Hi Steven,
Steven Rostedt wrote:
On Wed, 27 Apr 2022 15:01:20 +0530
"Naveen N. Rao" <naveen.n....@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....@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....@linux.vnet.ibm.com/
https://lore.kernel.org/all/cover.1656659357.git.naveen.n....@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....@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...@linux.ibm.com/
- Naveen
