Messages in this thread | | | Date | Tue, 16 Aug 2022 22:35:34 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH 0/2] ftrace/recordmcount: Handle object files without section symbols |
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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
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