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

@@ -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 */