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bpf: cgroup: Allow multi-attach program to replace itself

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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 1020c1f24a ("bpf: Simplify __cgroup_bpf_attach") and
commit d7bf2c10af ("bpf: allocate cgroup storage entries on attaching bpf programs")
as well as commit 324bda9e6c ("bpf: multi program support for cgroup+bpf")

Fixes: af6eea5743 ("bpf: Implement bpf_link-based cgroup BPF program attachment")
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200608162202.94002-1-lmb@cloudflare.com
This commit is contained in:
Lorenz Bauer 2020-06-08 17:22:01 +01:00 committed by Alexei Starovoitov
parent 26afa0a4eb
commit 248e00ac47
2 changed files with 8 additions and 1 deletions
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2
kernel/bpf/cgroup.c
View File

@ -378,7 +378,7 @@ static struct bpf_prog_list *find_attach_entry(struct list_head *progs,
}
list_for_each_entry(pl, progs, node) {
if (prog && pl->prog == prog)
if (prog && pl->prog == prog && prog != replace_prog)
/* disallow attaching the same prog twice */
return ERR_PTR(-EINVAL);
if (link && pl->link == link)

7
tools/testing/selftests/bpf/prog_tests/cgroup_attach_multi.c
View File

@ -230,6 +230,13 @@ void test_cgroup_attach_multi(void)
"prog_replace", "errno=%d\n", errno))
goto err;
/* replace program with itself */
attach_opts.replace_prog_fd = allow_prog[6];
if (CHECK(bpf_prog_attach_xattr(allow_prog[6], cg1,
BPF_CGROUP_INET_EGRESS, &attach_opts),
"prog_replace", "errno=%d\n", errno))
goto err;
value = 0;
CHECK_FAIL(bpf_map_update_elem(map_fd, &key, &value, 0));
CHECK_FAIL(system(PING_CMD));