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Message-ID: <87mtn1cosq.fsf@toke.dk>
Date: Fri, 22 Oct 2021 12:54:45 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Network Development <netdev@...r.kernel.org>,
bpf <bpf@...r.kernel.org>,
Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Subject: Re: [PATCH bpf] bpf: fix potential race in tail call compatibility
check
Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> On Thu, Oct 21, 2021 at 11:40 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>> + map_type = READ_ONCE(array->aux->type);
>> + if (!map_type) {
>> + /* There's no owner yet where we could check for compatibility.
>> + * Do an atomic swap to prevent racing with another invocation
>> + * of this branch (via simultaneous map_update syscalls).
>> */
>> - array->aux->type = fp->type;
>> - array->aux->jited = fp->jited;
>> + if (cmpxchg(&array->aux->type, 0, prog_type))
>> + return false;
>
> Other fields might be used in the compatibility check in the future.
> This hack is too fragile.
> Just use a spin_lock.
Well, yeah, we're adding another field for xdp_mb. I was just going to
eat more bits of the 'type' field, but OK, can switch to a spinlock
instead :)
-Toke
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