linux-kernel - Re: [PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of .text too

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Date:   Mon, 22 Jun 2020 16:15:36 -0700
From:   Fāng-ruì Sòng <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>,
        "# 3.4.x" <stable@...r.kernel.org>, Jian Cai <jiancai@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of
 .text too

On 2020-06-22, Nick Desaulniers wrote:
>On Wed, Jun 17, 2020 at 2:27 PM Fāng-ruì Sòng <maskray@...gle.com> wrote:
>>
>>
>> On 2020-06-17, Nick Desaulniers wrote:
>> >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.*.
>>
>> >ld.lld will produce .text.hot.*/.text.unlikely.* sections.
>>
>> Correction to this sentence. lld is not relevant here.
>>
>> -ffunction-sections combined with profile-guided optimization can
>> produce .text.hot.* .text.unlikely.* sections.  Newer clang may produce
>> .text.hot. .text.unlikely. (without suffix, but with a trailing dot)
>> when -fno-unique-section-names is specified, as an optimization to make
>> .strtab smaller.
>
>Then why was the bug report reporting https://reviews.llvm.org/D79600
>as the result of a bisection, if LLD is not relevant?  Was the
>bisection wrong?

https://reviews.llvm.org/D79600 is an LLVM codegen change, unrelated to LLD..
(As described in the patch, LLD's -z keep-text-section-prefix only
recognizes ".text.exit.*", not ".text.exit")

>The upstream report wasn't initially public, for no good reason.  So I
>didn't include it, but if we end up taking v1, this should have
>
>Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760

Thanks for making it public.

>The kernel doesn't use -fno-unique-section-names; is that another flag
>that's added by CrOS' compiler wrapper?
>https://source.chromium.org/chromiumos/chromiumos/codesearch/+/master:src/third_party/toolchain-utils/compiler_wrapper/config.go;l=110
>Looks like no.  It doesn't use `-fno-unique-section-names` or
>`-ffunction-sections`.

-fno-unique-section-names is a very rare option. It is not supported by GCC (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95095 ).
clang users use it very rarely, probably because not many people care
about additional strings taken by section names ".text.hot.a" ".text.hot.b" ".text.hot.c"
in the string table ".strtab" (clang since some point of 2018 uses
.strtab instead of .shstrtab which enables more string sharing).

>
>

>
>>
>> We've already seen that GCC can place main in .text.startup without
>> -ffunction-sections. There may be other non -ffunction-sections cases
>> for .text.hot.* or .text.unlikely.*. So it is definitely a good idea to
>> be more specific even if we don't care about -ffunction-sections for
>> now.
>>
>> >Make sure to group these together.  Otherwise these orphan sections may
>> >be placed outside of the the _stext/_etext boundaries.
>> >
>> >Cc: stable@...r.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
>> >Reported-by: Jian Cai <jiancai@...gle.com>
>> >Debugged-by: Luis Lozano <llozano@...gle.com>
>> >Suggested-by: Fāng-ruì Sòng <maskray@...gle.com>
>> >Tested-by: Luis Lozano <llozano@...gle.com>
>> >Tested-by: Manoj Gupta <manojgupta@...gle.com>
>> >Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> >---
>> > include/asm-generic/vmlinux.lds.h | 4 +++-
>> > 1 file changed, 3 insertions(+), 1 deletion(-)
>> >
>> >diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>> >index d7c7c7f36c4a..fe5aaef169e3 100644
>> >--- a/include/asm-generic/vmlinux.lds.h
>> >+++ b/include/asm-generic/vmlinux.lds.h
>> >@@ -560,7 +560,9 @@
>> >  */
>> > #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.*)                      \
>> >               NOINSTR_TEXT                                            \
>> >               *(.text..refcount)                                      \
>> >               *(.ref.text)                                            \
>> >--
>> >2.27.0.290.gba653c62da-goog
>> >
>
>
>
>--
>Thanks,
>~Nick Desaulniers