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    SubjectRe: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections
    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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