bpf: Use array_map_meta_equal for percpu array inner map replacement

percpu_array_map_ops.map_meta_equal points to the generic
bpf_map_meta_equal(), which does not compare max_entries.  When a
percpu array serves as an inner map, replacing it with one that has
fewer max_entries bypasses the check.  Since percpu_array_map_gen_lookup()
inlines the original template's index_mask as a JIT immediate, a lookup
on the replacement map can access pptrs[] out of bounds.

Point percpu_array_map_ops.map_meta_equal to array_map_meta_equal(),
which already enforces the max_entries equality check.

Add a selftest to verify that replacing a percpu array inner map with
a differently-sized one is rejected.

Fixes: db69718 ("bpf: inline bpf_map_lookup_elem() for PERCPU_ARRAY maps")
Signed-off-by: Guannan Wang <wgnbuaa@gmail.com>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260514074454.77491-1-wgnbuaa@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: keenanwgn

kernel/bpf/arraymap.c

@@ -827,7 +827,7 @@ const struct bpf_map_ops array_map_ops = {
 };
 
 const struct bpf_map_ops percpu_array_map_ops = {
-	.map_meta_equal = bpf_map_meta_equal,
+	.map_meta_equal = array_map_meta_equal,
 	.map_alloc_check = array_map_alloc_check,
 	.map_alloc = array_map_alloc,
 	.map_free = array_map_free,

tools/testing/selftests/bpf/prog_tests/percpu_array_inner_map.c

@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+/*
+ * Test that replacing an inner percpu array map with one that has different
+ * max_entries is rejected.  percpu_array_map_gen_lookup() inlines the
+ * template's index_mask, so allowing a smaller replacement would cause OOB.
+ */
+void test_percpu_array_inner_map(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts);
+	int outer_fd, tmpl_fd, good_fd, bad_fd, err;
+	int zero = 0;
+
+	/* Create template: percpu array with 8 entries */
+	tmpl_fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "tmpl",
+				 sizeof(int), sizeof(long), 8, NULL);
+	if (!ASSERT_OK_FD(tmpl_fd, "create_tmpl"))
+		return;
+
+	/* Create outer array-of-maps using template */
+	opts.inner_map_fd = tmpl_fd;
+	outer_fd = bpf_map_create(BPF_MAP_TYPE_ARRAY_OF_MAPS, "outer",
+				  sizeof(int), sizeof(int), 1, &opts);
+	if (!ASSERT_OK_FD(outer_fd, "create_outer"))
+		goto close_tmpl;
+
+	/* Insert template as initial inner map */
+	err = bpf_map_update_elem(outer_fd, &zero, &tmpl_fd, 0);
+	if (!ASSERT_OK(err, "insert_tmpl"))
+		goto close_outer;
+
+	/* Replacement with same max_entries should succeed */
+	good_fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "good",
+				 sizeof(int), sizeof(long), 8, NULL);
+	if (!ASSERT_OK_FD(good_fd, "create_good"))
+		goto close_outer;
+
+	err = bpf_map_update_elem(outer_fd, &zero, &good_fd, 0);
+	ASSERT_OK(err, "replace_same_max_entries");
+	close(good_fd);
+
+	/* Replacement with fewer max_entries must fail */
+	bad_fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "bad",
+				sizeof(int), sizeof(long), 2, NULL);
+	if (!ASSERT_OK_FD(bad_fd, "create_bad"))
+		goto close_outer;
+
+	err = bpf_map_update_elem(outer_fd, &zero, &bad_fd, 0);
+	ASSERT_ERR(err, "replace_smaller_max_entries");
+	close(bad_fd);
+
+close_outer:
+	close(outer_fd);
+close_tmpl:
+	close(tmpl_fd);
+}