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

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Message-ID: <55116FC1.1020400@redhat.com>
Date:	Tue, 24 Mar 2015 15:08:01 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <vda.linux@...glemail.com>
CC:	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 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).

Each flavor of fast kernel call does necessary massaging to
conform to the ABI. E.g. SYSCALL-based fast call clobbers ecx,
so its vDSO saves/restores ecx on stack.

Do you want a patch which adds such comment into every vDSO?
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