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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Kees Cook <keescook@chromium.org>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	linux-efi@vger.kernel.org,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Arvind Sankar" <nivedita@alum.mit.edu>,
	"Manoj Gupta" <manojgupta@google.com>,
	"Will Deacon" <will@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	linux-arch@vger.kernel.org, "Andi Kleen" <ak@linux.intel.com>,
	"Fāng-ruì Sòng" <maskray@google.com>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	x86@kernel.org, "Russell King" <linux@armlinux.org.uk>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	clang-built-linux@googlegroups.com,
	"Ingo Molnar" <mingo@redhat.com>,
	"Luis Lozano" <llozano@google.com>,
	"Borislav Petkov" <bp@suse.de>, "Arnd Bergmann" <arnd@arndb.de>,
	"Jian Cai" <jiancai@google.com>,
	"Nathan Chancellor" <natechancellor@gmail.com>,
	"Peter Collingbourne" <pcc@google.com>,
	linux-arm-kernel@lists.infradead.org,
	"Michal Marek" <michal.lkml@markovi.net>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	"James Morse" <james.morse@arm.com>
Subject: Re: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections
Date: Fri, 31 Jul 2020 23:51:28 -0400
Message-ID: <20200801035128.GB2800311@rani.riverdale.lan> (raw)
In-Reply-To: <20200731230820.1742553-14-keescook@chromium.org>

On Fri, Jul 31, 2020 at 04:07:57PM -0700, Kees Cook wrote:
> From: Nick Desaulniers <ndesaulniers@google.com>
> 
> Basically, consider .text.{hot|unlikely|unknown}.* part of .text, too.
> 
> When compiling with profiling information (collected via PGO
> instrumentations or AutoFDO sampling), Clang will separate code into
> .text.hot, .text.unlikely, or .text.unknown sections based on profiling
> information. After D79600 (clang-11), these sections will have a
> trailing `.` suffix, ie.  .text.hot., .text.unlikely., .text.unknown..
> 
> When using -ffunction-sections together with profiling infomation,
> either explicitly (FGKASLR) or implicitly (LTO), code may be placed in
> sections following the convention:
> .text.hot.<foo>, .text.unlikely.<bar>, .text.unknown.<baz>
> where <foo>, <bar>, and <baz> are functions.  (This produces one section
> per function; we generally try to merge these all back via linker script
> so that we don't have 50k sections).
> 
> For the above cases, we need to teach our linker scripts that such
> sections might exist and that we'd explicitly like them grouped
> together, otherwise we can wind up with code outside of the
> _stext/_etext boundaries that might not be mapped properly for some
> architectures, resulting in boot failures.
> 
> If the linker script is not told about possible input sections, then
> where the section is placed as output is a heuristic-laiden mess that's
> non-portable between linkers (ie. BFD and LLD), and has resulted in many
> hard to debug bugs.  Kees Cook is working on cleaning this up by adding
> --orphan-handling=warn linker flag used in ARCH=powerpc to additional
> architectures. In the case of linker scripts, borrowing from the Zen of
> Python: explicit is better than implicit.
> 
> Also, ld.bfd's internal linker script considers .text.hot AND
> .text.hot.* to be part of .text, as well as .text.unlikely and
> .text.unlikely.*. I didn't see support for .text.unknown.*, and didn't
> see Clang producing such code in our kernel builds, but I see code in
> LLVM that can produce such section names if profiling information is
> missing. That may point to a larger issue with generating or collecting
> profiles, but I would much rather be safe and explicit than have to
> debug yet another issue related to orphan section placement.
> 
> Reported-by: Jian Cai <jiancai@google.com>
> Suggested-by: Fāng-ruì Sòng <maskray@google.com>
> Tested-by: Luis Lozano <llozano@google.com>
> Tested-by: Manoj Gupta <manojgupta@google.com>
> Acked-by: Kees Cook <keescook@chromium.org>
> Cc: stable@vger.kernel.org
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
> Link: https://reviews.llvm.org/D79600
> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760
> Debugged-by: Luis Lozano <llozano@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/asm-generic/vmlinux.lds.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 2593957f6e8b..af5211ca857c 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -561,7 +561,10 @@
>   */
>  #define TEXT_TEXT							\
>  		ALIGN_FUNCTION();					\
> -		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
> +		*(.text.hot .text.hot.*)				\
> +		*(TEXT_MAIN .text.fixup)				\
> +		*(.text.unlikely .text.unlikely.*)			\
> +		*(.text.unknown .text.unknown.*)			\
>  		NOINSTR_TEXT						\
>  		*(.text..refcount)					\
>  		*(.ref.text)						\
> -- 
> 2.25.1
> 

This also changes the ordering to place all hot resp unlikely sections separate
from other text, while currently it places the hot/unlikely bits of each file
together with the rest of the code in that file. That seems like a reasonable
change and should be mentioned in the commit message.

However, the history of their being together comes from

  9bebe9e5b0f3 ("kbuild: Fix .text.unlikely placement")

which seems to indicate there was some problem with having them separated out,
although I don't quite understand what the issue was from the commit message.

Cc Andi and Michal to see if they remember.

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Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-31 23:07 [PATCH v5 00/36] Warn on orphan section placement Kees Cook
2020-07-31 23:07 ` [PATCH v5 01/36] x86/boot/compressed: Move .got.plt entries out of the .got section Kees Cook
2020-07-31 23:07 ` [PATCH v5 02/36] x86/boot/compressed: Force hidden visibility for all symbol references Kees Cook
2020-07-31 23:07 ` [PATCH v5 03/36] x86/boot/compressed: Get rid of GOT fixup code Kees Cook
2020-07-31 23:07 ` [PATCH v5 04/36] x86/boot: Add .text.* to setup.ld Kees Cook
2020-07-31 23:07 ` [PATCH v5 05/36] x86/boot: Remove run-time relocations from .head.text code Kees Cook
2020-07-31 23:42   ` Nick Desaulniers
2020-07-31 23:07 ` [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32, 64}.S Kees Cook
2020-08-07 18:12   ` Nick Desaulniers
2020-08-07 20:20     ` [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32,64}.S Arvind Sankar
2020-07-31 23:07 ` [PATCH v5 07/36] x86/boot: Check that there are no run-time relocations Kees Cook
2020-07-31 23:07 ` [PATCH v5 08/36] vmlinux.lds.h: Create COMMON_DISCARDS Kees Cook
2020-07-31 23:07 ` [PATCH v5 09/36] vmlinux.lds.h: Add .gnu.version* to COMMON_DISCARDS Kees Cook
2020-07-31 23:07 ` [PATCH v5 10/36] vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted sections Kees Cook
2020-07-31 23:07 ` [PATCH v5 11/36] vmlinux.lds.h: Split ELF_DETAILS from STABS_DEBUG Kees Cook
2020-07-31 23:07 ` [PATCH v5 12/36] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to ELF_DETAILS Kees Cook
2020-07-31 23:07 ` [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections Kees Cook
2020-08-01  3:51   ` Arvind Sankar [this message]
2020-08-01  6:18     ` Kees Cook
2020-08-01 17:27       ` Arvind Sankar
2020-08-03 19:05     ` Andi Kleen
2020-08-03 20:15       ` Arvind Sankar
2020-08-04  1:19         ` Fāng-ruì Sòng
2020-08-04  4:45         ` Andi Kleen
2020-08-04  5:32           ` Fāng-ruì Sòng
2020-08-04 16:06           ` Arvind Sankar
2020-08-21 19:18             ` Kees Cook
2020-07-31 23:07 ` [PATCH v5 14/36] efi/libstub: Disable -mbranch-protection Kees Cook
2020-07-31 23:07 ` [PATCH v5 15/36] arm64/mm: Remove needless section quotes Kees Cook
2020-07-31 23:08 ` [PATCH v5 16/36] arm64/kernel: Remove needless Call Frame Information annotations Kees Cook
2020-07-31 23:08 ` [PATCH v5 17/36] arm64/build: Remove .eh_frame* sections due to unwind tables Kees Cook
2020-07-31 23:08 ` [PATCH v5 18/36] arm64/build: Use common DISCARDS in linker script Kees Cook
2020-07-31 23:08 ` [PATCH v5 19/36] arm64/build: Add missing DWARF sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 20/36] arm64/build: Assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 21/36] arm64/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 22/36] arm/build: Refactor linker script headers Kees Cook
2020-07-31 23:08 ` [PATCH v5 23/36] arm/build: Explicitly keep .ARM.attributes sections Kees Cook
2020-08-03 19:02   ` Nick Desaulniers
2020-08-17 22:06     ` Fangrui Song
2020-07-31 23:08 ` [PATCH v5 24/36] arm/build: Add missing sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 25/36] arm/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 26/36] arm/boot: Handle all sections explicitly Kees Cook
2020-07-31 23:08 ` [PATCH v5 27/36] arm/boot: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 28/36] x86/asm: Avoid generating unused kprobe sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 29/36] x86/build: Enforce an empty .got.plt section Kees Cook
2020-08-01  2:12   ` Arvind Sankar
2020-08-01  5:32     ` Kees Cook
2020-08-21 17:49     ` Kees Cook
2020-07-31 23:08 ` [PATCH v5 30/36] x86/build: Assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 31/36] x86/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 32/36] x86/boot/compressed: Reorganize zero-size section asserts Kees Cook
2020-08-01  1:47   ` Arvind Sankar
2020-08-01  2:53     ` Arvind Sankar
2020-08-01  5:36       ` Kees Cook
2020-08-01 17:12         ` Arvind Sankar
2020-08-21 18:24           ` Kees Cook
2020-08-01  5:35     ` Kees Cook
2020-08-01 17:00       ` Arvind Sankar
2020-08-21 18:19     ` Kees Cook
2020-07-31 23:08 ` [PATCH v5 33/36] x86/boot/compressed: Remove, discard, or assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 34/36] x86/boot/compressed: Add missing debugging sections to output Kees Cook
2020-07-31 23:08 ` [PATCH v5 35/36] x86/boot/compressed: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 36/36] arm/build: Assert for unwanted sections Kees Cook

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