linux-kernel - [PATCH] libbpf: fix the name of a reused map

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Date:   Mon, 11 Jul 2022 18:25:21 +0800
From:   "anquan.wu" <leiqi96@...mail.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, john.fastabend@...il.com
Cc:     linux-kernel@...r.kernel.org, "anquan.wu" <leiqi96@...mail.com>
Subject: [PATCH] libbpf: fix the name of a reused map

BPF map name was limited to BPF_OBJ_NAME_LEN.
If a map name is defined as being longer than BPF_OBJ_NAME_LEN,
it will be truncated to BPF_OBJ_NAME_LEN
when a userspace program calls libbpf to create the map.
A pinned map also generates a path in the /sys.
If the previous program wanted to reuse the map，it can not get bpf_map
by name, because the name of the map is only partially the same as
the name which get from pinned path.

The syscall information below show that map name
"process_pinned_map" is truncated to process_pinned_"

bpf(BPF_OBJ_GET, {pathname="/sys/fs/bpf/process_pinned_map",
bpf_fd=0, file_flags=0}, 144) = -1 ENOENT (No such file or directory)

bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_HASH, key_size=4, value_size=4,
max_entries=1024, map_flags=0, inner_map_fd=0, map_name="process_pinned_",
map_ifindex=0, btf_fd=3, btf_key_type_id=6, btf_value_type_id=10,
btf_vmlinux_value_type_id=0}, 72) = 4

This patch check that if the name of pinned map are the same as the
actual name for the first (BPF_OBJ_NAME_LEN - 1),
bpf map still uses the name which is included in bpf object.

Signed-off-by: anquan.wu <leiqi96@...mail.com>
---
 tools/lib/bpf/libbpf.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e89cc9c885b3..5ad52a8accd1 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4328,6 +4328,7 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
 {
 	struct bpf_map_info info = {};
 	__u32 len = sizeof(info);
+	__u32 name_len;
 	int new_fd, err;
 	char *new_name;
 
@@ -4337,7 +4338,12 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
 	if (err)
 		return libbpf_err(err);
 
-	new_name = strdup(info.name);
+	name_len = strlen(info.name);
+	if ((BPF_OBJ_NAME_LEN - 1) == name_len && !strncmp(map->name, info.name, name_len))
+		new_name = strdup(map->name);
+	else
+		new_name = strdup(info.name);
+
 	if (!new_name)
 		return libbpf_err(-errno);
 
-- 
2.32.0