Re: [PATCH bpf-next 0/2] bpf,ftrace: bpf dispatcher function fix

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Tue, 2022-08-30 at 15:48 +0200, Jiri Olsa wrote:
> On Tue, Aug 30, 2022 at 12:25:25AM +0200, Daniel Borkmann wrote:
> > On 8/26/22 8:46 PM, Jiri Olsa wrote:
> > > hi,
> > > as discussed [1] sending fix that moves bpf dispatcher function
> > > of out
> > > ftrace locations together with Peter's
> > > HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
> > > dependency change.
> > 
> > Looks like the series breaks s390x builds; BPF CI link:
> > 
> > https://github.com/kernel-patches/bpf/runs/8079411784?check_suite_focus=true
> > 
> >   [...]
> >     CC      net/xfrm/xfrm_state.o
> >     CC      net/packet/af_packet.o
> >   {standard input}: Assembler messages:
> >   {standard input}:16055: Error: bad expression
> >   {standard input}:16056: Error: bad expression
> >   {standard input}:16057: Error: bad expression
> >   {standard input}:16058: Error: bad expression
> >   {standard input}:16059: Error: bad expression
> >     CC      drivers/s390/char/raw3270.o
> >     CC      net/ipv6/ip6_output.o
> >   [...]
> >     CC      net/xfrm/xfrm_output.o
> >     CC      net/ipv6/ip6_input.o
> >   {standard input}:16055: Error: invalid operands (*ABS* and *UND*
> > sections) for `%'
> >   {standard input}:16056: Error: invalid operands (*ABS* and *UND*
> > sections) for `%'
> >   {standard input}:16057: Error: invalid operands (*ABS* and *UND*
> > sections) for `%'
> >   {standard input}:16058: Error: invalid operands (*ABS* and *UND*
> > sections) for `%'
> >   {standard input}:16059: Error: invalid operands (*ABS* and *UND*
> > sections) for `%'
> >   make[3]: *** [scripts/Makefile.build:249: net/core/filter.o]
> > Error 1
> >   make[2]: *** [scripts/Makefile.build:465: net/core] Error 2
> >   make[2]: *** Waiting for unfinished jobs....
> >     CC      net/ipv4/tcp_fastopen.o
> >   [...]
> >     CC      lib/percpu-refcount.o
> >   make[1]: *** [Makefile:1855: net] Error 2
> >     CC      lib/rhashtable.o
> >   make[1]: *** Waiting for unfinished jobs....
> >     CC      lib/base64.o
> >   [...]
> >     AR      lib/built-in.a
> >     CC      kernel/kheaders.o
> >     AR      kernel/built-in.a
> >   make: *** [Makefile:353: __build_one_by_one] Error 2
> >   Error: Process completed with exit code 2.
> 
> 
> it does not break on my cross build with gcc 12, but I can
> reproduce with gcc 8 (CI seems to be on gcc 9)
> 
> the problem seems to be wrong assembler code with extra '%'
> that's generated for patchable_function_entry(5)
> 
> gcc 8 generates:
> 
> .LPFE1:
>         nopr    %%r0
>         nopr    %%r0
>         nopr    %%r0
>         nopr    %%r0
>         nopr    %%r0
> 
> and gcc 12 generates:
> 
> .LPFE1:
>         nopr    %r0
>         nopr    %r0
>         nopr    %r0
>         nopr    %r0
>         nopr    %r0
> 
> perhaps we need to upgrade gcc in CI? cc-ing Ilya, any idea?
> 
> thanks,
> jirka

It's not obvious to me which gcc commit fixed this; I will bisect and
find out. This will take some time.

However, officially, the kernel must be buildable by gcc 5.1+.
Whatever I find, it's unlikely that we'll be able to backport it
that far.

Therefore I think we need to find a way to conditionally
do something else when using broken gccs. Or maybe just keep this
x86-only after all.

Best regards,
Ilya