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

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Date:   Mon, 16 Oct 2023 23:29:44 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        David Kaplan <david.kaplan@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        llvm@...ts.linux.dev
Subject: Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't
 used at runtime

On Mon, Oct 16, 2023 at 02:10:40PM -0700, Nathan Chancellor wrote:
> I just bisected a boot failure that our continuous integration sees [1]
> with x86_64_defconfig + CONFIG_KCSAN=y to this change in -tip/-next. It
> does not appear to be clang specific, as I can reproduce it with GCC
> 13.2.0 from kernel.org [2] (the rootfs is available at [3], if it is
> necessary for reproducing).
> 
> $ make -skj"$(nproc)" ARCH=x86_64 CROSS_COMPILE=x86_64-linux- defconfig
> $ scripts/config -e KCSAN
> $ make -skj"$(nproc)" ARCH=x86_64 CROSS_COMPILE=x86_64-linux- olddefconfig bzImage
> $ qemu-system-x86_64 \
>     -display none \
>     -nodefaults \
>     -d unimp,guest_errors \
>     -append 'console=ttyS0 earlycon=uart8250,io,0x3f8' \
>     -kernel arch/x86/boot/bzImage \
>     -initrd x86_64-rootfs.cpio \
>     -cpu host \

What's the host?

> If there is any other information I can provide or patches I can test, I
> am more than happy to do so.

Yes, pls send your .config too because depending on the compiler, KCSAN
does get disabled with older ones. So I guess it has to be gcc 13 or so.

And full guest dmesg so that I can compare.

Thx.

-- 
Regards/Gruss,
    Boris.

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