[PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs

Denys Vlasenko
2016-03-11T16:53:43Z

Use of a temporary R8 register here seems to be unnecessary.

"push %r8" is a two-byte insn (it needs REX prefix to specify R8),
"push $0" is two-byte too. It seems just using the latter would be
no worse.

Thus, code had an unnecessary "xorq %r8,%r8" insn.
It probably costs nothing in execution time here since we are probably
limited by store bandwidth at this point, but still.

Run-tested under QEMU: 32-bit calls still work:

/ # ./test_syscall_vdso32
[RUN] Executing 6-argument 32-bit syscall via VDSO
[OK] Arguments are preserved across syscall
[NOTE] R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
[OK] R8..R15 did not leak kernel data
[RUN] Executing 6-argument 32-bit syscall via INT 80
[OK] Arguments are preserved across syscall
[OK] R8..R15 did not leak kernel data
[RUN] Running tests under ptrace
[RUN] Executing 6-argument 32-bit syscall via VDSO
[OK] Arguments are preserved across syscall
[NOTE] R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
[OK] R8..R15 did not leak kernel data
[RUN] Executing 6-argument 32-bit syscall via INT 80
[OK] Arguments are preserved across syscall
[OK] R8..R15 did not leak kernel data

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Ingo Molnar <mingo@kernel.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
arch/x86/entry/entry_64_compat.S | 45 +++++++++++++++++++---------------------
1 file changed, 21 insertions(+), 24 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 847f2f0..e1721da 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -72,24 +72,23 @@ ENTRY(entry_SYSENTER_compat)
pushfq /* pt_regs->flags (except IF = 0) */
orl $X86_EFLAGS_IF, (%rsp) /* Fix saved flags */
pushq $__USER32_CS /* pt_regs->cs */
- xorq %r8,%r8
- pushq %r8 /* pt_regs->ip = 0 (placeholder) */
+ pushq $0 /* pt_regs->ip = 0 (placeholder) */
pushq %rax /* pt_regs->orig_ax */
pushq %rdi /* pt_regs->di */
pushq %rsi /* pt_regs->si */
pushq %rdx /* pt_regs->dx */
pushq %rcx /* pt_regs->cx */
pushq $-ENOSYS /* pt_regs->ax */
- pushq %r8 /* pt_regs->r8 = 0 */
- pushq %r8 /* pt_regs->r9 = 0 */
- pushq %r8 /* pt_regs->r10 = 0 */
- pushq %r8 /* pt_regs->r11 = 0 */
+ pushq $0 /* pt_regs->r8 = 0 */
+ pushq $0 /* pt_regs->r9 = 0 */
+ pushq $0 /* pt_regs->r10 = 0 */
+ pushq $0 /* pt_regs->r11 = 0 */
pushq %rbx /* pt_regs->rbx */
pushq %rbp /* pt_regs->rbp (will be overwritten) */
- pushq %r8 /* pt_regs->r12 = 0 */
- pushq %r8 /* pt_regs->r13 = 0 */
- pushq %r8 /* pt_regs->r14 = 0 */
- pushq %r8 /* pt_regs->r15 = 0 */
+ pushq $0 /* pt_regs->r12 = 0 */
+ pushq $0 /* pt_regs->r13 = 0 */
+ pushq $0 /* pt_regs->r14 = 0 */
+ pushq $0 /* pt_regs->r15 = 0 */
cld

/*
@@ -205,17 +204,16 @@ ENTRY(entry_SYSCALL_compat)
pushq %rdx /* pt_regs->dx */
pushq %rbp /* pt_regs->cx (stashed in bp) */
pushq $-ENOSYS /* pt_regs->ax */
- xorq %r8,%r8
- pushq %r8 /* pt_regs->r8 = 0 */
- pushq %r8 /* pt_regs->r9 = 0 */
- pushq %r8 /* pt_regs->r10 = 0 */
- pushq %r8 /* pt_regs->r11 = 0 */
+ pushq $0 /* pt_regs->r8 = 0 */
+ pushq $0 /* pt_regs->r9 = 0 */
+ pushq $0 /* pt_regs->r10 = 0 */
+ pushq $0 /* pt_regs->r11 = 0 */
pushq %rbx /* pt_regs->rbx */
pushq %rbp /* pt_regs->rbp (will be overwritten) */
- pushq %r8 /* pt_regs->r12 = 0 */
- pushq %r8 /* pt_regs->r13 = 0 */
- pushq %r8 /* pt_regs->r14 = 0 */
- pushq %r8 /* pt_regs->r15 = 0 */
+ pushq $0 /* pt_regs->r12 = 0 */
+ pushq $0 /* pt_regs->r13 = 0 */
+ pushq $0 /* pt_regs->r14 = 0 */
+ pushq $0 /* pt_regs->r15 = 0 */

/*
* User mode is traced as though IRQs are on, and SYSENTER
@@ -316,11 +314,10 @@ ENTRY(entry_INT80_compat)
pushq %rdx /* pt_regs->dx */
pushq %rcx /* pt_regs->cx */
pushq $-ENOSYS /* pt_regs->ax */
- xorq %r8,%r8
- pushq %r8 /* pt_regs->r8 = 0 */
- pushq %r8 /* pt_regs->r9 = 0 */
- pushq %r8 /* pt_regs->r10 = 0 */
- pushq %r8 /* pt_regs->r11 = 0 */
+ pushq $0 /* pt_regs->r8 = 0 */
+ pushq $0 /* pt_regs->r9 = 0 */
+ pushq $0 /* pt_regs->r10 = 0 */
+ pushq $0 /* pt_regs->r11 = 0 */
pushq %rbx /* pt_regs->rbx */
pushq %rbp /* pt_regs->rbp */
pushq %r12 /* pt_regs->r12 */
--
1.8.1.4