netdev - Re: [PATCH bpf-next] libbpf: handle ENOTSUPP errno in libbpf

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Date:   Tue, 27 Apr 2021 18:18:27 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Pedro Tammela <pctammela@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH bpf-next] libbpf: handle ENOTSUPP errno in
 libbpf_strerror()

On 4/25/21 12:16 AM, Pedro Tammela wrote:
> The 'bpf()' syscall is leaking the ENOTSUPP errno that is internal to the kernel[1].
> More recent code is already using the correct EOPNOTSUPP, but changing
> older return codes is not possible due to dependency concerns, so handle ENOTSUPP
> in libbpf_strerror().
> 
> [1] https://lore.kernel.org/netdev/20200511165319.2251678-1-kuba@kernel.org/
> 
> Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
> ---
>   tools/lib/bpf/libbpf_errno.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf_errno.c b/tools/lib/bpf/libbpf_errno.c
> index 0afb51f7a919..7de8bbc34a37 100644
> --- a/tools/lib/bpf/libbpf_errno.c
> +++ b/tools/lib/bpf/libbpf_errno.c
> @@ -13,6 +13,9 @@
>   
>   #include "libbpf.h"
>   
> +/* This errno is internal to the kernel but leaks in the bpf() syscall. */
> +#define ENOTSUPP 524
> +
>   /* make sure libbpf doesn't use kernel-only integer typedefs */
>   #pragma GCC poison u8 u16 u32 u64 s8 s16 s32 s64
>   
> @@ -43,6 +46,12 @@ int libbpf_strerror(int err, char *buf, size_t size)
>   
>   	err = err > 0 ? err : -err;
>   
> +	if (err == ENOTSUPP) {
> +		snprintf(buf, size, "Operation not supported");
> +		buf[size - 1] = '\0';
> +		return 0;
> +	}
> +
>   	if (err < __LIBBPF_ERRNO__START) {
>   		int ret;

Could you fold this into the __LIBBPF_ERRNO__START test body to denote that it
belongs outside the libbpf error range? For example, could be simplified like this:

         if (err < __LIBBPF_ERRNO__START) {
                 int ret;

                 /* Handle ENOTSUPP separate here given it's kernel internal,
                  * but for sake of error string it has the same meaning as
                  * the EOPNOTSUPP error.
                  */
                 if (err == ENOTSUPP)
                         err = EOPNOTSUPP;
                 ret = strerror_r(err, buf, size);
                 buf[size - 1] = '\0';
                 return ret;
         }

Thanks,
Daniel