Re: [PATCH] efi/x86-mixed: move unmitigated RET into .rodata

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 16, 2022 at 09:04:56AM +0200, Ard Biesheuvel wrote:
> (cc Peter and Josh)
> 
> On Mon, 15 Aug 2022 at 22:18, Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Mon, Aug 15, 2022 at 09:57:50PM +0200, Ard Biesheuvel wrote:
> > > I'm not seeing that warning. Any config in particular beyond
> > > x86_64_defconfig that you have enabled?
> >
> > It is my workstation's tailored config. Attached.
> >
> > > I'm using Debian GCC 12.1.0 btw
> >
> > gcc (Debian 11.2.0-19) 11.2.0
> >
> 
> Complete thread here:
> https://lore.kernel.org/all/20220815132028.732531-1-ardb@kernel.org/
> 
> On Mon, 15 Aug 2022 at 22:18, Borislav Petkov <bp@alien8.de> wrote:
> >
> > For some reason, objtool is not happy here:
> > vmlinux.o: warning: objtool: efi_thunk_query_variable_info_nonblocking+0x1ba: unreachable instruction
> 
> [which is the instruction right after the call to __efi_thunk64()]
> 
> However, with the same config but without the patch (i.e., v6.0-rc1
> with nothing on top), I see:
> 
> vmlinux.o: warning: objtool: sme_enable+0x71: unreachable instruction
> 
> It appears that objtool is making inferences about whether or not
> __efi_thunk64() returns, even though it is marked as
> STACK_FRAME_NON_STANDARD. And note that I am not seeing any of these
> with x86_64_defconfig, only with Boris's config (attached)

STACK_FRAME_NON_STANDARD has no bearing on a call to that symbol being
noreturn or not.

noreturn is a bit of a pain point in that the compiler leaves no clue
in the object file. Objtool has a bunch of heuristics to guess at
noreturn, but the only surefire way to make things consistent is to
annotate the function with __noreturn and add it to the
global_noreturns[] array in tools/objtool/check.c

Alternatively, if objtool guesses wrong, you can annotate the assembler
with 'REACHABLE'.

Josh; should we create a config file for objtool to contain many of this
stuff? Then again, parsing a config file over and over and over again
isn't going to make it faster :/