Re: [PATCH v2] livepatch: old_name.number scheme in livepatch sysfs directory

On Wed, 4 Nov 2015, Josh Poimboeuf wrote:

On Wed, Nov 04, 2015 at 10:52:52AM +0100, Miroslav Benes wrote:

quoted

On Tue, 3 Nov 2015, Josh Poimboeuf wrote:

quoted

quoted

Object entry would be empty for not loaded object. I would not 
dare to propose to remove such object entries. It would make things worse. 

Why would removing an empty object entry make things worse?

I think it all comes down to a question whether the sysfs entries say what 
a patch is capable to patch or what this patch is currently patching in 
the system. I am inclined to the former so the removal would make me 
nervous. But I am not against the second approach. We are still in testing 
mode as far as sysfs is concerned so we can try even harsh changes and see 
how it's gonna go.

I see your point.  This approach only describes what is patched now, but
it doesn't describe what *will* be patched.  Ideally we could find a way
to describe both.

Speaking of harsh changes, here's an idea.

Which is the very same you proposed last year when I tried to persuade you 
to get rid off old_addr and stuff. I called it crazy I remember :D. So 
here we are again...

What if we require the patch author to supply the value of 'n' instead
of supplying the symbol address?  We could get rid of 'old_addr' as an
input in klp_func and and replace it with 'old_sympos' which has the
value of 'n'.  Or alternatively we could require old_name to be of the
format "func,n".

That would uniquely identify each patched function, even _before_ the
object is loaded.

I find it reasonable and we should try it. I think that old_sympos should 
have this semantics

0 - default, preserve more or less current behaviour. If the symbol is 
    unique there is no problem. If it is not the patching would fail.
1, 2, ... - occurrence of the symbol in kallsyms.

The advantage is that if the user does not care and is certain that the 
symbol is unique he doesn't have to do anything. If the symbol is not 
unique he still has means how to solve it.

Does it make sense?

It would also fix another big problem we have today, where there's no
way to disambiguate duplicate symbols in modules, for both function
addresses and for relocs.

True.

It would simplify the code in other places as well: no special handling
for kASLR, no need for klp_verify_vmlinux_symbol() vs
klp_find_object_symbol().

Which would be great.

A drawback is that it requires the patch author to do a little more due
diligence when filling out klp_func.  But we already require them to be
careful.

Yes, I don't think this should be a problem.

Thoughts?

Yup, we should try it. I suppose that the order of the symbols in kallsyms 
table is stable for once-built kernel. It is the order of the symbols in 
the object files, isn't it? And since each livepatch module is built 
against the specific kernel there should be no issues with this.

Regards,
Miroslav