Re: ia32_sysenter_target does not preserve EFLAGS

[Ingo Molnar]

* Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:

> On Fri, Mar 27, 2015 at 9:00 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, Mar 27, 2015 at 7:25 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> >>
> >> Apparently, users *don't* depend on arithmetic flags
> >> to survive over syscall. They also okay with DF flag
> >> being cleared.
> >
> > Generally, users probably dont' care about many registers at all being
> > saved, but it's worth noting that the reason system calls save/restore
> > even caller-saved registers is at least partly in order to avoid any
> > kernel information leaks.
> >
> > I don't believe that user mode will ever reasonably care about the
> > arithmetic flags being changed, but at the same time I also don't it
> > is something we should ever consider a "feature" we should try to take
> > advantage of. Generally we should try to not mess with the flag state,
> > and I'd *much* rather make the rule be that all the system call return
> > paths restore flags as much as possible.
> 
> "We don't clobber anything" ABI has its appeal.
> OTOH, fulfilling ABI's promises has cost which hast to be paid
> on every syscall, regardless whether userspace needed it or not.
> 
> Example. This is the uclibc implementation of write():
> 
> 00000000004acfc4 <__libc_write>:
>   4acfc4:       53                      push   %rbx
>   4acfc5:       48 63 ff                movslq %edi,%rdi
>   4acfc8:       b8 01 00 00 00          mov    $0x1,%eax
>   4acfcd:       0f 05                   syscall
>   4acfcf:       48 89 c3                mov    %rax,%rbx
>   4acfd2:       48 81 fb 00 f0 ff ff    cmp    $0xfffffffffffff000,%rbx
>   4acfd9:       76 0f                   jbe    4acfea <__libc_write+0x26>
>   4acfdb:       e8 64 15 00 00          callq  4ae544 <__GI___errno_location>
>   4acfe0:       89 da                   mov    %ebx,%edx
>   4acfe2:       f7 da                   neg    %edx
>   4acfe4:       89 10                   mov    %edx,(%rax)
>   4acfe6:       48 83 c8 ff             or     $0xffffffffffffffff,%rax
>   4acfea:       5b                      pop    %rbx
>   4acfeb:       c3                      retq
> 
> This is a C function. [...]

Arguably that's a self-inflicted wound of uclibc: nothing keeps it 
from taking advantage of the syscall ABI and avoiding the double 
save/restores.

> [...] Therefore any its caller assumes that C-clobbered registers 
> can be, indeed, clobbered here, so if that caller uses any of them, 
> it saves/restores them.
> 
> All efforts by kernel code to save/restore C-clobbered registers, 
> eight of them, are in vain. It's just useless work. Userspace does 
> not benefit from that effort.

That's true only in this particular uclibc case, where user-space 
decided to not take advantage of the save/restore property of the 
kernel.

> If our syscall ABI would say that those regs are not preserved, we 
> could have a bit faster syscalls. Any userspace code which really 
> had to have those registers preserved across a particular syscall, 
> could push/pop them itself.

We'd at minimum have to zero out the registers to avoid the 
information leak and at that point it's in fact faster to just 
save/restore in the syscall and allow user-space to take advantage of 
that, if it wishes to.

We cannot do it the other way around.

Thanks,

	Ingo
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