i386/audit: stop scribbling on the stack frame

git commit b4f0d3755c was very very dumb.
It was writing over %esp/pt_regs semi-randomly on i686  with the expected
"system can't boot" results.  As noted in:

https://bugs.freedesktop.org/show_bug.cgi?id=85277

This patch stops fscking with pt_regs.  Instead it sets up the registers
for the call to __audit_syscall_entry in the most obvious conceivable
way.  It then does just a tiny tiny touch of magic.  We need to get what
started in PT_EDX into 0(%esp) and PT_ESI into 4(%esp).  This is as easy
as a pair of pushes.

After the call to __audit_syscall_entry all we need to do is get that
now useless junk off the stack (pair of pops) and reload %eax with the
original syscall so other stuff can keep going about it's business.

Reported-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Link: http://lkml.kernel.org/r/1414037043-30647-1-git-send-email-eparis@redhat.com
Cc: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/entry_32.S

@@ -447,15 +447,14 @@ sysenter_exit:
 sysenter_audit:
 	testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
 	jnz syscall_trace_entry
-	addl $4,%esp
+	/* movl PT_EAX(%esp), %eax	already set, syscall number: 1st arg to audit */
-	CFI_ADJUST_CFA_OFFSET -4
+	movl PT_EBX(%esp), %edx		/* ebx/a0: 2nd arg to audit */
-	movl %esi,4(%esp)		/* 5th arg: 4th syscall arg */
+	/* movl PT_ECX(%esp), %ecx	already set, a1: 3nd arg to audit */
-	movl %edx,(%esp)		/* 4th arg: 3rd syscall arg */
+	pushl_cfi PT_ESI(%esp)		/* a3: 5th arg */
-	/* %ecx already in %ecx		   3rd arg: 2nd syscall arg */
+	pushl_cfi PT_EDX+4(%esp)	/* a2: 4th arg */
-	movl %ebx,%edx			/* 2nd arg: 1st syscall arg */
-	/* %eax already in %eax		   1st arg: syscall number */
 	call __audit_syscall_entry
-	pushl_cfi %ebx
+	popl_cfi %ecx /* get that remapped edx off the stack */
+	popl_cfi %ecx /* get that remapped esi off the stack */
 	movl PT_EAX(%esp),%eax		/* reload syscall number */
 	jmp sysenter_do_call