Compile error ppc64le: Cannot find symbol for section 11: .text.unlikely.

[Coiby Xu <coxu@redhat.com>]

On Fri, Dec 03, 2021 at 04:54:19PM +0100, Veronika Kabatova wrote:
>On Wed, Dec 1, 2021 at 3:20 AM Coiby Xu <coxu@redhat.com> wrote:
>>
>> On Wed, Nov 24, 2021 at 09:47:43PM +0800, Baoquan He wrote:
>> >On 11/24/21 at 01:47pm, Veronika Kabatova wrote:
>> >> Hi,
>> >>
>> >> for a while we've been seen the following error when compiling
>> >> the mainline kernel with gcc 11.2 and binutils 2.37:
>> >>
>> >> 00:02:32 Cannot find symbol for section 11: .text.unlikely.
>> >> 00:02:32 kernel/kexec_file.o: failed
>> >> 00:02:32 make[3]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
>> >> 00:02:32 make[3]: *** Deleting file 'kernel/kexec_file.o'
>> >> 00:02:32 make[2]: *** [Makefile:1846: kernel] Error 2
>> >> 00:02:32 make[2]: *** Waiting for unfinished jobs....
>> >>
>> >> The error only happens with ppc64le. I've tested this with cross
>> >> compilation, but the only reference to the error I found suggests
>> >> the same happens with the native compiles as well:
>> >>
>> >> https://github.com/groeck/linux-build-test/commit/142cbefbc0d37962c9a6c7f28ee415ecd5fd1e98
>> >>
>> >> In case it matters, the config used is the Fedora config with
>> >> kselftest options enabled, which you can grab from
>> >>
>> >> https://gitlab.com/redhat/red-hat-ci-tools/kernel/cki-internal-pipelines/cki-trusted-contributors/-/jobs/1760752896/artifacts/raw/artifacts/kernel-mainline.kernel.org-ppc64le-e4e737bb5c170df6135a127739a9e6148ee3da82.config
>> >>
>> >>
>> >> I've reached out to the Fedora compiler folks and Nick Clifton
>> >> suggested this is a problem with the kernel:
>> >>
>> >>     This message comes from the recordmcount tool, which is part of the kernel
>> >>     sources:
>> >>
>> >>     linux/scripts/recordmcount.[ch]
>> >>
>> >>     It appears to be triggered when a compiler update causes code to be
>> >>     rearranged. The problem has been reported before in various forums,
>> >>     but in particular I found this reference:
>> >>
>> >>     https://lore.kernel.org/lkml/20201204165742.3815221-2-arnd at kernel.org/
>> >>
>> >>     The point of which to me at least is that this is a kernel issue rather than
>> >>     a compiler issue.  Ie there must be some weak symbols in kexec_file.o file
>> >>     which need to be moved elsewhere.
>> >
>> >It could be arch_kexec_kernel_verify_sig() in kernel/kexec_file.c which
>> >is __weak, but not implemented in any ARCH. If true, this has been
>> >pointed out by Eric in one patch thread from Coiby.
>> >
>> >[PATCH v3 1/3] kexec: clean up arch_kexec_kernel_verify_sig
>> >http://lkml.kernel.org/r/20211018083137.338757-2-coxu at redhat.com
>> >
>> >Maybe Coiby can fetch above config file and run the test to check.
>>
>> "[PATCH v3 1/3] kexec: clean up arch_kexec_kernel_verify_sig" alone
>> would fix the error. If I turn arch_kexec_apply_relocations{_add,} into

Sorry I meant "alone won't fix the error".

>> static function, the error would be gone. As attached is the patch would
>> make this error disappear.
>>
>
>Thank you! I can confirm the attached patch fixes the problem.
>
>
>Veronika
>
>> However, s390 and x86 have its own implementation of
>> arch_kexec_apply_relocations_add. This makes it looks like to be gcc's
>> issue.

Based on the above point and further investigation, I think the root cause is
find_secsym_ndx in linux/scripts/recordmcount.h,
  /*
   * Find a symbol in the given section, to be used as the base for relocating
   * the table of offsets of calls to mcount.  A local or global symbol suffices,
   * but avoid a Weak symbol because it may be overridden; the change in value
   * would invalidate the relocations of the offsets of the calls to mcount.
   * Often the found symbol will be the unnamed local symbol generated by
   * GNU 'as' for the start of each section.  For example:
   *    Num:    Value  Size Type    Bind   Vis      Ndx Name
   *      2: 00000000     0 SECTION LOCAL  DEFAULT    1
   */
  static int find_secsym_ndx(unsigned const txtndx,
  				char const *const txtname,
  				uint_t *const recvalp,
  				unsigned int *sym_index,
  				Elf_Shdr const *const symhdr,
  				Elf32_Word const *symtab,
  				Elf32_Word const *symtab_shndx,
  				Elf_Ehdr const *const ehdr)
  {
         ...
		if (txtndx == get_symindex(symp, symtab, symtab_shndx)
			/* avoid STB_WEAK */

	 fprintf(stderr, "Cannot find symbol for section %u: %s.\n",
		txtndx, txtname);

This function prints the above warning after failing to find
arch_kexec_kernel_verify_sig or arch_kexec_apply_relocations{_add,} in
section 11: .text.unlikely. because it ignores the weak symbol and ppc64le
doesn't its arch implementations of these functions. I'll see if I can fix
it in linux/scripts/recordmcount.h.

>>
>>
>> >
>> >Thanks
>> >Baoquan
>> >
>>
>> --
>> Best regards,
>> Coiby
>

-- 
Best regards,
Coiby