BACKPORT: bpf: introduce BPF_F_ALLOW_OVERRIDE flag

If BPF_F_ALLOW_OVERRIDE flag is used in BPF_PROG_ATTACH command
to the given cgroup the descendent cgroup will be able to override
effective bpf program that was inherited from this cgroup.
By default it's not passed, therefore override is disallowed.

Examples:
1.
prog X attached to /A with default
prog Y fails to attach to /A/B and /A/B/C
Everything under /A runs prog X

2.
prog X attached to /A with allow_override.
prog Y fails to attach to /A/B with default (non-override)
prog M attached to /A/B with allow_override.
Everything under /A/B runs prog M only.

3.
prog X attached to /A with allow_override.
prog Y fails to attach to /A with default.
The user has to detach first to switch the mode.

In the future this behavior may be extended with a chain of
non-overridable programs.

Also fix the bug where detach from cgroup where nothing is attached
was not throwing error. Return ENOENT in such case.

Add several testcases and adjust libbpf.

Fixes: 3007098494be ("cgroup: add support for eBPF programs")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Fixes: Change-Id: I3df35d8d3b1261503f9b5bcd90b18c9358f1ac28
       ("cgroup: add support for eBPF programs")
[AmitP: Refactored original patch for android-4.9 where libbpf sources
        are in samples/bpf/ and test_cgrp2_attach2, test_cgrp2_sock,
        and test_cgrp2_sock2 sample tests do not exist.]
(cherry picked from commit 7f677633379b4abb3281cdbe7e7006f049305c03)
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Chatur27 <jasonbright2709@gmail.com>

3 files changed, 6 insertions, 3 deletions

diff --git a/samples/bpf/libbpf.c b/samples/bpf/libbpf.c
index 9ce707bf02a7..9cbc786e48e5 100644
--- a/samples/bpf/libbpf.c
+++ b/samples/bpf/libbpf.c
@@ -104,12 +104,14 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 	return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
 }
 
-int bpf_prog_attach(int prog_fd, int target_fd, enum bpf_attach_type type)
+int bpf_prog_attach(int prog_fd, int target_fd, enum bpf_attach_type type,
+		    unsigned int flags)
 {
 	union bpf_attr attr = {
 		.target_fd = target_fd,
 		.attach_bpf_fd = prog_fd,
 		.attach_type = type,
+		.attach_flags  = flags;
 	};
 
 	return syscall(__NR_bpf, BPF_PROG_ATTACH, &attr, sizeof(attr));
diff --git a/samples/bpf/libbpf.h b/samples/bpf/libbpf.h
index d0a799a52eaf..b06cf5aea097 100644
--- a/samples/bpf/libbpf.h
+++ b/samples/bpf/libbpf.h
@@ -15,7 +15,8 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 		  const struct bpf_insn *insns, int insn_len,
 		  const char *license, int kern_version);
 
-int bpf_prog_attach(int prog_fd, int attachable_fd, enum bpf_attach_type type);
+int bpf_prog_attach(int prog_fd, int attachable_fd, enum bpf_attach_type type,
+		    unsigned int flags);
 int bpf_prog_detach(int attachable_fd, enum bpf_attach_type type);
 
 int bpf_obj_pin(int fd, const char *pathname);
diff --git a/samples/bpf/test_cgrp2_attach.c b/samples/bpf/test_cgrp2_attach.c
index 63ef2083f766..9de4896edeef 100644
--- a/samples/bpf/test_cgrp2_attach.c
+++ b/samples/bpf/test_cgrp2_attach.c
@@ -124,7 +124,7 @@ int main(int argc, char **argv)
 	ret = bpf_prog_detach(cg_fd, type);
 	printf("bpf_prog_detach() returned '%s' (%d)\n", strerror(errno), errno);
 
-	ret = bpf_prog_attach(prog_fd, cg_fd, type);
+	ret = bpf_prog_attach(prog_fd, cg_fd, type, 0);
 	if (ret < 0) {
 		printf("Failed to attach prog to cgroup: '%s'\n",
 		       strerror(errno));