On Sat, Apr 16, 2022 at 10:42:22AM -0700, Linus Torvalds wrote:
> On Sat, Apr 16, 2022 at 10:28 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > you also need a _fsrm() one which checks X86_FEATURE_FSRM. That one
> > should simply do rep; stosb regardless of the size. For that you can
> > define an alternative_call_3 similar to how the _2 variant is defined.
>
> Honestly, my personal preference would be that with FSRM, we'd have an
> alternative that looks something like
>
> asm volatile(
> "1:"
> ALTERNATIVE("call __stosb_user", "rep movsb", X86_FEATURE_FSRM)
> "2:"
> _ASM_EXTABLE_UA(1b, 2b)
> :"=c" (count), "=D" (dest),ASM_CALL_CONSTRAINT
> :"0" (count), "1" (dest), "a" (0)
> :"memory");
>
> iow, the 'rep stosb' case would be inline.
I knew you were gonna say that - we have talked about this in the past.
And I'll do you one better -- we have the patch-if-bit-not-set thing now
too, so I think it should work if we did:
alternative_call_3(__clear_user_fsrm,
__clear_user_erms, ALT_NOT(X86_FEATURE_FSRM),
__clear_user_string, ALT_NOT(X86_FEATURE_ERMS),
__clear_user_orig, ALT_NOT(X86_FEATURE_REP_GOOD),
: "+&c" (size), "+&D" (addr)
:: "eax");
and yeah, you wanna get rid of the CALL even and I guess that could
be made to work - I just need to play with it a bit to hammer out the
details.
I.e., it would be most optimal if it ended up being
ALTERNATIVE_3("rep stosb",
"call ... ", ALT_NOT(X86_FEATURE_FSRM),
...
> Note that the above would have a few things to look out for:
>
> - special 'stosb' calling convention:
>
> %rax/%rcx/%rdx as inputs
> %rcx as "bytes not copied" return value
> %rdi can be clobbered
>
> so the actual functions would look a bit odd and would need to
> save/restore some registers, but they'd basically just emulate "rep
> stosb".
Right.
> - since the whole point is that the "rep movsb" is inlined, it also
> means that the "call __stosb_user" is done within the STAC/CLAC
> region, so objdump would have to be taught that's ok
>
> but wouldn't it be lovely if we could start moving towards a model
> where we can just inline 'memset' and 'memcpy' like this?
Yeah, inlined insns without even a CALL insn would be the most optimal
thing to do.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
