linux-kernel - Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32

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Message-ID: <CAMzpN2huVk1nNGLYsYXCOyCvpYBTwKYCyn29JQWZ+r8ZZmyskA@mail.gmail.com>
Date:	Tue, 24 Mar 2015 12:55:32 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

On Tue, Mar 24, 2015 at 10:08 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 03/24/2015 07:34 AM, Ingo Molnar wrote:
>>
>> * Denys Vlasenko <vda.linux@...glemail.com> wrote:
>>
>>> On Mon, Mar 23, 2015 at 9:38 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> Actually, I want to remove the added comment in the code.  I don't see
>>>> why we should have a specific comment about SS and not about, say, CS,
>>>> ESP, or anything else.  OK?
>>>
>>> Ok.
>>
>> Might be nice to place a more generic description there, which
>> registers are expected to be saved by user-space calling in here, etc.
>
> __kernel_vsyscall entry point has the same ABI in any 32-bit vDSO,
> the good old int 0x80 calling convention:
>
> syscall# in eax,
> params in ebx/ecx/edx/esi/edi/ebp,
> all registers are preserved by the syscall.
>
> (I think we don't guarantee that all flags are preserved:
> I have a testcase where DF gets cleared).

DF should always be clear on any function call per the C ABI.  But,
eflags should be preserved, at least the non-privileged bits.  I'd
like to see that testcase.

--
Brian Gerst
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