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

On Wed, 27 Apr 2022 15:01:20 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> 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
> 
>

There's been work to handle weak functions, but I'm not sure that work
handled the issues here. Are these patches still needed, or was there
another workaround to handle the problems this addressed?

-- Steve

Hi Steven,

Steven Rostedt wrote:
> On Wed, 27 Apr 2022 15:01:20 +0530
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> 
>> 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
>> 
>>
> 
> There's been work to handle weak functions, but I'm not sure that work
> handled the issues here. Are these patches still needed, or was there
> another workaround to handle the problems this addressed?

I'm afraid these patches are still needed to address issues in 
recordmcount.

I submitted patches to remove use of weak functions in the kexec 
subsystem, but those have only enabled building ppc64le defconfig 
without errors:
https://lore.kernel.org/all/20220519091237.676736-1-naveen.n.rao@linux.vnet.ibm.com/
https://lore.kernel.org/all/cover.1656659357.git.naveen.n.rao@linux.vnet.ibm.com/

The patch adding support for FTRACE_MCOUNT_MAX_OFFSET to powerpc only 
helps ignore weak functions during runtime:
https://lore.kernel.org/all/20220809105425.424045-1-naveen.n.rao@linux.vnet.ibm.com/

We still see errors from recordmcount when trying to build certain 
powerpc configs.

We are pursuing support for objtool, which doesn't have the same issues:
https://lore.kernel.org/all/20220808114908.240813-1-sv@linux.ibm.com/


- Naveen