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

[Brian Gerst]

On Tue, May 3, 2016 at 1:55 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, May 3, 2016 at 10:45 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> 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.
>
> Hmm.  We could do this or we could zero them on the way out from the
> fast path.  I have no real preference.  Preserving the values is
> probably a bit nicer.

It's no difference to push the register vs. pushing a zero (it was
pushing r8 before).  It would cost extra to explicitly zero them on
SYSRET.

--
Brian Gerst