Re: [PATCH RESEND] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs
From: Brian Gerst
Date: Tue May 03 2016 - 13:45:41 EST
On Mon, May 2, 2016 at 10:56 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>
> Resending. Still applies to current Ingo's tip tree
>
> 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 */
I think it actually should push r12-r15, since they are callee-saved
and we don't explicitly zero them out on SYSRET like r8-r10. If it
exited via IRET it would reload them as zero, so there is an
inconsistency there.
--
Brian Gerst