linux-kernel - Re: [PATCH v3 04/13] x86: Handle KCOV _

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Message-ID: <aH42--h-ARsvX5Wk@willie-the-truck>
Date: Mon, 21 Jul 2025 13:47:55 +0100
From: Will Deacon <will@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Kees Cook <kees@...nel.org>, Mike Rapoport <rppt@...nel.org>,
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Subject: Re: [PATCH v3 04/13] x86: Handle KCOV __init vs inline mismatches

On Sun, Jul 20, 2025 at 04:10:01PM +1000, Ard Biesheuvel wrote:
> On Sat, 19 Jul 2025 at 08:51, Kees Cook <kees@...nel.org> wrote:
> > On Fri, Jul 18, 2025 at 11:36:32AM +0300, Mike Rapoport wrote:
> > > 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@...nel.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.

Got it, thanks for the explanation!

> > 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.
> >
> 
> Given that 'inline' is already a macro in the kernel, could we just
> add __attribute__((__always_inline__)) to it when KCOV is enabled?

That sounds like a more robust approach and, by the sounds of it, we
could predicate it on GCC too. That would also provide a neat place for
a comment describing the problem.

Kees, would that work for you?

Will