linux-kernel - Re: [PATCH 0/7] x86 vdso32 cleanups

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Date:	Sat, 29 Aug 2015 09:10:45 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/7] x86 vdso32 cleanups

On Sat, Aug 29, 2015 at 8:20 AM, Brian Gerst <brgerst@...il.com> wrote:
> This patch set contains several cleanups to the 32-bit VDSO.  The
> main change is to only build one VDSO image, and select the syscall
> entry point at runtime.

Oh no, we have dueling patches!

I have a 2/3 finished series that cleans up the AT_SYSINFO mess
differently, as I outlined earlier.  I've only done the compat and
common bits (no 32-bit native support quite yet), and it enters
successfully on Intel using SYSENTER and on (fake) AMD using SYSCALL.
The SYSRET bit isn't there yet.

Other than some ifdeffery, the final system_call.S looks like this:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/tree/arch/x86/entry/vdso/vdso32/system_call.S?h=x86/entry_compat

The meat is (sorry for whitespace damage):

.text
.globl __kernel_vsyscall
.type __kernel_vsyscall,@function
ALIGN
__kernel_vsyscall:
CFI_STARTPROC
/*
* Reshuffle regs so that all of any of the entry instructions
* will preserve enough state.
*/
pushl %edx
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET edx, 0
pushl %ecx
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET ecx, 0
movl %esp, %ecx

#ifdef CONFIG_X86_64
/* If SYSENTER is available, use it. */
ALTERNATIVE_2 "", "sysenter", X86_FEATURE_SYSENTER32, \
                 "syscall",  X86_FEATURE_SYSCALL32
#endif

/* Enter using int $0x80 */
movl (%esp), %ecx
int $0x80
GLOBAL(int80_landing_pad)

/* Restore ECX and EDX in case they were clobbered. */
popl %ecx
CFI_RESTORE ecx
CFI_ADJUST_CFA_OFFSET -4
popl %edx
CFI_RESTORE edx
CFI_ADJUST_CFA_OFFSET -4
ret
CFI_ENDPROC

.size __kernel_vsyscall,.-__kernel_vsyscall
.previous

And that's it.

What do you think?  This comes with massively cleaned up kernel-side
asm as well as a test case that actually validates the CFI directives.

Certainly, a bunch of your patches make sense regardless, and I'll
review them and add them to my queue soon.

--Andy
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