linux-kernel - [PATCH 0/2] ftrace/recordmcount: Handle object files without section symbols

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Message-Id: <cover.1651047542.git.naveen.n.rao@linux.vnet.ibm.com>
Date:   Wed, 27 Apr 2022 15:01:20 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        llvm@...ts.linux.dev
Subject: [PATCH 0/2] ftrace/recordmcount: Handle object files without section symbols

This solves a build issue on powerpc with binutils v2.36 and newer [1].
Since commit d1bcae833b32f1 ("ELF: Don't generate unused section
symbols") [2], binutils started dropping section symbols that it thought
were unused. Due to this, in certain scenarios, recordmcount is unable 
to find a non-weak symbol to generate a relocation record against.

Clang integrated assembler is also aggressive in dropping section 
symbols [3].

In the past, there have been various workarounds to address this. See 
commits 55d5b7dd6451b5 ("initramfs: fix clang build failure") and 
6e7b64b9dd6d96 ("elfcore: fix building with clang") and a recent patch:
https://lore.kernel.org/linuxppc-dev/20220425174128.11455-1-naveen.n.rao@linux.vnet.ibm.com/T/#u

Fix this issue by using the weak symbol in the relocation record. This 
can result in duplicate locations in the mcount table if those weak 
functions are overridden, so have ftrace skip dupicate entries.

Objtool already follows this approach, so patch 2 updates recordmcount 
to do the same. Patch 1 updates ftrace to skip duplicate entries.

- Naveen

[1] https://github.com/linuxppc/issues/issues/388
[2] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1
[3] https://github.com/ClangBuiltLinux/linux/issues/981

Naveen N. Rao (2):
  ftrace: Drop duplicate mcount locations
  recordmcount: Handle sections with no non-weak symbols

 kernel/trace/ftrace.c  | 13 ++++++-
 scripts/recordmcount.h | 86 +++++++++++++++++++++++++++++++++++-------
 2 files changed, 85 insertions(+), 14 deletions(-)

base-commit: 83d8a0d166119de813cad27ae7d61f54f9aea707
-- 
2.35.1