Re: [PATCH 5/5] bpf: Allow to resolve bpf trampoline in unwind

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Jan 07, 2020 at 09:30:12AM +0100, Daniel Borkmann wrote:
> On 1/7/20 12:46 AM, Alexei Starovoitov wrote:
> > On Sun, Dec 29, 2019 at 03:37:40PM +0100, Jiri Olsa wrote:
> > > When unwinding the stack we need to identify each
> > > address to successfully continue. Adding latch tree
> > > to keep trampolines for quick lookup during the
> > > unwind.
> > > 
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ...
> > > +bool is_bpf_trampoline(void *addr)
> > > +{
> > > +	return latch_tree_find(addr, &tree, &tree_ops) != NULL;
> > > +}
> > > +
> > >   struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
> > >   {
> > >   	struct bpf_trampoline *tr;
> > > @@ -65,6 +98,7 @@ struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
> > >   	for (i = 0; i < BPF_TRAMP_MAX; i++)
> > >   		INIT_HLIST_HEAD(&tr->progs_hlist[i]);
> > >   	tr->image = image;
> > > +	latch_tree_insert(&tr->tnode, &tree, &tree_ops);
> > 
> > Thanks for the fix. I was thinking to apply it, but then realized that bpf
> > dispatcher logic has the same issue.
> > Could you generalize the fix for both?
> > May be bpf_jit_alloc_exec_page() can do latch_tree_insert() ?
> > and new version of bpf_jit_free_exec() is needed that will do latch_tree_erase().
> > Wdyt?
> 
> Also this patch is buggy since your latch lookup happens under RCU, but
> I don't see anything that waits a grace period once you remove from the
> tree. Instead you free the trampoline right away.

thanks, did not think of that.. will (try to) fix ;-)

> 
> On a different question, given we have all the kallsym infrastructure
> for BPF already in place, did you look into whether it's feasible to
> make it a bit more generic to also cover JITed buffers from trampolines?
> 

hum, it did not occur to me that we want to see it in kallsyms,
but sure.. how about: bpf_trampoline_<key> ?

key would be taken from bpf_trampoline::key as function's BTF id

jirka