libbpf: Wire up BPF token support at BPF object level

Add BPF token support to BPF object-level functionality.

BPF token is supported by BPF object logic either as an explicitly
provided BPF token from outside (through BPF FS path), or implicitly
(unless prevented through bpf_object_open_opts).

Implicit mode is assumed to be the most common one for user namespaced
unprivileged workloads. The assumption is that privileged container
manager sets up default BPF FS mount point at /sys/fs/bpf with BPF token
delegation options (delegate_{cmds,maps,progs,attachs} mount options).
BPF object during loading will attempt to create BPF token from
/sys/fs/bpf location, and pass it for all relevant operations
(currently, map creation, BTF load, and program load).

In this implicit mode, if BPF token creation fails due to whatever
reason (BPF FS is not mounted, or kernel doesn't support BPF token,
etc), this is not considered an error. BPF object loading sequence will
proceed with no BPF token.

In explicit BPF token mode, user provides explicitly custom BPF FS mount
point path. In such case, BPF object will attempt to create BPF token
from provided BPF FS location. If BPF token creation fails, that is
considered a critical error and BPF object load fails with an error.

Libbpf provides a way to disable implicit BPF token creation, if it
causes any troubles (BPF token is designed to be completely optional and
shouldn't cause any problems even if provided, but in the world of BPF
LSM, custom security logic can be installed that might change outcome
depending on the presence of BPF token). To disable libbpf's default BPF
token creation behavior user should provide either invalid BPF token FD
(negative), or empty bpf_token_path option.

BPF token presence can influence libbpf's feature probing, so if BPF
object has associated BPF token, feature probing is instructed to use
BPF object-specific feature detection cache and token FD.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20240124022127.2379740-26-andrii@kernel.org

1 files changed, 12 insertions, 1 deletions

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 6cd9c501624f..535ae15ed493 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -177,10 +177,21 @@ struct bpf_object_open_opts {
 	 * logs through its print callback.
 	 */
 	__u32 kernel_log_level;
+	/* Path to BPF FS mount point to derive BPF token from.
+	 *
+	 * Created BPF token will be used for all bpf() syscall operations
+	 * that accept BPF token (e.g., map creation, BTF and program loads,
+	 * etc) automatically within instantiated BPF object.
+	 *
+	 * Setting bpf_token_path option to empty string disables libbpf's
+	 * automatic attempt to create BPF token from default BPF FS mount
+	 * point (/sys/fs/bpf), in case this default behavior is undesirable.
+	 */
+	const char *bpf_token_path;
 
 	size_t :0;
 };
-#define bpf_object_open_opts__last_field kernel_log_level
+#define bpf_object_open_opts__last_field bpf_token_path
 
 /**
  * @brief **bpf_object__open()** creates a bpf_object by opening