Re: [PATCH v4 1/5] livepatch: Fail object initialization on duplicate patched function

[Petr Mladek]

On Tue 2026-09-08 12:17:44, sashiko-bot@xxxxxxxxxx wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Atomic replace patches will fail to load if previous patches targeted aliased functions.
> --
> 
> commit e5553650270f93e11fe885d264a3829802223d3a
> Author: Harry Hsu <x90613@xxxxxxxxx>
> 
> livepatch: Fail object initialization on duplicate patched function
> 
> This commit introduces a check to prevent patching multiple aliases of a
> function within a single livepatch. It fails object initialization if aliases
> resolving to the same address are detected.
> 
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index a240d1144e89c..a6762cbe74b70 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> [ ... ]
> > @@ -885,6 +885,21 @@ static int klp_init_object_loaded(struct klp_patch *patch,
> >  		if (ret)
> >  			return ret;
> >  
> > +		/*
> > +		 * Aliased symbols share one address, so they would resolve to
> > +		 * the same klp_ops and stack up on a single ops->func_stack,
> > +		 * leaving the redirection ambiguous.
> > +		 */
> > +		klp_for_each_func(obj, prev_func) {
> > +			if (prev_func == func)
> > +				break;
> > +			if (prev_func->old_func == func->old_func) {
> > +				pr_err("'%s' and '%s' resolve to the same address, aliased symbols are not supported\n",
> > +				       prev_func->old_name, func->old_name);
> > +				return -EINVAL;
> 
> [Severity: High]
> Will this check inadvertently reject nop functions generated by an atomic
> replace patch when previous patches targeted aliased functions?
> 
> If previously loaded livepatches targeted aliased functions (for example, one
> patch targets __do_sys_fork and another targets __x64_sys_fork), they were
> permitted since they resided in different patches.
> 
> When an atomic replace patch is loaded, klp_add_nops() loops over all
> previously patched functions and adds nops for them to the new patch. Since
> nop deduplication is based on old_name, nops for both aliases are added.
> 
> Later, this check in klp_init_object_loaded() resolves their addresses and
> finds duplicate old_func addresses. This causes the initialization to fail
> with -EINVAL, which prevents the system from atomically replacing existing
> livepatches and might force a reboot if the old patches cannot be safely
> disabled manually.

Wow, this is a nice catch. I would say that it is a corner case but
the problem seems to exist. Now, the question is how to deal with it.

I see three possibilities:

1. We might detect the aliases in klp_find_func() and create only
   one "nop" entry or do not create it all.

2. The problem might be easier to solve after adding
   the provides/obsoletes, aka replace set, feature [1].
   It should prevent loading the other livepatch for the aliased
   symbol in the 1st place.

3. Just document the problem as a limitation. It might be hit only
   when installing more liveapatches in parallel that current
   wild way.

I personally prefer to wait for the provides/requires feature.

[1] https://lore.kernel.org/all/20260607131659.29281-1-laoar.shao@xxxxxxxxx/

Best Regards,
Petr

> > +			}
> > +		}
> > +
> >  		ret = kallsyms_lookup_size_offset((unsigned long)func->old_func,
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260908120325.299649-1-pmladek@xxxxxxxx?part=1