[tip:x86/asm] x86/asm/entry/32: Shorten __audit_syscall_entry() args preparation

tip-bot for Denys Vlasenko
2015-06-10T07:11:27Z

Commit-ID: a92fde25231a89d7d10895482556260c1b63767d
Gitweb: http://git.kernel.org/tip/a92fde25231a89d7d10895482556260c1b63767d
Author: Denys Vlasenko <dvlasenk@redhat.com>
AuthorDate: Tue, 9 Jun 2015 20:54:09 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 10 Jun 2015 08:42:13 +0200

x86/asm/entry/32: Shorten __audit_syscall_entry() args preparation

We use three MOVs to swap edx and ecx. We can use one XCHG
instead.

Expand the comments. It's difficult to keep track which arg#
every register corresponds to, so spell it out.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1433876051-26604-3-git-send-email-dvlasenk@redhat.com
[ Expanded the comments some more. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/entry/entry_64_compat.S | 27 ++++++++++++++++++---------
1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 0fa108c..bb187a6 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -178,17 +178,26 @@ sysexit_from_sys_call:

#ifdef CONFIG_AUDITSYSCALL
.macro auditsys_entry_common
- movl %esi, %r8d /* 5th arg: 4th syscall arg */
- movl %ecx, %r9d /* swap with edx */
- movl %edx, %ecx /* 4th arg: 3rd syscall arg */
- movl %r9d, %edx /* 3rd arg: 2nd syscall arg */
- movl %ebx, %esi /* 2nd arg: 1st syscall arg */
- movl %eax, %edi /* 1st arg: syscall number */
+ /*
+ * At this point, registers hold syscall args in the 32-bit syscall ABI:
+ * EAX is syscall number, the 6 args are in EBX,ECX,EDX,ESI,EDI,EBP.
+ *
+ * We want to pass them to __audit_syscall_entry(), which is a 64-bit
+ * C function with 5 parameters, so shuffle them to match what
+ * the function expects: RDI,RSI,RDX,RCX,R8.
+ */
+ movl %esi, %r8d /* arg5 (R8 ) <= 4th syscall arg (ESI) */
+ xchg %ecx, %edx /* arg4 (RCX) <= 3rd syscall arg (EDX) */
+ /* arg3 (RDX) <= 2nd syscall arg (ECX) */
+ movl %ebx, %esi /* arg2 (RSI) <= 1st syscall arg (EBX) */
+ movl %eax, %edi /* arg1 (RDI) <= syscall number (EAX) */
call __audit_syscall_entry
+
/*
- * We are going to jump back to syscall dispatch.
- * Prepare syscall args as required by 64-bit C ABI.
- * Clobbered registers are loaded from pt_regs on stack.
+ * We are going to jump back to the syscall dispatch code.
+ * Prepare syscall args as required by the 64-bit C ABI.
+ * Registers clobbered by __audit_syscall_entry() are
+ * loaded from pt_regs on stack:
*/
movl ORIG_RAX(%rsp), %eax /* syscall number */
movl %ebx, %edi /* arg1 */