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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
Date: Tue, 12 Jan 2021 15:56:26 -0800
Message-ID: <202101121555.C929315@keescook> (raw)
In-Reply-To: <20210112220634.GA1377517@ubuntu-m3-large-x86>

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

  reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-12 20:55 Nathan Chancellor
2021-01-12 21:15 ` Nick Desaulniers
2021-01-12 21:37   ` Nathan Chancellor
2021-01-12 21:53     ` Nick Desaulniers
2021-01-12 22:06       ` Nathan Chancellor
2021-01-12 23:56         ` Kees Cook [this message]
2021-01-13  0:12 ` [PATCH v2] " Nathan Chancellor
2021-01-27 22:44   ` [PATCH v3] " Nathan Chancellor
2021-01-27 22:54     ` Nick Desaulniers
2021-01-13  0:18 ` [PATCH] " kernel test robot
2021-01-13  0:39 ` kernel test robot
2021-01-13  1:31   ` Nathan Chancellor
2021-01-13  1:39     ` Nick Desaulniers
2021-01-27 22:26       ` Nick Desaulniers

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