On Wed, May 13, 2015 at 08:19:55AM +0200, Ingo Molnar wrote:
> Looks nice. Would be useful to do before/after analysis of the
> generated asm with a defconfig and document that in the changelog.
Right, so I'm looking at what we have now:
/* Standard copy_to_user with segment limit checking */
ENTRY(_copy_to_user)
CFI_STARTPROC
GET_THREAD_INFO(%rax)
movq %rdi,%rcx
addq %rdx,%rcx
jc bad_to_user
cmpq TI_addr_limit(%rax),%rcx
ja bad_to_user
This is adding @to (in %rdi) with size (in %rdx) and then looking at the
carry flag. __chk_range_not_ok() does the same thing, but with a single
operation, AFAICT:
static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size,
unsigned long limit)
{
/*
* If we have used "sizeof()" for the size,
* we know it won't overflow the limit (but
* it might overflow the 'addr', so it's
* important to subtract the size from the
* limit, not add it to the address).
*/
if (__builtin_constant_p(size))
return addr > limit - size;
and we're avoiding the addr overflow by subtracting size from limit.
So the resulting asm looks like this:
.file 22 "./arch/x86/include/asm/uaccess.h"
.loc 22 54 0
movq -16360(%r14), %rax # _208->addr_limit.seg, tmp347
%r14 contains thread_info
subq $88, %rax #, D.37904
88 is the size
.file 23 "./arch/x86/include/asm/uaccess_64.h"
.loc 23 165 0
cmpq %rax, %r12 # D.37904, ubuf
%r12 contains the user ptr
ja .L493 #,
movq %r12, %rdi # ubuf, to
prep args for copy_user...
movl $88, %edx #, len
alternative starts here
#APP
# 36 "./arch/x86/include/asm/uaccess_64.h" 1
661:
call copy_user_generic_unrolled #
....
so we end up replacing
MOV
ADD
JC
CMP
JA
JMP (alternative)
with
MOV
SUB
CMP
JA
MOV
MOV
CALL (alternative)
The only problem I see here is that we have to do two MOVs to put args
in proper registers before calling the copy_user* version. But we end
up with a single conditional instead of two. And the MOVs are cheaper.
Also, we gets rid of asm glue, even betterer :-)
> I'd keep any changes to inlining decisions a separate patch and do
> vmlinux before/after size analysis as well, so that we don't mix the
> effects of the various enhancements.
Yap.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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