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From: Nick Desaulniers <ndesaulniers@google.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
	Kees Cook <keescook@chromium.org>, Ingo Molnar <mingo@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Collingbourne <pcc@google.com>,
	James Morse <james.morse@arm.com>, Borislav Petkov <bp@suse.de>,
	Ingo Molnar <mingo@redhat.com>,
	Russell King <linux@armlinux.org.uk>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Linux-Arch <linux-arch@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	linux-toolchains@vger.kernel.org
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections
Date: Tue, 27 Oct 2020 13:28:02 -0700
Message-ID: <CAKwvOdm9kuKoVnQoVo7T91gRb9QiCTp2G_PnwbdPM=o710Lx5A@mail.gmail.com> (raw)
In-Reply-To: <CAMj1kXEhcQ_ngNVWddV76NqEz6d0tDhfStYGd5diydefzVLvdQ@mail.gmail.com>

(+ right linux-toolchains mailing list, apologies for adding the wrong
one, I'm forever doomed to have gmail autocomplete to the wrong one
now that I've sent to it before)

On Tue, Oct 27, 2020 at 1:15 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 27 Oct 2020 at 21:12, Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Tue, Oct 27, 2020 at 12:25 PM Geert Uytterhoeven
> > <geert@linux-m68k.org> wrote:
> > >
> > > Hi Nick,
> > >
> > > CC Josh
> > >
> > > On Mon, Oct 26, 2020 at 6:49 PM Nick Desaulniers
> > > <ndesaulniers@google.com> wrote:
> > > > On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
> > > > <geert@linux-m68k.org> wrote:
> > > > > On Mon, Oct 26, 2020 at 6:39 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > > > On Mon, 26 Oct 2020 at 17:01, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > > > On Mon, Oct 26, 2020 at 2:29 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > > > > On Mon, Oct 26, 2020 at 1:29 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > > > > > I.e. including the ".eh_frame" warning. I have tried bisecting that
> > > > > > > > > warning (i.e. with be2881824ae9eb92 reverted), but that leads me to
> > > > > > > > > commit b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section
> > > > > > > > > placement"), which is another red herring.
> > > > > > > >
> > > > > > > > kernel/bpf/core.o is the only file containing an eh_frame section,
> > > > > > > > causing the warning.
> > > >
> > > > When I see .eh_frame, I think -fno-asynchronous-unwind-tables is
> > > > missing from someone's KBUILD_CFLAGS.
> > > > But I don't see anything curious in kernel/bpf/Makefile, unless
> > > > cc-disable-warning is somehow broken.
> > >
> > > I tracked it down to kernel/bpf/core.c:___bpf_prog_run() being tagged
> > > with __no_fgcse aka __attribute__((optimize("-fno-gcse"))).
> > >
> > > Even if the function is trivially empty ("return 0;"), a ".eh_frame" section
> > > is generated.  Removing the __no_fgcse tag fixes that.
> >
> > That's weird.  I feel pretty strongly that unless we're working around
> > a well understood compiler bug with a comment that links to a
> > submitted bug report, turning off rando compiler optimizations is a
> > terrible hack for which one must proceed straight to jail; do not pass
> > go; do not collect $200.  But maybe I'd feel differently for this case
> > given the context of the change that added it.  (Ard mentions
> > retpolines+orc+objtool; can someone share the relevant SHA if you have
> > it handy so I don't have to go digging?)
>
> commit 3193c0836f203a91bef96d88c64cccf0be090d9c
> Author: Josh Poimboeuf <jpoimboe@redhat.com>
> Date:   Wed Jul 17 20:36:45 2019 -0500
>
>     bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()
>
> has
>
> Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code")
>
> and mentions objtool and CONFIG_RETPOLINE.

Thanks for the context.  It might be time to revisit the above commit.
If I revert it (small conflict that's easy to fixup),
kernel/bpf/core.o builds cleanly with defconfig+GCC-9.3, so maybe
obtool did get smart enough to handle that case?  Probably regresses
the performance of that main dispatch loop for BPF, but not sure what
folks are expecting when retpolines are enabled.
-- 
Thanks,
~Nick Desaulniers

  parent reply index

Thread overview: 92+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 19:42 [PATCH v6 00/29] Warn on orphan section placement Kees Cook
2020-08-21 19:42 ` [PATCH v6 01/29] vmlinux.lds.h: Create COMMON_DISCARDS Kees Cook
2020-09-01 11:48   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 02/29] vmlinux.lds.h: Add .gnu.version* to COMMON_DISCARDS Kees Cook
2020-09-01 11:48   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 03/29] vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted sections Kees Cook
2020-09-01 11:48   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 04/29] vmlinux.lds.h: Split ELF_DETAILS from STABS_DEBUG Kees Cook
2020-09-01 11:48   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 05/29] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to ELF_DETAILS Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 06/29] vmlinux.lds.h: add PGO and AutoFDO input sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] vmlinux.lds.h: Add " tip-bot2 for Nick Desaulniers
2020-08-21 19:42 ` [PATCH v6 07/29] efi/libstub: Disable -mbranch-protection Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 08/29] arm64/mm: Remove needless section quotes Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 09/29] arm64/kernel: Remove needless Call Frame Information annotations Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 10/29] arm64/build: Remove .eh_frame* sections due to unwind tables Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 11/29] arm64/build: Use common DISCARDS in linker script Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 12/29] arm64/build: Add missing DWARF sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 13/29] arm64/build: Assert for unwanted sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-10-26 12:29   ` [PATCH v6 13/29] " Geert Uytterhoeven
2020-10-26 13:29     ` Geert Uytterhoeven
2020-10-26 16:01       ` Geert Uytterhoeven
2020-10-26 17:38         ` Ard Biesheuvel
2020-10-26 17:43           ` Geert Uytterhoeven
2020-10-26 17:48             ` Nick Desaulniers
2020-10-26 17:53               ` Geert Uytterhoeven
2020-10-27  8:37                 ` Geert Uytterhoeven
2020-10-27 19:25               ` Geert Uytterhoeven
2020-10-27 19:33                 ` Ard Biesheuvel
2020-10-27 20:00                   ` Arvind Sankar
2020-10-27 20:12                 ` Nick Desaulniers
2020-10-27 20:15                   ` Ard Biesheuvel
2020-10-27 20:17                     ` Nick Desaulniers
2020-10-27 20:30                       ` Arvind Sankar
2020-10-27 20:40                         ` Nick Desaulniers
2020-10-27 21:24                           ` Arvind Sankar
2020-10-27 20:28                     ` Nick Desaulniers [this message]
2020-10-27 20:32                       ` Arvind Sankar
2020-10-27 20:36                         ` Nick Desaulniers
2020-10-28 21:36                     ` Alexei Starovoitov
2020-10-27  8:51             ` Geert Uytterhoeven
2020-10-27 10:08           ` Jean-Philippe Brucker
2020-10-27 10:20             ` Geert Uytterhoeven
2020-10-27 11:29               ` Ard Biesheuvel
2020-10-27 11:53                 ` Naresh Kamboju
2020-10-27 13:56                 ` Ard Biesheuvel
2020-08-21 19:42 ` [PATCH v6 14/29] arm64/build: Warn on orphan section placement Kees Cook
2020-08-21 19:42 ` [PATCH v6 15/29] arm/build: Refactor linker script headers Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 16/29] arm/build: Explicitly keep .ARM.attributes sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 17/29] arm/build: Add missing sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:42 ` [PATCH v6 18/29] arm/build: Assert for unwanted sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 19/29] arm/build: Warn on orphan section placement Kees Cook
2020-08-21 19:43 ` [PATCH v6 20/29] arm/boot: Handle all sections explicitly Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 21/29] arm/boot: Warn on orphan section placement Kees Cook
2020-08-21 19:43 ` [PATCH v6 22/29] x86/asm: Avoid generating unused kprobe sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 23/29] x86/build: Enforce an empty .got.plt section Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 24/29] x86/build: Assert for unwanted sections Kees Cook
2020-09-01 11:47   ` [tip: core/build] x86/build: Add asserts " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 25/29] x86/build: Warn on orphan section placement Kees Cook
2020-08-21 19:43 ` [PATCH v6 26/29] x86/boot/compressed: Reorganize zero-size section asserts Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 27/29] x86/boot/compressed: Remove, discard, or assert for unwanted sections Kees Cook
2020-08-21 20:01   ` Arvind Sankar
2020-08-21 21:21     ` Kees Cook
2020-08-21 21:28       ` Arvind Sankar
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 28/29] x86/boot/compressed: Add missing debugging sections to output Kees Cook
2020-09-01 11:47   ` [tip: core/build] " tip-bot2 for Kees Cook
2020-08-21 19:43 ` [PATCH v6 29/29] x86/boot/compressed: Warn on orphan section placement Kees Cook
2020-08-31 19:41 ` [PATCH v6 00/29] " Kees Cook
2020-09-01  7:11   ` Ingo Molnar
2020-09-01  7:59     ` Ingo Molnar
2020-09-01  8:16       ` Ingo Molnar
2020-09-01 15:17         ` Kees Cook
2020-09-01 18:02           ` Nick Desaulniers
2020-09-01 23:18             ` Kees Cook
2020-09-01 23:54               ` Nick Desaulniers

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