On Wed, 26 Aug 2020 at 10:22, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Lorenz Bauer <lmb@xxxxxxxxxxxxxx> writes: > > > On Tue, 25 Aug 2020 at 19:06, Alexei Starovoitov > > <alexei.starovoitov@xxxxxxxxx> wrote: > >> > >> 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? > > > > The idea is to prevent concurrent access / modification of pinned maps > > + pinned link from a command line tool. I could just as well lock one > > of the maps for this, but conceptually the link is the thing that > > actually controls what maps are used via the attached BPF program. > > FWIW I'm using flock(EX) on the link for now, which is fine for my use > > case. I just thought I'd raise this in case it was an oversight :) > > FWIW I'm doing something similar in libxdp, except I'm using flock(EX) > on the parent directory (i.e., /sys/fs/bpf/xdp) since I need to protect > multiple modifications inside it: Thank you for the suggestion, that is indeed much nicer! Now why did I bother with fcntl in the first place? :) Best -- Lorenz Bauer | Systems Engineer 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK www.cloudflare.com
