Re: [PATCH] samples/bpf: Fix cross compiler error with bpf sample

[Daniel Borkmann]

On 08/04/2017 08:33 PM, Joel Fernandes wrote:
> On Fri, Aug 4, 2017 at 6:58 AM, Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> > On 08/04/2017 07:46 AM, Joel Fernandes wrote:
> > > When cross-compiling the bpf sample map_perf_test for aarch64, I find that
> > > __NR_getpgrp is undefined. This causes build errors. Fix it by allowing
> > > the
> > > deprecated syscall in the sample.
> > >
> > > Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> > > ---
> > >    samples/bpf/map_perf_test_user.c | 2 ++
> > >    1 file changed, 2 insertions(+)
> > >
> > > diff --git a/samples/bpf/map_perf_test_user.c
> > > b/samples/bpf/map_perf_test_user.c
> > > index 1a8894b5ac51..6e6fc7121640 100644
> > > --- a/samples/bpf/map_perf_test_user.c
> > > +++ b/samples/bpf/map_perf_test_user.c
> > > @@ -8,7 +8,9 @@
> > >    #include <sched.h>
> > >    #include <stdio.h>
> > >    #include <sys/types.h>
> > > +#define __ARCH_WANT_SYSCALL_DEPRECATED
> > >    #include <asm/unistd.h>
> > > +#undef __ARCH_WANT_SYSCALL_DEPRECATED
> >
> >
> > So the only arch that sets __ARCH_WANT_SYSCALL_DEPRECATED
> > is score:
> >
> >    $ git grep -n __ARCH_WANT_SYSCALL_DEPRECATED
> >    arch/score/include/uapi/asm/unistd.h:7:#define
> > __ARCH_WANT_SYSCALL_DEPRECATED
> >    include/uapi/asm-generic/unistd.h:837:#ifdef
> > __ARCH_WANT_SYSCALL_DEPRECATED
> >    include/uapi/asm-generic/unistd.h:899:#endif /*
> > __ARCH_WANT_SYSCALL_DEPRECATED */
> >
> > But even if this would make aarch64 compile, the syscall
> > numbers don't match up:
> >
> >    $ git grep -n __NR_getpgrp include/uapi/asm-generic/unistd.h
> >    include/uapi/asm-generic/unistd.h:841:#define __NR_getpgrp 1060
> >    include/uapi/asm-generic/unistd.h:843:__SYSCALL(__NR_getpgrp, sys_getpgrp)
> >
> > The only thing that can be found on arm64 is:
> >
> >    $ git grep -n __NR_getpgrp arch/arm64/
> >    arch/arm64/include/asm/unistd32.h:154:#define __NR_getpgrp 65
> >    arch/arm64/include/asm/unistd32.h:155:__SYSCALL(__NR_getpgrp, sys_getpgrp)
> >
> > In arch/arm64/include/asm/unistd.h, it does include the
> > uapi/asm/unistd.h when compat is not set, but without the
> > __ARCH_WANT_SYSCALL_DEPRECATED. That doesn't look correct
> > unless I'm missing something, hmm, can't we just attach the
> > kprobes to a different syscall, one that is not deprecated,
> > so that we don't run into this in the first place?
>
> Yes, I agree that's better. I think we can use getpgid. I'll try to
> whip something today and send it out.

Ok, cool. Please make sure that this doesn't clash with anything
else attached to map_perf_test_kern.c already given the obj file
is loaded first with the attachment points.

> I also wanted to fix something else, HOSTCC is set to gcc, but I want
> the boostrap part of the sample to run an ARM so I have to make HOSTCC
> my cross-compiler. Right now I'm hacking it to point to the arm64 gcc
> however I think I'd like to add a 'cross compile mode' or something
> whether HOSTCC points to CROSS_COMPILE instead. I'm happy to discuss
> any ideas to get this fixed too.

Yeah, sounds like a good idea to add such possibility. In case of
cross compiling to a target arch with different endianess, you might
also need to specifically select bpfeb (big endian) resp. bpfel
(little endian) as clang target. (Just bpf target uses host endianess.)

Thanks, Joel!