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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	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>,
	kernel-toolchains@vger.kernel.org,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections
Date: Wed, 28 Oct 2020 14:36:14 -0700
Message-ID: <20201028213614.kqk7atvk6fcjely4@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <CAMj1kXEhcQ_ngNVWddV76NqEz6d0tDhfStYGd5diydefzVLvdQ@mail.gmail.com>

On Tue, Oct 27, 2020 at 09:15:17PM +0100, Ard Biesheuvel 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")

That commit is broken.
I had this patch in my queue:
-#define __no_fgcse __attribute__((optimize("-fno-gcse")))
+#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer")))

Sounds like you want to add -fno-asynchronous-unwind-tables to the above list?

> and mentions objtool and CONFIG_RETPOLINE.
> 
> >  (I feel the same about there
> > being an empty asm(); statement in the definition of asm_volatile_goto
> > for compiler-gcc.h).  Might be time to "fix the compiler."
> >
> > (It sounds like Arvind is both in agreement with my sentiment, and has
> > the root cause).
> >
> 
> I agree that the __no_fgcse hack is terrible. Does Clang support the
> following pragmas?
> 
> #pragma GCC push_options
> #pragma GCC optimize ("-fno-gcse")
> #pragma GCC pop_options

That will work too, but optimize("-fno...,-fno..,-fno..") is imo cleaner.

  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
2020-10-27 20:32                       ` Arvind Sankar
2020-10-27 20:36                         ` Nick Desaulniers
2020-10-28 21:36                     ` Alexei Starovoitov [this message]
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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