selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 46 insertions, 0 deletions

diff --git a/arch/hexagon/lib/modsi3.S b/arch/hexagon/lib/modsi3.S
new file mode 100644
index 000000000..9ea1c86ef
--- /dev/null
+++ b/arch/hexagon/lib/modsi3.S
@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2021, The Linux Foundation. All rights reserved.
+ */
+
+#include <linux/linkage.h>
+
+SYM_FUNC_START(__hexagon_modsi3)
+        {
+                p2 = cmp.ge(r0,#0)
+                r2 = abs(r0)
+                r1 = abs(r1)
+        }
+        {
+                r3 = cl0(r2)
+                r4 = cl0(r1)
+                p0 = cmp.gtu(r1,r2)
+        }
+        {
+                r3 = sub(r4,r3)
+                if (p0) jumpr r31
+        }
+        {
+                p1 = cmp.eq(r3,#0)
+                loop0(1f,r3)
+                r0 = r2
+                r2 = lsl(r1,r3)
+        }
+        .falign
+1:
+        {
+                p0 = cmp.gtu(r2,r0)
+                if (!p0.new) r0 = sub(r0,r2)
+                r2 = lsr(r2,#1)
+                if (p1) r1 = #0
+        }:endloop0
+        {
+                p0 = cmp.gtu(r2,r0)
+                if (!p0.new) r0 = sub(r0,r1)
+                if (p2) jumpr r31
+        }
+        {
+                r0 = neg(r0)
+                jumpr r31
+        }
+SYM_FUNC_END(__hexagon_modsi3)