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 14:48:10 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
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 11:29:44PM +0200, Borislav Petkov wrote:
> 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?

A Threadripper 3990X, although I will say that this is reproducible with
QEMU's TCG, as that is what our CI tests with.

> > 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.

I attached the config and full dmesg from this change and the one
directly before it.

Cheers,
Nathan

View attachment "config" of type "text/plain" (140498 bytes)

View attachment "bad-boot.log" of type "text/plain" (14814 bytes)

View attachment "good-boot.log" of type "text/plain" (23385 bytes)