Re: [PATCH bpf] bpf: Take module reference for ip in module code

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Mar 24, 2021 at 12:31:42PM +0100, Jiri Olsa wrote:
> On Tue, Mar 23, 2021 at 06:22:37PM -0700, Alexei Starovoitov wrote:
> > On Tue, Mar 23, 2021 at 10:15:33PM +0100, Jiri Olsa wrote:
> > > Currently module can be unloaded even if there's a trampoline
> > > register in it. It's easily reproduced by running in parallel:
> > > 
> > >   # while :; do ./test_progs -t module_attach; done
> > >   # while :; do ./test_progs -t fentry_test; done
> > > 
> > > Taking the module reference in case the trampoline's ip is
> > > within the module code. Releasing it when the trampoline's
> > > ip is unregistered.
> > > 
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >  kernel/bpf/trampoline.c | 32 ++++++++++++++++++++++++++++++++
> > >  1 file changed, 32 insertions(+)
> > > 
> > > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > > index 1f3a4be4b175..f6cb179842b2 100644
> > > --- a/kernel/bpf/trampoline.c
> > > +++ b/kernel/bpf/trampoline.c
> > > @@ -87,6 +87,27 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
> > >  	return tr;
> > >  }
> > >  
> > > +static struct module *ip_module_get(unsigned long ip)
> > > +{
> > > +	struct module *mod;
> > > +	int err = 0;
> > > +
> > > +	preempt_disable();
> > > +	mod = __module_text_address(ip);
> > > +	if (mod && !try_module_get(mod))
> > > +		err = -ENOENT;
> > > +	preempt_enable();
> > > +	return err ? ERR_PTR(err) : mod;
> > > +}
> > > +
> > > +static void ip_module_put(unsigned long ip)
> > > +{
> > > +	struct module *mod = __module_text_address(ip);
> > 
> > Conceptually looks correct, but how did you test it?!
> > Just doing your reproducer:
> > while :; do ./test_progs -t module_attach; done & while :; do ./test_progs -t fentry_test; done
> > 
> > I immediately hit:
> > [   19.461162] WARNING: CPU: 1 PID: 232 at kernel/module.c:264 module_assert_mutex_or_preempt+0x2e/0x40
> > [   19.477126] Call Trace:
> > [   19.477464]  __module_address+0x28/0xf0
> > [   19.477865]  ? __bpf_trace_bpf_testmod_test_write_bare+0x10/0x10 [bpf_testmod]
> > [   19.478711]  __module_text_address+0xe/0x60
> > [   19.479156]  bpf_trampoline_update+0x2ff/0x470
> 
> I don't have lockdep enabled.. ah the module_mutex is held
> during module init, that's why all the code I was using as
> a reference did not take it.. sorry, will fix

ah it's the missing preempt_disable ;-) ok

jirka

> 
> > 
> > Which points to an obvious bug above.
> > 
> > How did you debug it to this module going away issue?
> > Why does test_progs -t fentry_test help to repro?
> > Or does it?
> > It doesn't touch anything in modules.
> 
> test_prog also loads/unloads that module, but it could be
> just insmod/rmmod instead, will change
> 
> jirka