On Mon, Nov 23, 2020 at 9:49 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Nov 19, 2020 at 03:22:39PM -0800, Andrii Nakryiko wrote:
> > __module_address() needs to be called with preemption disabled or with
> > module_mutex taken. preempt_disable() is enough for read-only uses, which is
> > what this fix does.
> >
> > Fixes: a38d1107f937 ("bpf: support raw tracepoints in modules")
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> > kernel/trace/bpf_trace.c | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index d255bc9b2bfa..bb98a377050a 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2060,7 +2060,11 @@ struct bpf_raw_event_map *bpf_get_raw_tracepoint(const char *name)
> >
> > void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp)
> > {
> > - struct module *mod = __module_address((unsigned long)btp);
> > + struct module *mod;
> > +
> > + preempt_disable();
> > + mod = __module_address((unsigned long)btp);
> > + preempt_enable();
> >
> > if (mod)
> > module_put(mod);
>
> I don't understand why 'mod' cannot become dangling pointer after preempt_enable().
> Either it needs a comment explaining why it's ok or module_put() should
> be in preempt disabled section.
Yeah, I think it can, assuming the kernel module can be unloaded
despite non-zero refcnt (probably happens with force unload?). I'll
drop the `if (mod)` part (module_put() checks that internally) and
will move module_put(mod) inside the preempt disable/enable region.
