On Wed, May 13, 2015 at 12:46:30PM +0200, Ingo Molnar wrote:
>
> * Borislav Petkov <b...@alien8.de> wrote:
>
> > On Wed, May 13, 2015 at 12:31:40PM +0200, Ingo Molnar wrote:
> > > So why should an alternatives-CALL, inlined directly into call sites,
> > > cost more kernel space?
> >
> > Not the alternatives CALL alone but inlining _copy_*_user with all
> > the preparation glue around it would. Basically what we're doing
> > currently.
>
> So I reacted to this comment of yours:
>
> > > > The disadvantage is that we have CALL after CALL [...]
>
> Is the CALL after CALL caused by us calling an alternatives patched
> function? If yes then we probably should not do that: alternatives
> switching should IMHO happen at the highest possible level.
Right, so I was trying to analyze Linus' suggestion to uninline stuff
and put it in uaccess_64.c. And that does save us some size and
alternatives patch sites but produces the CALL ... CALL thing.
So let me show you what we have now:
ffffffff8102a774: 0f 1f 40 00 nopl 0x0(%rax)
ffffffff8102a778: ba 58 00 00 00 mov $0x58,%edx
ffffffff8102a77d: 4c 89 ff mov %r15,%rdi
ffffffff8102a780: 49 83 c7 58 add $0x58,%r15
ffffffff8102a784: e8 b7 19 2f 00 callq ffffffff8131c140
<_copy_to_user>
...
ffffffff8131c140 <_copy_to_user>:
ffffffff8131c140: 65 48 8b 04 25 88 a9 mov %gs:0xa988,%rax
ffffffff8131c147: 00 00
ffffffff8131c149: 48 2d 00 40 00 00 sub $0x4000,%rax
ffffffff8131c14f: 48 89 f9 mov %rdi,%rcx
ffffffff8131c152: 48 01 d1 add %rdx,%rcx
ffffffff8131c155: 0f 82 bb c5 57 00 jb ffffffff81898716
<bad_to_user>
ffffffff8131c15b: 48 3b 48 18 cmp 0x18(%rax),%rcx
ffffffff8131c15f: 0f 87 b1 c5 57 00 ja ffffffff81898716
<bad_to_user>
ffffffff8131c165: e9 36 00 00 00 jmpq ffffffff8131c1a0
<copy_user_generic_unrolled>
ffffffff8131c16a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
so we prep args, call _copy_to_user, do checks and then JMP to the
optimal alternative function.
What I'd like to do is (hypothetically copy'pasted together):
ffffffff8102a778: ba 58 00 00 00 mov $0x58,%edx
ffffffff8102a77d: 4c 89 ff mov %r15,%rdi
movq -16360(%r14), %rax
# _208->addr_limit.seg, tmp347
subq $88, %rax #,
D.37904
cmpq %rax, %r15 #
D.37904, ubuf
ja .L493 #,
call copy_user_generic_unrolled
#
which saves us the first CALL to _copy_to_user and we do the
alternatives <copy_user_generic_unrolled> CALL directly.
This would mean that we will have to inline _copy_*_user() and switch it
to use copy_user_generic() which already does the proper alternatives.
For the price of some minor size increase and more alternatives patch
sites.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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