netdev - Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog

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Date:   Wed, 8 May 2019 10:56:45 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        ast@...nel.org, daniel@...earbox.net
Subject: Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog_array

On Wed, May 08, 2019 at 10:18:41AM -0700, Stanislav Fomichev wrote:
> Right now we are not using rcu api correctly: we pass __rcu pointers
> to bpf_prog_array_xyz routines but don't use rcu_dereference on them
> (see bpf_prog_array_delete_safe and bpf_prog_array_copy in particular).
> Instead of sprinkling rcu_dereferences, let's just get rid of those
> __rcu annotations and move rcu handling to a higher level.
> 
> It looks like all those routines are called from the rcu update
> side and we can use simple rcu_dereference_protected to get a
> reference that is valid as long as we hold a mutex (i.e. no other
> updater can change the pointer, no need for rcu read section and
> there should not be a use-after-free problem).
> 
> To be fair, there is currently no issue with the existing approach
> since the calls are mutex-protected, pointer values don't change,
> __rcu annotations are ignored. But it's still nice to use proper api.
> 
> The series fixes the following sparse warnings:

Absolutely not.
please fix it properly.
Removing annotations is not a fix.