bpf: read proc exe at tracepoint/sys_execve

This is needed for matchBinaries in the next commit. Previously we were
using the tracepoint argugement filename, which is potentially a
relative path.  This change has two main benefits: all paths checked
against matchBinaries will be absolutes and all symbolic links will be
resolved by the kernel.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>

bpf/lib/bpf_task.h

@@ -10,6 +10,9 @@
 
 /* __d_path_local flags */
 // #define UNRESOLVED_MOUNT_POINTS	   0x01 // (deprecated)
+// this error is returned by __d_path_local in the following cases:
+// - the path walk did not conclude (too many dentry)
+// - the path was too long to fit in the buffer
 #define UNRESOLVED_PATH_COMPONENTS 0x02
 
 #ifdef __LARGE_BPF_PROG

bpf/lib/process.h

@@ -270,6 +270,19 @@ struct msg_k8s {
 	char docker_id[DOCKER_ID_LENGTH];
 }; // All fields aligned so no 'packed' attribute.
 
+#define BINARY_PATH_MAX_LEN 256
+
+struct heap_exe {
+	// because of verifier limitations, this has to be 2 * 256 bytes while 256
+	// should be theoretically sufficient, and actually is, in unit tests.
+	char buf[BINARY_PATH_MAX_LEN * 2];
+	// offset points to the start of the path in the above buffer. Use offset to
+	// read the path in the buffer since it's written from the end.
+	char *off;
+	__u32 len;
+	__u32 error;
+}; // All fields aligned so no 'packed' attribute.
+
 struct msg_execve_event {
 	struct msg_common common;
 	struct msg_k8s kube;
@@ -289,10 +302,9 @@ struct msg_execve_event {
 	/* below fields are not part of the event, serve just as
 	 * heap for execve programs
 	 */
+	struct heap_exe exe;
 }; // All fields aligned so no 'packed' attribute.
 
-#define BINARY_PATH_MAX_LEN 256
-
 // This structure stores the binary path that was recorded on execve.
 // Technically PATH_MAX is 4096 but we limit the length we store since we have
 // limits on the length of the string to compare:

bpf/process/bpf_execve_event.c

@@ -144,6 +144,21 @@ read_execve_shared_info(void *ctx, __u64 pid)
 	return secureexec;
 }
 
+static inline __attribute__((always_inline)) __u32
+read_exe(struct task_struct *task, struct heap_exe *exe)
+{
+	struct file *file = BPF_CORE_READ(task, mm, exe_file);
+	struct path *path = __builtin_preserve_access_index(&file->f_path);
+
+	exe->len = BINARY_PATH_MAX_LEN;
+	exe->off = (char *)&exe->buf;
+	exe->off = __d_path_local(path, exe->off, (int *)&exe->len, (int *)&exe->error);
+	if (exe->len > 0)
+		exe->len = BINARY_PATH_MAX_LEN - exe->len;
+
+	return exe->len;
+}
+
 __attribute__((section("tracepoint/sys_execve"), used)) int
 event_execve(struct sched_execve_args *ctx)
 {
@@ -184,6 +199,11 @@ event_execve(struct sched_execve_args *ctx)
 	p->auid = get_auid();
 	p->uid = get_current_uid_gid();
 
+	// Reading the absolute path of the process exe for matchBinaries.
+	// Historically we used the filename, a potentially relative path (maybe to
+	// a symlink) coming from the execve tracepoint.
+	read_exe(task, &event->exe);
+
 	p->size += read_path(ctx, event, filename);
 	p->size += read_args(ctx, event);
 	p->size += read_cwd(ctx, p);