Re: [PATCH v3 04/13] x86: Handle KCOV __init vs inline mismatches

Mike Rapoport
2025-07-22T08:27:05Z

On Fri, Jul 18, 2025 at 03:51:28PM -0700, Kees Cook wrote:
> On Fri, Jul 18, 2025 at 11:36:32AM +0300, Mike Rapoport wrote:
> > Hi Kees,
> >
> > On Thu, Jul 17, 2025 at 04:25:09PM -0700, Kees Cook wrote:
> > > When KCOV is enabled all functions get instrumented, unless the
> > > __no_sanitize_coverage attribute is used. To prepare for
> > > __no_sanitize_coverage being applied to __init functions, we have to
> > > handle differences in how GCC's inline optimizations get resolved. For
> > > x86 this means forcing several functions to be inline with
> > > __always_inline.
> > >
> > > Signed-off-by: Kees Cook <kees@kernel.org>
> >
> > ...
> >
> > > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > > index bb19a2534224..b96746376e17 100644
> > > --- a/include/linux/memblock.h
> > > +++ b/include/linux/memblock.h
> > > @@ -463,7 +463,7 @@ static inline void *memblock_alloc_raw(phys_addr_t size,
> > > NUMA_NO_NODE);
> > > }
> > >
> > > -static inline void *memblock_alloc_from(phys_addr_t size,
> > > +static __always_inline void *memblock_alloc_from(phys_addr_t size,
> > > phys_addr_t align,
> > > phys_addr_t min_addr)
> >
> > I'm curious why from all memblock_alloc* wrappers this is the only one that
> > needs to be __always_inline?
>
> Thread-merge[1], adding Will Deacon, who was kind of asking the same
> question.
>
> Based on what I can tell, GCC has kind of fragile inlining logic, in the
> sense that it can change whether or not it inlines something based on
> optimizations. It looks like the kcov instrumentation being added (or in
> this case, removed) from a function changes the optimization results,
> and some functions marked "inline" are _not_ inlined. In that case, we end up
> with __init code calling a function not marked __init, and we get the
> build warnings I'm trying to eliminate.
>
> So, to Will's comment, yes, the problem is somewhat fragile (though
> using either __always_inline or __init will deterministically solve it).
> We've tripped over this before with GCC and the solution has usually
> been to just use __always_inline and move on.
>
> For memblock_alloc*, it appears to be that the heuristic GCC uses
> resulted in only memblock_alloc_from() being a problem in this case. I
> can certainly mark them all as __always_inline if that is preferred.

We had a few of those already converted to __always_inline, so I'm ok with
continuing to fix them one at at time. Gives a feeling of job security ;-)

> --
> Kees Cook

--
Sincerely yours,
Mike.