Messages in this thread | | | Subject | Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC | From | Andy Lutomirski <> | Date | Fri, 1 Mar 2019 07:27:47 -0800 |
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> On Mar 1, 2019, at 6:38 AM, Peter Zijlstra <peterz@infradead.org> wrote: > >> On Fri, Mar 01, 2019 at 01:57:18PM +0100, Peter Zijlstra wrote: >>> On Fri, Mar 01, 2019 at 01:27:45PM +0100, Peter Zijlstra wrote: >>> arch/x86/lib/usercopy_64.o: warning: objtool: .altinstr_replacement+0x30: redundant UACCESS disable >> >>> The usercopy one is difficult, that's copy_user_handle_tail(), it is >>> buggered though, because that lacks notrace and thus has a __fentry__ >>> call in. >>> >>> Also, afaict all exception jumps into copy_user_handle_tail() will have >>> AC=1, but the __{get,put}_user_nocheck() things do STAC/CLAC all over >>> again. >>> >>> So what do we do? Annotate that we start with AC=1 and then immediately >>> do the clac, and then let __{get,put}_user_nocheck() do their own thing? >>> or make it use the unsafe stuff? >> >> Or.. we move the thing to assembly. Of course, I suck at (writing) asm, >> so the below is probably broken in various ways. > > The advantage is that it now all lives in the same .o file and objtool > can actually follow and find the complete control flow. > > I've made it ENDPROC() such that it becomes STT_FUNC and objtool does > all the normal things. I've also moved the ALIGN_DESTINATION macro into > the .S file. > > Andy, do we have a sensible self-test for this path?
Not that I know of. Something like my (rejected) strncpy_from_user test could probably be added fairly easily.
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