On Tue, Aug 25, 2020 at 6:39 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> I was playing around a bit, and noticed that trying to acquire an
> exclusive POSIX record lock on a bpf_link fd fails. I've traced this
> to the call to anon_inode_getfile from bpf_link_prime which
> effectively specifies O_RDONLY on the bpf_link struct file. This makes
> check_fmode_for_setlk return EBADF.
>
> This means the following:
> * flock(link, LOCK_EX): works
> * fcntl(link, SETLK, F_RDLCK): works
> * fcntl(link, SETLK, F_WRLCK): doesn't work
>
> Especially the discrepancy between flock(EX) and fcntl(WRLCK) has me
> puzzled. Should fcntl(WRLCK) work on a link?
>
> program fds are always O_RDWR as far as I can tell (so all locks
> work), while maps depend on map_flags.
Because for links fd/file flags are reserved for the future use.
progs are rdwr for historical reasons while maps can have three combinations:
/* Flags for accessing BPF object from syscall side. */
BPF_F_RDONLY = (1U << 3),
BPF_F_WRONLY = (1U << 4),
by default they are rdwr.
What is your use case to use flock on bpf_link fd?
