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Date:   Tue, 12 Jan 2021 15:56:26 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] ubsan: Implement __ubsan_handle_alignment_assumption
Message-ID: <202101121555.C929315@keescook>
References: <20210112205542.1375847-1-natechancellor@gmail.com>
 <CAKwvOd=yrVKBn9TN2cP8SiB7A8=c2g41PyodKGJu+xEQwAmnDA@mail.gmail.com>
 <20210112213703.GA1376568@ubuntu-m3-large-x86>
 <CAKwvOdkA5kmXhKFDFTApLyT5LcUX2-Xr6vJJ0b8wePunMpLu0g@mail.gmail.com>
 <20210112220634.GA1377517@ubuntu-m3-large-x86>
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On Tue, Jan 12, 2021 at 03:06:34PM -0700, Nathan Chancellor wrote:
> On Tue, Jan 12, 2021 at 01:53:30PM -0800, Nick Desaulniers wrote:
> > On Tue, Jan 12, 2021 at 1:37 PM Nathan Chancellor
> > <natechancellor@gmail.com> wrote:
> > >
> > > > if real_ptr is an unsigned long, do we want to use `__ffs(real_ptr) +
> > > > 1` here rather than ffs which takes an int?  It seems the kernel is
> > > > missing a definition of ffsl. :(
> > >
> > > Why the + 1? I think if we use __ffs (which it seems like we should), I
> > > think that needs to become
> > 
> > This came up recently in an internal code review; ffs and __ffs differ
> > in output by one.  See also the definition of ffs for alpha in
> > arch/alpha/include/asm/bitops.h.
> 
> Interesting, thanks for bringing it up! Looks like ffs returns 1-32 and
> __ffs returns 0-31. I think that we want __ffs here because we are
> shifting (1UL << 32 overflows on 32-bit architectures) and the code in
> LLVM appears to agree. LeastSignificantSetBitIndex evaluates to
> __builtin_ctzl, which is the asm-generic implementation of __ffs.

Sounds good. With __ffs, consider your v2:

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook