Fwd: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' when compiling with -flto

Fwd: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' when compiling with -flto

[Bagas Sanjaya]

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Hi,

mrwizardwizard <terrym3201@protonmail.com> reported kernel FTBFS on bugzilla
(https://bugzilla.kernel.org/show_bug.cgi?id=219099) when the kernel
is built with LTO:

> Hello,
>
> when compiling linux kernel 6.9-6.10 with -flto
>
> compiler outputs:
>
> ./arch/x86/include/asm/current.h:42:25: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' const_pcpu_hot);
>

He could reproduce the build error on mainline:

>> Can you reproduce above on current mainline (v6.11-rc1)?
> 
> 
> same messages
> 
> mkdir -p /home/mrwizardwizard/Documents/Kernel/linux-6.11-rc1/tools/objtool && make O=/home/mrwizardwizard/Documents/Kernel/linux-6.11-rc1 subdir=tools/objtool --no-print-directory -C objtool 
>   INSTALL libsubcmd_headers
>   CALL    scripts/checksyscalls.sh
>   CC      init/main.o
>   AS      arch/x86/entry/entry.o
>   AS      arch/x86/entry/entry_64.o
> arch/x86/entry/entry_64.o: warning: objtool: __switch_to_asm+0x18: stack layout conflict in alternatives: .altinstr_replacement+0x1d
>   CC      arch/x86/entry/syscall_64.o
> In file included from ./include/asm-generic/percpu.h:7,
>                  from ./arch/x86/include/asm/percpu.h:616,
>                  from ./arch/x86/include/asm/current.h:11,
>                  from ./arch/x86/include/asm/processor.h:17,
>                  from ./arch/x86/include/asm/timex.h:5,
>                  from ./include/linux/timex.h:67,
>                  from ./include/linux/time32.h:13,
>                  from ./include/linux/time.h:60,
>                  from ./include/linux/stat.h:19,
>                  from ./include/linux/module.h:13,
>                  from init/main.c:17:
> ./arch/x86/include/asm/current.h:42:25: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot'
>    42 |                         const_pcpu_hot);
>       |                         ^~~~~~~~~~~~~~
> ./arch/x86/include/asm/current.h:38:42: note: 'pcpu_hot' was declared here
>    38 | DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
>       |                                          ^~~~~~~~
> make[3]: *** [scripts/Makefile.build:244: init/main.o] Error 1
> make[2]: *** [scripts/Makefile.build:485: init] Error 2
> make[2]: *** Waiting for unfinished jobs....
>   CC      arch/x86/entry/common.o
>   CC      arch/x86/events/core.o
>   CC      arch/x86/entry/vdso/vma.o
> In file included from ./include/asm-generic/percpu.h:7,
>                  from ./arch/x86/include/asm/percpu.h:616,
>                  from ./arch/x86/include/asm/current.h:11,
>                  from ./include/linux/sched.h:12,
>                  from ./include/linux/ptrace.h:6,
>                  from ./include/uapi/asm-generic/bpf_perf_event.h:4,
>                  from ./arch/x86/include/generated/uapi/asm/bpf_perf_event.h:1,
>                  from ./include/uapi/linux/bpf_perf_event.h:11,
>                  from ./include/linux/perf_event.h:18,
>                  from arch/x86/events/core.c:15:
> ./arch/x86/include/asm/current.h:42:25: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot'
>    42 |                         const_pcpu_hot);
>       |                         ^~~~~~~~~~~~~~
> ./arch/x86/include/asm/current.h:38:42: note: 'pcpu_hot' was declared here
>    38 | DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
>       |                                          ^~~~~~~~
> make[4]: *** [scripts/Makefile.build:244: arch/x86/events/core.o] Error 1
> make[3]: *** [scripts/Makefile.build:485: arch/x86/events] Error 2
> make[3]: *** Waiting for unfinished jobs....
>   AR      arch/x86/entry/vsyscall/built-in.a
>   CC      arch/x86/entry/vdso/extable.o
>   LDS     arch/x86/entry/vdso/vdso.lds
>   AS      arch/x86/entry/vdso/vdso-note.o
>   CC      arch/x86/entry/vdso/vclock_gettime.o
>   CC      arch/x86/entry/vdso/vgetcpu.o
>   CC      arch/x86/entry/vdso/vgetrandom.o
>   AS      arch/x86/entry/vdso/vgetrandom-chacha.o
>   HOSTCC  arch/x86/entry/vdso/vdso2c
>   VDSO    arch/x86/entry/vdso/vdso64.so.dbg
>   OBJCOPY arch/x86/entry/vdso/vdso64.so
>   VDSO2C  arch/x86/entry/vdso/vdso-image-64.c
>   CC      arch/x86/entry/vdso/vdso-image-64.o
>   AR      arch/x86/entry/vdso/built-in.a
>   AS      arch/x86/entry/thunk.o
>   AR      arch/x86/entry/built-in.a
> make[2]: *** [scripts/Makefile.build:485: arch/x86] Error 2
> make[1]: *** [/home/mrwizardwizard/Documents/Kernel/linux-6.11-rc1/Makefile:1927: .] Error 2
> make: *** [Makefile:224: __sub-make] Error 

For the full build log, see Bugzilla link.

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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Re: Fwd: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' when compiling with -flto

[Thomas Gleixner]

On Sat, Aug 03 2024 at 10:55, Bagas Sanjaya wrote:
>> when compiling linux kernel 6.9-6.10 with -flto
>>
>> compiler outputs:
>>
>> ./arch/x86/include/asm/current.h:42:25: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' const_pcpu_hot);
>
> He could reproduce the build error on mainline:

That's caused by:

  ed2f752e0e0a ("x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code generation")

Uros?

Thanks,

        tglx

Re: Fwd: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' when compiling with -flto

[Uros Bizjak]

On Wed, Aug 7, 2024 at 4:08 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Sat, Aug 03 2024 at 10:55, Bagas Sanjaya wrote:
> >> when compiling linux kernel 6.9-6.10 with -flto
> >>
> >> compiler outputs:
> >>
> >> ./arch/x86/include/asm/current.h:42:25: error: 'const_pcpu_hot' causes a section type conflict with 'pcpu_hot' const_pcpu_hot);
> >
> > He could reproduce the build error on mainline:
>
> That's caused by:
>
>   ed2f752e0e0a ("x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code generation")
>
> Uros?

I have taken the discussion to the bugreport, where the potential
patch is also posted.

For reference, my answer is as follows:

--cut here--
(In reply to mrwizardwizard from comment #0)

> when compiling linux kernel 6.9-6.10 with -flto

You need a whole lot more patches to successfully compile kernel with
GCC LTO. The last revision can be found at:

https://lore.kernel.org/lkml/20221114114344.18650-1-jirislaby@kernel.org/

The patchset defines CONFIG_LTO, which can be used to disable
offending optimization, something like the to-be attached patch.

Ideally, a LTO-compatible "const __seg_gs" qualified alias to struct
pcpu_hot should be declared, so LTO can retain the disabled
optimization, but ...

... the LTO patchset was rejected mainly due to:
- the performance is the same
- the resulting image is bigger
- we need a whole lot of ugly hacks to placate the linker.
--cut here--

Thanks,
Uros.