Messages in this thread | | | From | Andy Lutomirski <> | Subject | Re: objtool clac/stac handling change.. | Date | Wed, 1 Jul 2020 13:36:22 -0700 |
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> On Jul 1, 2020, at 12:35 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Wed, Jul 1, 2020 at 11:29 AM Andy Lutomirski <luto@kernel.org> wrote: >> >> Do we really want the exception handling to do the CLAC? Having >> unsafe_get_user() do CLAC seems surprising to me, and it will break >> use cases like: >> >> if (!user_access_begin(...) >> goto out; >> >> ret = unsafe_get_user(...); >> >> user_access_end(); >> >> check ret; > > That's not how unsafe_get_user() works. > > unsafe_get_user() always jumps to the error label, it never returns a > value. So the code is actually now what you claim above, but > > if (!user_access_begin(...) > goto out; > > unsafe_get_user(..., out_fault); > user_access_end(); > .. this is good, use the value we got.. > > out_fault: > user_access_end(); > out: > return -EFAULT;
Ugh, right. But maybe, with the asm goto magic, we can’t get rid of this. I’ve always disliked the pattern where we enable user access, do a bunch of accesses that branch on error, and finish up. We ought to be able to do it the way I described and get decent code generation too.
If we do this extable change, we end up with a different mess: some exception handlers will clear AC and some won’t. I’m sure objtool can deal with this with some effort, but I’m not convinced it’s worth it.
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