| Message ID | cover.1651047542.git.naveen.n.rao@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | ftrace/recordmcount: Handle object files without section symbols | expand |
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
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