[RFC PATCH bpf-next] bpf: Check percpu map value size first

[Tao Chen]

Percpu map is often used, but the map value size limit often ignored,
like issue: https://github.com/iovisor/bcc/issues/2519. Actually,
percpu map value size is bound by PCPU_MIN_UNIT_SZIE, so we
can check the value size whether it exceeds PCPU_MIN_UNIT_SZIE first,
like percpu map of local_storage. Maybe the error message seems clearer
compared with "cannot allocate memory".

the test case we create a percpu map with large value like:
struct test_t {
        int a[100000];
};
struct {
        __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
        __uint(max_entries, 1);
        __type(key, u32);
        __type(value, struct test_t);
} start SEC(".maps");

test on ubuntu24.04 vm:
libbpf: Error in bpf_create_map_xattr(start):Argument list too long(-7).
Retrying without BTF.

Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
---
 kernel/bpf/arraymap.c | 3 +++
 kernel/bpf/hashtab.c  | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index a43e62e2a8bb..7c3c32f156ff 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -73,6 +73,9 @@ int array_map_alloc_check(union bpf_attr *attr)
 	/* avoid overflow on round_up(map->value_size) */
 	if (attr->value_size > INT_MAX)
 		return -E2BIG;
+	/* percpu map value size is bound by PCPU_MIN_UNIT_SIZE */
+	if (percpu && attr->value_size > PCPU_MIN_UNIT_SIZE)
+		return -E2BIG;
 
 	return 0;
 }
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 45c7195b65ba..16d590fe1ffb 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -462,6 +462,9 @@ static int htab_map_alloc_check(union bpf_attr *attr)
 		 * kmalloc-able later in htab_map_update_elem()
 		 */
 		return -E2BIG;
+	/* percpu map value size is bound by PCPU_MIN_UNIT_SIZE */
+	if (percpu && attr->value_size > PCPU_MIN_UNIT_SIZE)
+		return -E2BIG;
 
 	return 0;
 }
-- 
2.25.1