Re: [PATCH] rwonce: handle KCSAN like KASAN in read_word_at_a_time()

[Jann Horn]

On Wed, Mar 26, 2025 at 9:39 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
> On Tue, Mar 25, 2025 at 05:01:34PM +0100, Jann Horn wrote:
> > Also, since this read can be racy by design, we should technically do
> > READ_ONCE(), so add that.
> >
> > Fixes: dfd402a4c4ba ("kcsan: Add Kernel Concurrency Sanitizer infrastructure")
> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ...
> > diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
> > index 8d0a6280e982..e9f2b84d2338 100644
> > --- a/include/asm-generic/rwonce.h
> > +++ b/include/asm-generic/rwonce.h
> > @@ -79,11 +79,14 @@ unsigned long __read_once_word_nocheck(const void *addr)
> >       (typeof(x))__read_once_word_nocheck(&(x));                      \
> >  })
> >
> > -static __no_kasan_or_inline
> > +static __no_sanitize_or_inline
> >  unsigned long read_word_at_a_time(const void *addr)
> >  {
> > +     /* open-coded instrument_read(addr, 1) */
> >       kasan_check_read(addr, 1);
> > -     return *(unsigned long *)addr;
> > +     kcsan_check_read(addr, 1);
> > +
> > +     return READ_ONCE(*(unsigned long *)addr);
>
> I bisected a boot hang that I see on arm64 with LTO enabled to this
> change as commit ece69af2ede1 ("rwonce: handle KCSAN like KASAN in
> read_word_at_a_time()") in -next. With LTO, READ_ONCE() gets upgraded to
> ldar / ldapr, which requires an aligned address to access, but
> read_word_at_a_time() can be called with an unaligned address. I
> confirmed this should be the root cause by removing the READ_ONCE()
> added above or removing the selects of DCACHE_WORD_ACCESS and
> HAVE_EFFICIENT_UNALIGNED_ACCESS in arch/arm64/Kconfig, which avoids
> the crash.

Oh, bleeeh. Thanks for figuring that out. I guess that means we should
remove that READ_ONCE() again to un-break the build. I'll send a patch
in a bit...