x86/asm/entry/32: Simplify the zeroing of pt_regs->r8..r11 in the int80 code path

32-bit syscall entry points do not save the complete pt_regs struct,
they leave some fields uninitialized. However, they must be
careful to not leak uninitialized data in pt_regs->r8..r11 to
ptrace users.

CLEAR_RREGS macro is used to zero these fields out when needed.

However, in the int80 code path this zeroing is unconditional.
This patch simplifies it by storing zeroes there right away,
when pt_regs is constructed on stack.

This uses shorter instructions:

   text    data     bss     dec     hex filename
   1423       0       0    1423     58f ia32entry.o.before
   1407       0       0    1407     57f ia32entry.o

Compile-tested.

Signed-off-by: Denys Vlasenko <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Drewry <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: dvlasenk

arch/x86/entry/ia32entry.S

@@ -421,6 +421,10 @@ ia32_badarg:
 	movq $-EFAULT,%rax
 	jmp ia32_sysret
 
+ia32_ret_from_sys_call:
+	CLEAR_RREGS
+	jmp int_ret_from_sys_call
+
 /*
  * Emulated IA32 system calls via int 0x80.
  *
@@ -462,8 +466,12 @@ ENTRY(ia32_syscall)
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rcx			/* pt_regs->cx */
 	pushq	$-ENOSYS		/* pt_regs->ax */
+	pushq	$0			/* pt_regs->r8 */
+	pushq	$0			/* pt_regs->r9 */
+	pushq	$0			/* pt_regs->r10 */
+	pushq	$0			/* pt_regs->r11 */
 	cld
-	sub	$(10*8),%rsp /* pt_regs->r8-11,bp,bx,r12-15 not saved */
+	sub	$(6*8),%rsp /* pt_regs->bp,bx,r12-15 not saved */
 
 	orl $TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
 	testl $_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
@@ -481,13 +489,10 @@ ia32_do_call:
 ia32_sysret:
 	movq %rax,RAX(%rsp)
 1:
-ia32_ret_from_sys_call:
-	CLEAR_RREGS
 	jmp int_ret_from_sys_call
 
 ia32_tracesys:
 	SAVE_EXTRA_REGS
-	CLEAR_RREGS
 	movq %rsp,%rdi        /* &pt_regs -> arg1 */
 	call syscall_trace_enter
 	LOAD_ARGS32	/* reload args from stack in case ptrace changed it */