bpf: fix next_id decoding for BPF_*_GET_NEXT_ID

It is an R/W attribute and is supposed to be handled as such.

* src/bpf.c (BEGIN_BPF_CMD_DECODER(BPF_PROG_GET_NEXT_ID)): Store next_id
field value on entering, print the new value (if it has been modified)
on exiting.
* NEWS: Mention it.

2 files changed, 19 insertions, 4 deletions

diff --git a/NEWS b/NEWS
index 1a5b47fe2..7a55a37fd 100644
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,8 @@ Noteworthy changes in release ?.?? (????-??-??)
   * Updated lists of ioctl commands from Linux 5.18.
 
 * Bug fixes
+  * Fixed printing of the updated value of union bpf_attr.next_id
+    on the exiting of bpf(BPF_*_GET_NEXT_ID) calls.
 
 Noteworthy changes in release 5.17 (2022-03-26)
 ===============================================
diff --git a/src/bpf.c b/src/bpf.c
index c9b4161ce..2fef61686 100644
--- a/src/bpf.c
+++ b/src/bpf.c
@@ -537,10 +537,23 @@ END_BPF_CMD_DECODER(RVAL_DECODED)
 
 BEGIN_BPF_CMD_DECODER(BPF_PROG_GET_NEXT_ID)
 {
-	tprint_struct_begin();
-	PRINT_FIELD_U(attr, start_id);
-	tprint_struct_next();
-	PRINT_FIELD_U(attr, next_id);
+	if (entering(tcp)) {
+		set_tcb_priv_ulong(tcp, attr.next_id);
+
+		tprint_struct_begin();
+		PRINT_FIELD_U(attr, start_id);
+		tprint_struct_next();
+		PRINT_FIELD_U(attr, next_id);
+
+		return 0;
+	}
+
+	uint32_t saved_next_id = get_tcb_priv_ulong(tcp);
+
+	if (saved_next_id != attr.next_id) {
+		tprint_value_changed();
+		PRINT_VAL_U(attr.next_id);
+	}
 
 	/* open_flags field has been added in Linux v4.15-rc1~84^2~384^2~4 */
 	if (len <= offsetof(struct BPF_PROG_GET_NEXT_ID_struct, open_flags))