On Tue, Sep 15, 2020 at 5:44 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> Running selftest
> ./btf_btf -p
> the kernel had the following warning:
> [ 51.528185] WARNING: CPU: 3 PID: 1756 at kernel/bpf/hashtab.c:717 htab_map_get_next_key+0x2eb/0x300
> [ 51.529217] Modules linked in:
> [ 51.529583] CPU: 3 PID: 1756 Comm: test_btf Not tainted 5.9.0-rc1+ #878
> [ 51.530346] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.el7.centos 04/01/2014
> [ 51.531410] RIP: 0010:htab_map_get_next_key+0x2eb/0x300
> ...
> [ 51.542826] Call Trace:
> [ 51.543119] map_seq_next+0x53/0x80
> [ 51.543528] seq_read+0x263/0x400
> [ 51.543932] vfs_read+0xad/0x1c0
> [ 51.544311] ksys_read+0x5f/0xe0
> [ 51.544689] do_syscall_64+0x33/0x40
> [ 51.545116] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> The related source code in kernel/bpf/hashtab.c:
> 709 static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> 710 {
> 711 struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
> 712 struct hlist_nulls_head *head;
> 713 struct htab_elem *l, *next_l;
> 714 u32 hash, key_size;
> 715 int i = 0;
> 716
> 717 WARN_ON_ONCE(!rcu_read_lock_held());
>
> In kernel/bpf/inode.c, bpffs map pretty print calls map->ops->map_get_next_key()
> without holding a rcu_read_lock(), hence causing the above warning.
> To fix the issue, just surrounding map->ops->map_get_next_key() with rcu read lock.
>
> Reported-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andriin@xxxxxx>
> Cc: Martin KaFai Lau <kafai@xxxxxx>
> Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap")
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
LGTM.
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> kernel/bpf/inode.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index fb878ba3f22f..18f4969552ac 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -226,10 +226,12 @@ static void *map_seq_next(struct seq_file *m, void *v, loff_t *pos)
> else
> prev_key = key;
>
> + rcu_read_lock();
> if (map->ops->map_get_next_key(map, prev_key, key)) {
> map_iter(m)->done = true;
> - return NULL;
> + key = NULL;
> }
> + rcu_read_unlock();
> return key;
> }
>
> --
> 2.24.1
>
