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Date:   Tue, 12 Jan 2021 15:06:34 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ubsan: Implement __ubsan_handle_alignment_assumption

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@...il.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.

Cheers,
NAthan

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