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Message-ID: <Y08M4+GxoqvuZ+bq@hirez.programming.kicks-ass.net>
Date: Tue, 18 Oct 2022 22:30:27 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Joao Moreira <joao@...rdrivepizza.com>
Cc: Kees Cook <keescook@...omium.org>, x86@...nel.org,
Sami Tolvanen <samitolvanen@...gle.com>,
linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
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).
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