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Date:   Thu, 5 May 2022 09:36:22 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Peter Collingbourne <pcc@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-hardening@vger.kernel.org, andrew.cooper3@citrix.com,
        Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>, hjl.tools@gmail.com,
        alyssa.milburn@linux.intel.com,
        Nick Desaulniers <ndesaulniers@google.com>,
        gabriel.gomes@linux.intel.com, rick.p.edgecombe@intel.com
Subject: Re: [RFC PATCH 01/11] x86: kernel FineIBT
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On Wed, May 04, 2022 at 05:28:57PM -0700, Sami Tolvanen wrote:
> On Wed, May 4, 2022 at 11:17 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > __cfi_\sym:                                     __cfi_\sym:
> >                                                         int3; int3                              # 2
> >         endbr                   # 4                     mov     \hash, %eax                     # 5
> >         call    __fineibt_\hash # 5                     int3; int3                              # 2
> > \sym:                                           \sym:
> 
> OK, that looks reasonable to me.
> 
> > It seems to preserve the properties of the last one in that direct calls
> > will already be correct and we don't need linker fixups, and objtool can
> > simply parse the preamble as regular instructions without needing
> > further help.
> 
> Wouldn't objtool still print out unreachable instruction warnings here?

Depends a bit on what kind of symbol they end up being, if they're
STT_FUNC we'll probably get the complaint that it falls through into the
next symbol, while if they're STT_NOTYPE then yes, we'll get the
unreachable thing.

So either way we need to special case the __cfi_\sym things anyway.

But that should be relatively straight forward. I think I would lean
towards making then STT_FUNC (they really are for FineIBT anyway) and
then supressing the fallthrough warning for all functions that start
with "__cfi_". This way we get an ORC entry for them and the unwinder
will be happy.