Re: [PATCH 10/14] bpf: Re-initialize lnode in bpf_ksym_del

On Tue, Feb 11, 2020 at 10:28:50AM -0800, Andrii Nakryiko wrote:

On Sat, Feb 8, 2020 at 7:43 AM Jiri Olsa [off-list ref] wrote:

quoted

When bpf_prog is removed from kallsyms it's on the way
out to be removed, so we don't care about lnode state.

However the bpf_ksym_del will be used also by bpf_trampoline
and bpf_dispatcher objects, which stay allocated even when
they are not in kallsyms list, hence the lnode re-init.

The list_del_rcu commentary states that we need to call
synchronize_rcu, before we can change/re-init the list_head
pointers.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---

Wouldn't it make more sense to have patches 7 though 10 as a one
patch? It's a generalization of ksym from being bpf_prog-specific to
be more general (which this initialization fix is part of, arguably).

it was my initial change ;-) but then I realized I have to explain
several things in the changelog, and that's usually the sign that
you need to split the patch.. also I think now it's easier for review
and backporting

so I prefer it split like this, but if you guys want to squash it
together, I'll do it ;-)

jirka

quoted

 kernel/bpf/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 73242fd07893..66b17bea286e 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c

@@ -676,6 +676,13 @@ void bpf_ksym_del(struct bpf_ksym *ksym)
        spin_lock_bh(&bpf_lock);
        __bpf_ksym_del(ksym);
        spin_unlock_bh(&bpf_lock);
+
+       /*
+        * As explained in list_del_rcu, We must call synchronize_rcu
+        * before changing list_head pointers.
+        */
+       synchronize_rcu();
+       INIT_LIST_HEAD_RCU(&ksym->lnode);
 }

 static bool bpf_prog_kallsyms_candidate(const struct bpf_prog *fp)
--

2.24.1