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Date: Tue, 15 Sep 2020 17:57:57 -0700 From: Andrii Nakryiko <andrii.nakryiko@...il.com> To: Yonghong Song <yhs@...com> Cc: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Kernel Team <kernel-team@...com>, Andrii Nakryiko <andriin@...com>, Martin KaFai Lau <kafai@...com> Subject: Re: [PATCH bpf] bpf: fix a rcu warning for bpffs map pretty-print On Tue, Sep 15, 2020 at 5:44 PM Yonghong Song <yhs@...com> 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@...nel.org> > Cc: Andrii Nakryiko <andriin@...com> > Cc: Martin KaFai Lau <kafai@...com> > Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap") > Signed-off-by: Yonghong Song <yhs@...com> > --- LGTM. Acked-by: Andrii Nakryiko <andriin@...com> > 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 >
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