* Borislav Petkov <b...@alien8.de> wrote:
> From: Borislav Petkov <b...@suse.de>
>
> Use the asm macro and drop the locally grown version.
> @@ -73,9 +49,11 @@ ENTRY(_copy_to_user)
> jc bad_to_user
> cmpq TI_addr_limit(%rax),%rcx
> ja bad_to_user
> + ALTERNATIVE_2 "jmp copy_user_generic_unrolled", \
> + "jmp copy_user_generic_string", \
> + X86_FEATURE_REP_GOOD, \
> + "jmp copy_user_enhanced_fast_string", \
> + X86_FEATURE_ERMS
Btw., as a future optimization, wouldn't it be useful to patch this
function at its first instruction, i.e. to have three fully functional
copy_user_generic_ variants and choose to jmp to one of them in the
first instruction of the original function?
The advantage would be two-fold:
1) right now: smart microarchitectures that are able to optimize
jump-after-jump (and jump-after-call) targets in their branch
target cache can do so in this case, reducing the overhead of the
patching, possibly close to zero in the cached case.
2) in the future: we could actually do a (limited) re-link of the
kernel during bootup, and patch up the original copy_to_user call
sites directly to one of the three variants. Alternatives patching
done at the symbol level. Does current tooling allow something
like this already?
Thanks,
Ingo
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