From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
"H. Peter Anvin" <hpa@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-tip-commits@vger.kernel.org,
Fenghua Yu <fenghua.yu@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [tip:x86/asm] x86-64, copy_user: Remove zero byte check before copy user buffer.
Date: Wed, 20 Nov 2013 12:36:20 -0800 [thread overview]
Message-ID: <528D1D44.3000600@zytor.com> (raw)
In-Reply-To: <CA+55aFz9-bh=L8vf9qd3kx7wMtYqTtqx8YzULABX5reGtPBExw@mail.gmail.com>
On 11/20/2013 12:13 PM, Linus Torvalds wrote:
> On Wed, Nov 20, 2013 at 11:28 AM, H. Peter Anvin <hpa@linux.intel.com> wrote:
>>>
>>> .section .fixup,"ax"
>>> 11: lea (%rdx,%rcx,8),%rcx
>>> 12: movl %ecx,%edx /* ecx is zerorest also */
>>
>> -> Even if %rdx+%rcx*8 > 2^32 we end up truncating at 12: -- not that it
>> matters, since both arguments are prototyped as "unsigned" and therefore
>> the C compiler is supposed to guarantee the upper 32 bits are ignored.
>
> Ahh. That was the one I thought was broken, but yes, while the upper
> bits of %rcx are calculated and not zeroed, they end up not actually
> getting used. So yeah, I'll believe it's correct.
>
That being said, "lea (%rdx,%rcx,8),%ecx" (leal, as opposed to leaq) is
a perfectly legitimate instruction and actually one byte shorter. The
big question is if some broken version of gas will choke on it.
-hpa
next prev parent reply other threads:[~2013-11-20 20:36 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-16 20:37 [PATCH] x86/copy_user.S: Remove zero byte check before copy user buffer Fenghua Yu
2013-11-17 6:18 ` [tip:x86/asm] x86-64, copy_user: " tip-bot for Fenghua Yu
[not found] ` <CA+55aFyFz_6oy8-1jb5Jzk+4VC5MLA5d0KbqhZxki0=+DmggBg@mail.gmail.com>
2013-11-17 6:51 ` H. Peter Anvin
2013-11-19 4:37 ` H. Peter Anvin
2013-11-19 19:38 ` Linus Torvalds
2013-11-20 19:12 ` H. Peter Anvin
2013-11-20 19:28 ` H. Peter Anvin
2013-11-20 20:13 ` Linus Torvalds
2013-11-20 20:36 ` H. Peter Anvin [this message]
2013-11-20 21:44 ` Linus Torvalds
2013-11-20 20:54 ` [tip:x86/asm] x86-64, copy_user: Use leal to produce 32-bit results tip-bot for H. Peter Anvin
2013-11-20 22:00 ` tip-bot for H. Peter Anvin
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