Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

[Ingo Molnar <mingo@elte.hu>]

* Andy Lutomirski <luto@MIT.EDU> wrote:

> This patch is not perfect: the vread_tsc and vread_hpet functions 
> are still at a fixed address.  Fixing that might involve making 
> alternative patching work in the vDSO.

Can you see any problem with them? Here is how they are looking like 
currently:

ffffffffff600100 <vread_tsc>:
ffffffffff600100:	55                   	push   %rbp
ffffffffff600101:	48 89 e5             	mov    %rsp,%rbp
ffffffffff600104:	66 66 90             	data32 xchg %ax,%ax
ffffffffff600107:	66 66 90             	data32 xchg %ax,%ax
ffffffffff60010a:	0f 31                	rdtsc  
ffffffffff60010c:	89 c1                	mov    %eax,%ecx
ffffffffff60010e:	48 89 d0             	mov    %rdx,%rax
ffffffffff600111:	48 8b 14 25 28 0d 60 	mov    0xffffffffff600d28,%rdx
ffffffffff600118:	ff 
ffffffffff600119:	48 c1 e0 20          	shl    $0x20,%rax
ffffffffff60011d:	48 09 c8             	or     %rcx,%rax
ffffffffff600120:	48 39 d0             	cmp    %rdx,%rax
ffffffffff600123:	73 03                	jae    ffffffffff600128 <vread_tsc+0x28>
ffffffffff600125:	48 89 d0             	mov    %rdx,%rax
ffffffffff600128:	5d                   	pop    %rbp
ffffffffff600129:	c3                   	retq   

ffffffffff60012a <vread_hpet>:
ffffffffff60012a:	55                   	push   %rbp
ffffffffff60012b:	48 89 e5             	mov    %rsp,%rbp
ffffffffff60012e:	8b 04 25 f0 f0 5f ff 	mov    0xffffffffff5ff0f0,%eax
ffffffffff600135:	89 c0                	mov    %eax,%eax
ffffffffff600137:	5d                   	pop    %rbp
ffffffffff600138:	c3                   	retq   

There's no obvious syscall instruction in them that i can see. No 
0x0f 0x05 pattern (even misaligned), no 0xcd-anything.

We could even 'tie down' the actual assembly by moving this all to a 
.S - this way we protect against GCC accidentally generating 
something dangerous in there. I suggested that before.

Thanks,

	Ingo