bpf: Add support to specify uprobe_multi target via file descriptor

Allow uprobe_multi link to identify the target binary by an already
opened file descriptor.

Adding new BPF_F_UPROBE_MULTI_PATH_FD flag and the path_fd field for
the attr.link_create.uprobe_multi struct.

When the flag is set, we resolve the target from path_fd, without the
flag, we keep the existing string path behavior.

I don't see a use case for supporting O_PATH file descriptors, because
we need to read the binary first to get probes offsets, so I'm using
the CLASS(fd, f), which fails for O_PATH fds.

Assisted-by: Codex:GPT-5.4
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260611114230.950379-4-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -1327,7 +1327,11 @@ enum {
  * BPF_TRACE_UPROBE_MULTI attach type to create return probe.
  */
 enum {
-	BPF_F_UPROBE_MULTI_RETURN = (1U << 0)
+	/* Get return uprobe. */
+	BPF_F_UPROBE_MULTI_RETURN     = (1U << 0),
+
+	/* Get path from provided path_fd. */
+	BPF_F_UPROBE_MULTI_PATH_FD    = (1U << 1),
 };
 
 /* link_create.netfilter.flags used in LINK_CREATE command for
@@ -1864,6 +1868,7 @@ union bpf_attr {
 				__u32		cnt;
 				__u32		flags;
 				__u32		pid;
+				__u32		path_fd;
 			} uprobe_multi;
 			struct {
 				union {