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Date:   Tue, 18 Oct 2022 22:30:27 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joao Moreira <joao@overdrivepizza.com>
Cc:     Kees Cook <keescook@chromium.org>, x86@kernel.org,
        Sami Tolvanen <samitolvanen@google.com>,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
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On Tue, Oct 18, 2022 at 01:17:28PM -0700, Joao Moreira wrote:
> > 
> > > Tangent: why are these nop instead of 0xcc? These bytes aren't
> > > executed
> > > ever are they?
> > 
> > Because that's what the compiler gets us through
> > -fpatchable-function-entry.
> 
> Is it useful to get the compiler to emit 0xcc with
> -fpatchable-function-entry under any circumstance? I can probably change
> that quickly if needed/useful.

Having it emit 0xcc for the bytes in front of the symbol might be
interesting. It would mean a few kernel changes, but nothing too hard.

That is, -fpatchable-function-entry=N,M gets us N-M bytes in at the
start of the symbol and M bytes in front of it. The N-M bytes at the
start of the function *are* executed and should obviously not become
0xcc (GCC keeps them 0x90 while LLVM makes them large NOPs).