linux-kernel - Re: [PATCH 1/2] x86: unify copy_from

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Date:	Sat, 26 Oct 2013 09:05:09 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Ingo Molnar <mingo@...nel.org>, Jan Beulich <JBeulich@...e.com>
CC:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] x86: unify copy_from_user() size checking

On 10/26/2013 03:31 AM, Ingo Molnar wrote:
>
> * Jan Beulich <JBeulich@...e.com> wrote:
>
>> Commits 4a3127693001c61a21d1ce680db6340623f52e93 ("x86: Turn the
>> copy_from_user check into an (optional) compile time warning") and
>> 63312b6a6faae3f2e5577f2b001e3b504f10a2aa ("x86: Add a Kconfig option to
>> turn the copy_from_user warnings into errors") touched only the 32-bit
>> variant of copy_from_user(), whereas the original commit
>> 9f0cf4adb6aa0bfccf675c938124e68f7f06349d ("x86: Use
>> __builtin_object_size() to validate the buffer size for
>> copy_from_user()") also added the same code to the 64-bit one.
>>
>> Further the earlier conversion from an inline WARN() to the call to
>> copy_from_user_overflow() went a little too far: When the number of
>> bytes to be copied is not a constant (e.g. [looking at 3.11] in
>> drivers/net/tun.c:__tun_chr_ioctl() or
>> drivers/pci/pcie/aer/aer_inject.c:aer_inject_write()), the compiler
>> will always have to keep the funtion call, and hence there will always
>> be a warning. By using __builtin_constant_p() we can avoid this.
>>
>> And then this slightly extends the effect of
>> CONFIG_DEBUG_STRICT_USER_COPY_CHECKS in that apart from converting
>> warnings to errors in the constant size case, it retains the (possibly
>> wrong) warnings in the non-constant size case, such that if someone is
>> prepared to get a few false positives, (s)he'll be able to recover the
>> current behavior (except that these diagnostics now will never be
>> converted to errors).
>>
>> Since the 32-bit variant (intentionally) didn't call might_fault(), the
>> unification results in this being called twice now. Adding a suitable
>> #ifdef would be the alternative if that's a problem.
>
> Addressing that sanely would be nice.
>
>> I'd like to point out though that with __compiletime_object_size()
>> being restricted to gcc before 4.6, the whole construct is going to
>> become more and more pointless going forward. I would question
>> however that commit 2fb0815c9ee6b9ac50e15dd8360ec76d9fa46a2 ("gcc4:
>> disable __compiletime_object_size for GCC 4.6+") was really necessary,
>> and instead this should have been dealt with as is done here from the
>> beginning.
>
> Can we now revert 2fb0815c9ee6?
>

I sure don't mind if there is a different and better solution.

Guenter

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