linux-kernel - Re: ia32_sysenter_target does not preserve EFLAGS

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Message-ID: <CAK1hOcMhFU45-Ycbvhh8g1pA6cJQoDWKUL6AMMk5m+o6p0PzBA@mail.gmail.com>
Date:	Sat, 28 Mar 2015 12:17:13 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: ia32_sysenter_target does not preserve EFLAGS

On Sat, Mar 28, 2015 at 10:46 AM, Ingo Molnar <mingo@...nel.org> wrote:
> * Denys Vlasenko <vda.linux@...glemail.com> wrote:
>> This is a C function. [...]
>
> Arguably that's a self-inflicted wound of uclibc: nothing keeps it
> from taking advantage of the syscall ABI and avoiding the double
> save/restores.

It's not uclibc who calls write(), it's user program.

IIRC uclibc can't make user program aware that write()
is not clobbering registers.

Even if it could do that via an __attribute__ somehow,
it would be a violation of standards: write() is supposed to be
an ordinary C function, users must be able to take its address
and assign it to a pointer declared as

    ssize_t (*ptr)(int, const void *, size_t);

Slapping __attribute__((different_abi))
onto write() makes that impossible, the signature
no longer matches.

Let me go back from hypothetics to the actual situation.
We can't do such a drastic ABI change now, it's too big.

But is looks like we can relax ABI wrt saving flags,
because it's broken for some time, and no one complains.

If we say that arith flags and DF are not saved, it may
mean that we don't need to kill ourselves with
awkward and costly (popf is 20 cycles) EFLAGS
manipulations.
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