linux-kernel - Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't used at runtime

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Message-ID: <20231024201913.GHZTgmwf6QMkX8BGbo@fat_crate.local>
Date:   Tue, 24 Oct 2023 22:19:13 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     "Kaplan, David" <David.Kaplan@....com>,
        Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't
 used at runtime

On Thu, Oct 19, 2023 at 08:20:51AM -0700, Josh Poimboeuf wrote:
> GCC doesn't read asm.  Even if it did that wouldn't fix things for
> callers of custom-ABI return-thunk-using functions.
> 
> The below seems to work.

Right, I guess we can do something like that. Linker is not happy here
about that symbol, tho:

ld: arch/x86/lib/retpoline.o:(.altinstr_replacement+0x95): undefined reference to `warn_thunk_thunk'
make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
make[1]: *** [/mnt/kernel/kernel/5th/linux/Makefile:1165: vmlinux] Error 2
make: *** [Makefile:234: __sub-make] Error 2

-- 
Regards/Gruss,
    Boris.

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