Re: [PATCH 07/22] x86/uaccess: Remove ELF function annotation from copy_user_handle_tail()

[Peter Zijlstra]

On Tue, Jul 16, 2019 at 11:16:48AM -0700, Nick Desaulniers wrote:
> On Sun, Jul 14, 2019 at 5:37 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > After an objtool improvement, it's complaining about the CLAC in
> > copy_user_handle_tail():
> >
> >   arch/x86/lib/copy_user_64.o: warning: objtool: .altinstr_replacement+0x12: redundant UACCESS disable
> >   arch/x86/lib/copy_user_64.o: warning: objtool:   copy_user_handle_tail()+0x6: (alt)
> >   arch/x86/lib/copy_user_64.o: warning: objtool:   copy_user_handle_tail()+0x2: (alt)
> >   arch/x86/lib/copy_user_64.o: warning: objtool:   copy_user_handle_tail()+0x0: <=== (func)
> >
> > copy_user_handle_tail() is incorrectly marked as a callable function, so
> > objtool is rightfully concerned about the CLAC with no corresponding
> > STAC.
> >
> > Remove the ELF function annotation.  The copy_user_handle_tail() code
> > path is already verified by objtool because it's jumped to by other
> > callable asm code (which does the corresponding STAC).
> 
> What is CLAC and STAC?

CLear AC flag and SeT AC flag, SMAP repurposed the EFLAGS.AC for CPL0.

Also see commit: ea24213d8088 ("objtool: Add UACCESS validation")