Re: [PATCH v4 4/5] livepatch: Clear relocations when klp_apply_object_relocs() fails

[Miroslav Benes]

On Fri, 18 Sep 2026, Miroslav Benes wrote:

> On Tue, 8 Sep 2026, Petr Mladek wrote:
> 
> > When a module is loaded, klp_module_coming() updates all enabled
> > livepatches. If an error occurs, it delegates cleanup to
> > klp_cleanup_module_patches_limited(). However, this cleanup loop skips
> > the partially updated patch, leaving any changes made prior to failure
> > unreverted.
> > 
> > One unhandled failure path occurs inside klp_apply_object_relocs(). On
> > architectures like x86_64, apply_relocate_add() performs a verification
> > step using memcmp() to check that memory contains the expected relocated
> > or zeroed value. If relocations left behind by a failed patch are not
> > cleared, subsequent patch operations or reloads can fail this validation.
> > 
> > Introduce klp_write_object_relocs_limited() to unwind and clear only the
> > relocations that were successfully applied before klp_write_object_relocs()
> > encountered an error.
> > 
> > There is no need to clear relocations for other objects in the failing
> > patch because klp_module_coming() operates strictly on the specific
> > module being loaded.
> 
> Hm, I spent some quite time on that and I am not sure if I deciphered 
> everything correctly.
> 
> It seems to me that all error handling in those paths you are mentioning 
> above is correct and the only problematic thing is that 
> klp_apply_object_relocs() in klp_init_object_loaded() returns ret 
> immediately which leaves partially applied relocations in case of an 
> error. Following calls in klp_init_object_loaded() go through err: label 
> where klp_free_object_loaded() is called which should be fine.
> 
> Is it correct?
> 
> Wouldn't be better to be somehow consistent and clean up right in the 
> error path of klp_apply_object_relocs() call in klp_init_object_loaded() 
> similarly to what is already there. After all we want to clean the object.
> 
> But I also see why you want to do it this way. It only seems more fragile 
> to me.

And only now I realized that I checked with 5/5 already applied by 
mistake. The above still stands though and at least I reviewed 5/5 as 
well.

Miroslav