Messages in this thread | | | Date | Wed, 13 May 2015 12:28:32 +0200 | From | Borislav Petkov <> | Subject | Re: [RFC PATCH] Drop some asm from copy_user_64.S |
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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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