Re: [PATCH 0/2] ftrace/recordmcount: Handle object files without section symbols

Naveen N. Rao
2022-08-16T17:06:14Z

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