Received-SPF: pass (google.com: domain of linux-kernel+bounces-150440-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Precedence: bulk
MIME-Version: 1.0
References: <20240417-x86-fix-kexec-with-llvm-18-v1-0-5383121e8fb7@kernel.org>
 <46343CD5-24D6-46BF-92A7-0B0FA0E6937D@alien8.de> <CAKwvOd=3Jrzju++=Ve61=ZdeshxUM=K3-bGMNREnGOQgNw=aag@mail.gmail.com>
In-Reply-To: <CAKwvOd=3Jrzju++=Ve61=ZdeshxUM=K3-bGMNREnGOQgNw=aag@mail.gmail.com>
From: Ard Biesheuvel <ardb@kernel.org>
Date: Thu, 18 Apr 2024 17:59:53 +0200
Message-ID: <CAMj1kXH+rNU4pa0ywcgkohXBu4G+iCXML_s9WVJW03ZRwtwW-w@mail.gmail.com>
Subject: Re: [PATCH 0/2] x86/purgatory: Avoid kexec runtime warning with LLVM 18
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Borislav Petkov <bp@alien8.de>, Arthur Eubanks <aeubanks@google.com>, 
	Nathan Chancellor <nathan@kernel.org>, tglx@linutronix.de, mingo@redhat.com, 
	dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, morbo@google.com, 
	justinstitt@google.com, song@kernel.org, ribalda@chromium.org, 
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev, patches@lists.linux.dev, 
	ns <0n-s@users.noreply.github.com>, Fangrui Song <maskray@google.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Thu, 18 Apr 2024 at 17:44, Nick Desaulniers <ndesaulniers@google.com> wr=
ote:
>
> On Thu, Apr 18, 2024 at 4:15=E2=80=AFAM Borislav Petkov <bp@alien8.de> wr=
ote:
> > How much of this silliness should we expect now for other parts of the =
kernel?
>
> Looks like ARCH=3Dpowerpc sets -mcmodel=3Dlarge for modules and ARCH=3Dum
> does for the whole kernel. So that LLVM change may have implications
> for those 2 other architectures.  Not sure we've had any bug reports
> or breakage in CI yet, like we have for x86+kexec.
>
> > Can we turn this off?
>
> Maybe we need to revisit
> commit e16c2983fba0 ("x86/purgatory: Change compiler flags from
> -mcmodel=3Dkernel to -mcmodel=3Dlarge to fix kexec relocation errors")
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit=
/?id=3De16c2983fba0fa6763e43ad10916be35e3d8dc05
>
> at least the -mcmodel=3Dkernel addition (since that patch added a few
> additional compiler flags that still LGTM).
>
..

> + Fangrui, Ard, who might know of alternative solutions to
> -mcmodel=3Dlarge for e16c2983fba0.
>

I think it would be better to use -mcmodel=3Dsmall -fpic. As Nick
explains, the large code model is really more suitable for executables
that span a large memory range. The issue with the purgatory seems to
be that it can be placed anywhere in memory, not that it is very big.

-mcmodel=3Dsmall -fpic is what user space typically uses, so it is much
less likely to create problems.

Note that I have been looking into whether we can build the entire
kernel with -fpic (for various reasons). There are some issues to
resolve there, mostly related to per-CPU variables and the per-CPU
stack protector, but beyond that, things work happily and the number
of boot time relocations drops dramatically, due to the use of
RIP-relative references. So for the purgatory, I wouldn't expect too
many surprises.

> Otherwise, I think the dedicated linker script is the way to go. We
> really want tight control over what is or is not in the purgatory
> image.

Linker scripts are a bit tedious when it comes to maintenance,
especially with weird executables such as this one and needing to
support different linkers. So I'd prefer to avoid this.