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, 80 insertions, 0 deletions

diff --git a/fs/nfs/symlink.c b/fs/nfs/symlink.c
new file mode 100644
index 000000000..74a072896
--- /dev/null
+++ b/fs/nfs/symlink.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  linux/fs/nfs/symlink.c
+ *
+ *  Copyright (C) 1992  Rick Sladkey
+ *
+ *  Optimization changes Copyright (C) 1994 Florian La Roche
+ *
+ *  Jun 7 1999, cache symlink lookups in the page cache.  -DaveM
+ *
+ *  nfs symlink handling code
+ */
+
+#include <linux/time.h>
+#include <linux/errno.h>
+#include <linux/sunrpc/clnt.h>
+#include <linux/nfs.h>
+#include <linux/nfs2.h>
+#include <linux/nfs_fs.h>
+#include <linux/pagemap.h>
+#include <linux/stat.h>
+#include <linux/mm.h>
+#include <linux/string.h>
+
+#include "internal.h"
+
+/* Symlink caching in the page cache is even more simplistic
+ * and straight-forward than readdir caching.
+ */
+
+static int nfs_symlink_filler(struct file *file, struct folio *folio)
+{
+	struct inode *inode = folio->mapping->host;
+	int error;
+
+	error = NFS_PROTO(inode)->readlink(inode, &folio->page, 0, PAGE_SIZE);
+	folio_end_read(folio, error == 0);
+	return error;
+}
+
+static const char *nfs_get_link(struct dentry *dentry,
+				struct inode *inode,
+				struct delayed_call *done)
+{
+	struct folio *folio;
+	void *err;
+
+	if (!dentry) {
+		err = ERR_PTR(nfs_revalidate_mapping_rcu(inode));
+		if (err)
+			return err;
+		folio = filemap_get_folio(inode->i_mapping, 0);
+		if (IS_ERR(folio))
+			return ERR_PTR(-ECHILD);
+		if (!folio_test_uptodate(folio)) {
+			folio_put(folio);
+			return ERR_PTR(-ECHILD);
+		}
+	} else {
+		err = ERR_PTR(nfs_revalidate_mapping(inode, inode->i_mapping));
+		if (err)
+			return err;
+		folio = read_cache_folio(&inode->i_data, 0, nfs_symlink_filler,
+				NULL);
+		if (IS_ERR(folio))
+			return ERR_CAST(folio);
+	}
+	set_delayed_call(done, page_put_link, folio);
+	return folio_address(folio);
+}
+
+/*
+ * symlinks can't do much...
+ */
+const struct inode_operations nfs_symlink_inode_operations = {
+	.get_link	= nfs_get_link,
+	.getattr	= nfs_getattr,
+	.setattr	= nfs_setattr,
+	.fileattr_get	= nfs_fileattr_get,
+};