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Date:   Tue, 18 Oct 2022 21:48:42 -0700
From:   Joao Moreira <joao@overdrivepizza.com>
To:     Peter Zijlstra <peterz@infradead.org>
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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>> 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).

Uhum, all makes sense. I drafted something here:

https://github.com/lvwr/llvm-project/commits/joao/int3

Let me know if this works for you or if there is something that should 
be tweaked, like adding a specific flag and such. This currently emits 
0xcc instead of 0x90 for the nops before the function entry symbol for 
kernel code on x86-64. It seems to be working (see generated snippet 
below), but let me know otherwise:

Generated with -fpatchable-function-entry=10,5

Disassembly of section .text:

0000000000000000 <save_processor_state-0x5>:
    0:   cc                      int3
    1:   cc                      int3
    2:   cc                      int3
    3:   cc                      int3
    4:   cc                      int3

0000000000000005 <save_processor_state>:
    5:   0f 1f 44 00 08          nopl   0x8(%rax,%rax,1)
    a:   41 57                   push   %r15
    c:   41 56                   push   %r14
...