linux-kernel - Re: [tip: objtool/core] start_kernel: Add __no_stack

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Message-ID: <20230519174215.GA2158284@maniforge>
Date:   Fri, 19 May 2023 12:42:15 -0500
From:   David Vernet <void@...ifault.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Miguel Ojeda <ojeda@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: [tip: objtool/core] start_kernel: Add __no_stack_protector
 function attribute

On Fri, May 19, 2023 at 10:18:40AM -0700, Nick Desaulniers wrote:
> On Fri, May 19, 2023 at 10:11 AM David Vernet <void@...ifault.com> wrote:
> >
> > On Thu, May 18, 2023 at 11:08:03AM -0000, tip-bot2 for ndesaulniers@...gle.com wrote:
> > > The following commit has been merged into the objtool/core branch of tip:
> > >
> > > Commit-ID:     514ca14ed5444b911de59ed3381dfd195d99fe4b
> > > Gitweb:        https://git.kernel.org/tip/514ca14ed5444b911de59ed3381dfd195d99fe4b
> > > Author:        ndesaulniers@...gle.com <ndesaulniers@...gle.com>
> > > AuthorDate:    Mon, 17 Apr 2023 15:00:05 -07:00
> > > Committer:     Josh Poimboeuf <jpoimboe@...nel.org>
> >
> > Hi Nick, Josh, Peter,
> >
> > Do you have an ETA for when this will make its way to Linus' tree?
> > clang-17 built kernels have failed to boot since [0], so it would be
> > nice to get this in sooner rather than later if possible.
> 
> David,
> Can you confirm that your version of clang-17 is updated? clang-17 is
> unreleased; ToT will become clang-17.
>
> https://reviews.llvm.org/rGfc4494dffa5422b2be5442c235554e76bed79c8a
> should have fixed any boot failures related to stack protectors.  That
> is to say that Josh's series is irrelevant to anyone using either an
> existing release of clang, or something closer to ToT than April 13.

Thanks for the quick reply, Nick. The latest clang-17 does indeed fix
the issue. Apologies for not trying that first -- I was using the only
tagged verson of clang-17 (which admittedly is not a released version),
and figured it wasn't a compiler bug given that the assembly looked
sane, compilers are allowed to do all sorts of interesting things with
__noreturn, and that [1] removes -fstack-protector from start_kernel()
altogether.

[1]: https://lore.kernel.org/lkml/20230412-no_stackp-v1-1-46a69b507a4b@google.com/

> LLVM commit fc4494dffa54 ("[StackProtector] don't check stack
> protector before calling nounwind functions")
> landed April 13, so please check that your build of clang-17 is after that date.
> 
> Either way, thanks for testing with clang, and the report. You can
> always file a bug at our issue tracker:
> https://github.com/ClangBuiltLinux/linux/issues or see our page for
> more ways to get in touch:
> https://clangbuiltlinux.github.io/
> We're very active on our mailing list, and on IRC.

Ack, thanks for letting me know for next time.

- David