Merge tag 'sched_ext-for-7.3-rc4-fixes-2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

Pull sched_ext fix from Tejun Heo:

 - The CPU topology helper for BPF schedulers took no buffer size, so
   its structure couldn't grow without breaking schedulers built against
   the older layout. Add a size argument.

* tag 'sched_ext-for-7.3-rc4-fixes-2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext:
  sched_ext: Add a size argument to scx_bpf_cid_topo() so struct scx_cid_topo can grow

2 files changed, 20 insertions, 10 deletions

diff --git a/kernel/sched/ext/cid.c b/kernel/sched/ext/cid.c
index bc4eee5bb4cb..4b08866d75f6 100644
--- a/kernel/sched/ext/cid.c
+++ b/kernel/sched/ext/cid.c
@@ -912,30 +912,36 @@ bool scx_cmask_empty(const struct scx_cmask *m)
 /**
  * scx_bpf_cid_topo - Copy out per-cid topology info
  * @cid: cid to look up
- * @out__uninit: where to copy the topology info; fully written by this call
+ * @out: where to copy the topology info
+ * @out__sz: size of @out, the program's sizeof(struct scx_cid_topo)
  * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
  *
- * Fill @out__uninit with the topology info for @cid. Trigger scx_error() if
- * @cid is out of range. If @cid is valid but in the no-topo section, all fields
- * are set to -1. All fields are also set to -1 when no cid tables have been
- * published yet, which a program may observe while racing the root enable.
+ * Fill @out with the topology info for @cid. Trigger scx_error() if @cid is out
+ * of range. If @cid is valid but in the no-topo section, all fields are set to
+ * -1. All fields are also set to -1 when no cid tables have been published yet,
+ * which a program may observe while racing the root enable.
+ *
+ * The program's struct may be older or newer than the kernel's. The smaller of
+ * @out__sz and the kernel's size is copied and the rest of @out is set to -1.
  */
-__bpf_kfunc void scx_bpf_cid_topo(s32 cid, struct scx_cid_topo *out__uninit,
+__bpf_kfunc void scx_bpf_cid_topo(s32 cid, struct scx_cid_topo *out, size_t out__sz,
 				  const struct bpf_prog_aux *aux)
 {
+	size_t len = min(out__sz, sizeof(*out));
 	struct scx_cid_topo *topo;
 	struct scx_sched *sch;
 
+	/* the error cases and fields the kernel lacks read as -1 */
+	memset(out, 0xff, out__sz);
+
 	guard(rcu)();
 
 	sch = scx_prog_sched(aux);
 	topo = rcu_dereference(scx_cid_topo);
-	if (unlikely(!sch) || !cid_valid(sch, cid) || unlikely(!topo)) {
-		*out__uninit = SCX_CID_TOPO_NEG;
+	if (unlikely(!sch) || !cid_valid(sch, cid) || unlikely(!topo))
 		return;
-	}
 
-	*out__uninit = topo[cid];
+	memcpy(out, &topo[cid], len);
 }
 
 __bpf_kfunc_end_defs();
diff --git a/kernel/sched/ext/types.h b/kernel/sched/ext/types.h
index 943d8d429a2c..139176cf9fc6 100644
--- a/kernel/sched/ext/types.h
+++ b/kernel/sched/ext/types.h
@@ -70,6 +70,10 @@ enum scx_consts {
  * smaller shards if the LLC exceeds the target size. No-topo cids are packed
  * into their own max-sized shards.
  *
+ * New fields are appended, never inserted: scx_bpf_cid_topo() copies this
+ * struct out sized by the program's own layout, and an older program's copy
+ * must stay a prefix of the kernel's.
+ *
  * @core_cid: first cid of this cid's core (smt-sibling group)
  * @core_idx: global index of that core, in [0, nr_cores_at_init)
  * @llc_cid: first cid of this cid's LLC