Re: [PATCH] kexec_file: Drop pr_err in weak implementations of arch_kexec_apply_relocations[_add]

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Eric W. Biederman wrote:
> Michael Ellerman <mpe@ellerman.id.au> writes:
> 
>> "Eric W. Biederman" <ebiederm@xmission.com> writes:
>>> Looking at this the pr_err is absolutely needed.  If an unsupported case
>>> winds up in the purgatory blob and the code can't handle it things
>>> will fail silently much worse later.
>>
>> It won't fail later, it will fail the syscall.
>>
>> sys_kexec_file_load()
>>   kimage_file_alloc_init()
>>     kimage_file_prepare_segments()
>>       arch_kexec_kernel_image_load()
>>         kexec_image_load_default()
>>           image->fops->load()
>>             elf64_load()        # powerpc
>>             bzImage64_load()    # x86
>>               kexec_load_purgatory()
>>                 kexec_apply_relocations()
>>
>> Which does:
>>
>> 	if (relsec->sh_type == SHT_RELA)
>> 		ret = arch_kexec_apply_relocations_add(pi, section,
>> 						       relsec, symtab);
>> 	else if (relsec->sh_type == SHT_REL)
>> 		ret = arch_kexec_apply_relocations(pi, section,
>> 						   relsec, symtab);
>> 	if (ret)
>> 		return ret;
>>
>> And that error is bubbled all the way back up. So as long as
>> arch_kexec_apply_relocations() returns an error the syscall will fail
>> back to userspace and there'll be an error message at that level.
>>
>> It's true that having nothing printed in dmesg makes it harder to work
>> out why the syscall failed. But it's a kernel bug if there are unhandled
>> relocations in the kernel-supplied purgatory code, so a user really has
>> no way to do anything about the error even if it is printed.
> 
> Good point.  I really hadn't noticed the error code in there when I
> looked.
> 
> I still don't think changing the functionality of the code because of
> a tool issue is the right solution.

Ok.

> 
> 
>>> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> writes:
>>>
>>>> Baoquan He wrote:
>>>>> On 04/25/22 at 11:11pm, Naveen N. Rao wrote:
>>>>>> kexec_load_purgatory() can fail for many reasons - there is no need to
>>>>>> print an error when encountering unsupported relocations.
>>>>>> 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
>>>>> I am not familiar with binutils, while wondering if this exists in other
>>>>> ARCHes except of ppc. Arm64 doesn't have the ARCH override either, do we
>>>>> have problem with it?
>>>>
>>>> I'm not aware of this specific file causing a problem on other architectures -
>>>> perhaps the config options differ enough. There are however more reports of
>>>> similar issues affecting other architectures with the llvm integrated assembler:
>>>> https://github.com/ClangBuiltLinux/linux/issues/981
>>>>
>>>>>
>>>>>> were unused.  This isn't an issue in general, but with kexec_file.c, gcc
>>>>>> is placing kexec_arch_apply_relocations[_add] into a separate
>>>>>> .text.unlikely section and the section symbol ".text.unlikely" is being
>>>>>> dropped. Due to this, recordmcount is unable to find a non-weak symbol
>>>>> But arch_kexec_apply_relocations_add is weak symbol on ppc.
>>>>
>>>> Yes. Note that it is just the section symbol that gets dropped. The section is
>>>> still present and will continue to hold the symbols for the functions
>>>> themselves.
>>>
>>> So we have a case where binutils thinks it is doing something useful
>>> and our kernel specific tool gets tripped up by it.
>>
>> It's not just binutils, the LLVM assembler has the same behavior.
>>
>>> Reading the recordmcount code it looks like it is finding any symbol
>>> within a section but ignoring weak symbols.  So I suspect the only
>>> remaining symbol in the section is __weak and that confuses
>>> recordmcount.
>>>
>>> Does removing the __weak annotation on those functions fix the build
>>> error?  If so we can restructure the kexec code to simply not use __weak
>>> symbols.
>>>
>>> Otherwise the fix needs to be in recordmcount or binutils, and we should
>>> loop whoever maintains recordmcount in to see what they can do.
>>
>> It seems that recordmcount is not really maintained anymore now that x86
>> uses objtool?
>>
>> There've been several threads about fixing recordmcount, but none of
>> them seem to have lead to a solution.
> 
> That is unfortunate.
> 
>> These weak symbol vs recordmcount problems have been worked around going
>> back as far as 2020:
>>
>>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/linux/elfcore.h?id=6e7b64b9dd6d96537d816ea07ec26b7dedd397b9
> 
> I am more than happy to adopt the kind of solution that was adopted
> there in elfcore.h and simply get rid of __weak symbols in the kexec
> code.
> 
> Using __weak symbols is really not the common kernel way of doing
> things.  Using __weak symbols introduces a bit of magic in how the
> kernel gets built that is unnecessary.
> 
> Can someone verify that deleting __weak is enough to get powerpc to
> build?  AKA:
> 
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 8347fc158d2b..7f4ca8dbe26f 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -117,7 +117,7 @@ int __weak arch_kexec_kernel_verify_sig(struct kimage *image, void *buf,
>   *
>   * Return: 0 on success, negative errno on error.
>   */
> -int __weak
> +int
>  arch_kexec_apply_relocations_add(struct purgatory_info *pi, Elf_Shdr *section,
>                                  const Elf_Shdr *relsec, const Elf_Shdr *symtab)
>  {
> @@ -134,7 +134,7 @@ arch_kexec_apply_relocations_add(struct purgatory_info *pi, Elf_Shdr *section,
>   *
>   * Return: 0 on success, negative errno on error.
>   */
> -int __weak
> +int
>  arch_kexec_apply_relocations(struct purgatory_info *pi, Elf_Shdr *section,
>                              const Elf_Shdr *relsec, const Elf_Shdr *symtab)
>  {

Yes, dropping the __weak attribute allows recordmcount to emit a 
relocation using those symbols, so that resolves the problem.

> 
> If that change is verified to work a proper patch that keeps x86 and
> s390 building that have actual implementations should not be too
> difficult to write.

Sure, I will post a patch for that.


Thanks,
Naveen