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Date:   Tue, 15 Sep 2020 18:19:42 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Yonghong Song <yhs@...com>, 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 5:58 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> 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>

Applied. Thanks!

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