netdev - Re: [PATCH bpf] samples/bpf: Fix the error return code of xdp

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Date:   Fri, 2 Jul 2021 12:29:35 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Wang Hai <wanghai38@...wei.com>, davem@...emloft.net,
        kuba@...nel.org, ast@...nel.org, daniel@...earbox.net,
        hawk@...nel.org, john.fastabend@...il.com, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, kpsingh@...nel.org
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf] samples/bpf: Fix the error return code of
 xdp_redirect's main()


On 16/06/2021 06.25, Wang Hai wrote:
> Fix to return a negative error code from the error handling
> case instead of 0, as done elsewhere in this function.

The main() function in C should never return a negative value on Unix 
POSIX systems.


There is a good explaination in exit(3p): `man 3p exit`

    The  value  of  status may be 0, EXIT_SUCCESS, EXIT_FAILURE, or any 
other value, though only the least significant 8 bits (that is, status & 
0377) shall be available to a waiting parent process.

Thus, negative values are often seen as 255 in the $? program exit 
status variable $?.


Also explained in exit(3):

     The C standard specifies two constants, EXIT_SUCCESS=0 and 
EXIT_FAILURE=1.

I see the 'samples/bpf/xdp_redirect_user.c' in most places just use 0 or 1.


> If bpf_map_update_elem() failed, main() should return a negative error.
>
> Fixes: 832622e6bd18 ("xdp: sample program for new bpf_redirect helper")
> Signed-off-by: Wang Hai <wanghai38@...wei.com>
> ---
>   samples/bpf/xdp_redirect_user.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/samples/bpf/xdp_redirect_user.c b/samples/bpf/xdp_redirect_user.c
> index 41d705c3a1f7..c903f1ccc15e 100644
> --- a/samples/bpf/xdp_redirect_user.c
> +++ b/samples/bpf/xdp_redirect_user.c
> @@ -213,5 +213,5 @@ int main(int argc, char **argv)
>   	poll_stats(2, ifindex_out);
>   
>   out:
> -	return 0;
> +	return ret;
>   }


(Sorry, I didn't complain it time as I see this patch is already applied)