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

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Message-ID: <CAJWu+oqY7KLbVGSmuedoZp0D9usXuP0JUfzws_aUt461M3gxQA@mail.gmail.com>
Date:   Fri, 4 Aug 2017 11:33:01 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>
Subject: Re: [PATCH] samples/bpf: Fix cross compiler error with bpf sample

On Fri, Aug 4, 2017 at 6:58 AM, Daniel Borkmann <daniel@...earbox.net> 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@...gle.com>
>> ---
>>   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.

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.

thanks!

-Joel


>
> Thanks,
> Daniel