote:
> On Thu, Mar 17, 2022 at 02:00:19PM +0000, Andrew Cooper wrote:
>> On 17/03/2022 11:08, Roger Pau Monne wrote:
>>> A side effect of ignoring such sections is that symbols belonging to
>>> them won't be resolved, and that could make relocations belonging to
>>> other sections that reference those symbols fail.
>>>
>>> For example it's likely to have an empty .altinstr_replacement with
>>> symbols pointing to it, and marking the section as ignored will
>>> prevent the symbols from being resolved, which in turn will cause any
>>> relocations against them to fail.
>> I agree this is a bug in livepatch handling, but it's also an error in
>> the generated livepatch.  We should not have relocations to an empty
>> altinstr_replacement section in the first place.
> Well, the relocation destination is in the .altinstructions section
> (which is not empty). It happens however to reference a symbol that
> points to the .altinstr_replacement section that's empty.
>
> We could likely avoid generating the altinstr_replacement section in
> the first place, but I think it's more robust to handle those properly
> in the elf loading code.

Actually, it turns out it's distinctly non-trivial to omit these
references.  We need to put the replacement somewhere, so we can
subtract the start from the end, and figure out if it is 0.

~Andrew