On Mon, Jun 8, 2020 at 9:22 AM Lorenz Bauer lmb@cloudflare.com wrote:
When using BPF_PROG_ATTACH to attach a program to a cgroup in BPF_F_ALLOW_MULTI mode, it is not possible to replace a program with itself. This is because the check for duplicate programs doesn't take the replacement program into account.
Replacing a program with itself might seem weird, but it has some uses: first, it allows resetting the associated cgroup storage. Second, it makes the API consistent with the non-ALLOW_MULTI usage, where it is possible to replace a program with itself. Third, it aligns BPF_PROG_ATTACH with bpf_link, where replacing itself is also supported.
Sice this code has been refactored a few times this change will only apply to v5.7 and later. Adjustments could be made to commit 1020c1f24a94 ("bpf: Simplify __cgroup_bpf_attach") and commit d7bf2c10af05 ("bpf: allocate cgroup storage entries on attaching bpf programs") as well as commit 324bda9e6c5a ("bpf: multi program support for cgroup+bpf")
Signed-off-by: Lorenz Bauer lmb@cloudflare.com Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment")
Applied. Thanks