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

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Message-ID: <59847D97.8080003@iogearbox.net>
Date:   Fri, 04 Aug 2017 15:58:47 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Joel Fernandes <joelaf@...gle.com>
CC:     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 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?

Thanks,
Daniel