Re: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

[Denys Vlasenko]

On Wed, Mar 11, 2015 at 4:35 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> Comparison of object code:
>>>     Old:
>>>      1e6:   8b 44 24 38             mov    0x38(%esp),%eax
>>>      1ea:   8a 64 24 40             mov    0x40(%esp),%ah
>>>      1ee:   8a 44 24 34             mov    0x34(%esp),%al
>>>      1f2:   25 03 04 02 00          and    $0x20403,%eax
>>>      1f7:   3d 03 04 00 00          cmp    $0x403,%eax
>>>      1fc:   74 0f                   je     20d <ldt_ss>
>>>     New:
>>>      1e6:   f6 44 24 3a 02          testb  $0x2,0x3a(%esp)
>>>      1eb:   75 0e                   jne    1fb <restore_nocheck>
>>>      1ed:   f6 44 24 34 03          testb  $0x3,0x34(%esp)
>>>      1f2:   74 07                   je     1fb <restore_nocheck>
>>>      1f4:   f6 44 24 40 04          testb  $0x4,0x40(%esp)
>>>      1f9:   75 0f                   jne    20a <ldt_ss>
>>
>> Please do some benchmarking of this: a tight loop of getpid or getppid
>> syscalls ought to be enough to be able to time this accurately.
>
> Before you benchmark, I think you should reorder it: check CS, then
> OLDSS, then EFLAGS.  The case where CS & 3 == 0, OLDSS & 4 == 4, and
> EFLAGS & VM == VM should be *extremely* rare.
>
> I'm going to hold off on resending my sp0/sp1/ss cleanups until we
> resolve this -- if it turns out that your code is the same or faster
> and therefore gets merged, then I think that all the -8 crap can just
> be deleted instead of being fixed.

I thought that I got NAKed by Linus on this change?

Basically, he wants to retain that padding because it guards against
_future_ bugs_ where someone would touch pt_regs->ss
and it will fail, very rarely. I think it's a sound reason to retain
the padding.


Anyway, since you ask. I benchmarked current code against this patch:

 restore_all_notrace:
 #ifdef CONFIG_X86_ESPFIX32
-       movl PT_EFLAGS(%esp), %eax      # mix EFLAGS, SS and CS
-       # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
-       # are returning to the kernel.
-       # See comments in process.c:copy_thread() for details.
-       movb PT_OLDSS(%esp), %ah
-       movb PT_CS(%esp), %al
-       andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
-       cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
        CFI_REMEMBER_STATE
-       je ldt_ss                       # returning to user-space with LDT SS
+       testb   $3, PT_CS(%esp)
+       jz      restore_nocheck         # CPL0, not it
+#ifdef CONFIG_VM86
+       testb   $2, (PT_EFLAGS+2)(%esp)
+       jnz     restore_nocheck         # EFLAGS.VM set, not it
+#endif
+       # Note: we access PT_OLDSS only when we know it exists.
+       # If PT_CS is from CPL0, it does not exist.
+       testb   $SEGMENT_LDT, PT_OLDSS(%esp)
+       jnz     ldt_ss                  # returning to user-space with LDT SS
 #endif

This code only gets executed on int80 path, not on sysenter.
The test was to run 10 million getpids.

The numbers I've got are *the same* before and after the patch:
227.99 ns per getpid().

I double-checked my test setup by adding a few PAUSE insns
in this code - which, as expected, was seen easily in the test run.
I did not forget to enable ESPFIX32 and VM86 in the .config.
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