On Mon, Nov 30, 2020 at 7:42 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> The kernel feature probing results in 'errno' being set if the probing
> fails (as is often the case). This can stick around and leak to the caller,
> which can lead to confusion later. So let's make sure we always reset errno
> after calling a probe function.
What specifically is the problem and what sort of confusion we are
talking about here? You are not supposed to check errno, unless the
function returned -1 or other error result.
In some cases, you have to reset errno manually just to avoid
confusion (see how strtol() is used, as an example).
I.e., I don't see the problem here, any printf() technically can set
errno to <0, we don't reset errno after each printf call though,
right?
>
> Fixes: 47b6cb4d0add ("libbpf: Make kernel feature probing lazy")
> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 28baee7ba1ca..8d05132e1945 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4021,6 +4021,8 @@ static bool kernel_supports(enum kern_feature_id feat_id)
> pr_warn("Detection of kernel %s support failed: %d\n", feat->desc, ret);
> WRITE_ONCE(feat->res, FEAT_MISSING);
> }
> + /* reset errno after probing to prevent leaking it to caller */
> + errno = 0;
> }
>
> return READ_ONCE(feat->res) == FEAT_SUPPORTED;
> --
> 2.29.2
>
