Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing programs

On Mon, Jan 11, 2021 at 7:57 PM Martin KaFai Lau [off-list ref] wrote:

On Fri, Jan 08, 2021 at 03:19:47PM -0800, Song Liu wrote:

[ ... ]

quoted

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index dd5aedee99e73..9bd47ad2b26f1 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c

@@ -140,17 +140,18 @@ static void __bpf_selem_unlink_storage(struct bpf_local_storage_elem *selem)
 {
      struct bpf_local_storage *local_storage;
      bool free_local_storage = false;
+     unsigned long flags;

      if (unlikely(!selem_linked_to_storage(selem)))
              /* selem has already been unlinked from sk */
              return;

      local_storage = rcu_dereference(selem->local_storage);
-     raw_spin_lock_bh(&local_storage->lock);
+     raw_spin_lock_irqsave(&local_storage->lock, flags);

It will be useful to have a few words in commit message on this change
for future reference purpose.

Please also remove the in_irq() check from bpf_sk_storage.c
to avoid confusion in the future.  It probably should
be in a separate patch.

[ ... ]

quoted

diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
index 4ef1959a78f27..f654b56907b69 100644
diff --git a/kernel/fork.c b/kernel/fork.c
index 7425b3224891d..3d65c8ebfd594 100644

[ ... ]

quoted

--- a/kernel/fork.c
+++ b/kernel/fork.c

@@ -96,6 +96,7 @@
 #include <linux/kasan.h>
 #include <linux/scs.h>
 #include <linux/io_uring.h>
+#include <linux/bpf.h>

 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>

@@ -734,6 +735,7 @@ void __put_task_struct(struct task_struct *tsk)
      cgroup_free(tsk);
      task_numa_free(tsk, true);
      security_task_free(tsk);
+     bpf_task_storage_free(tsk);
      exit_creds(tsk);

If exit_creds() is traced by a bpf and this bpf is doing
bpf_task_storage_get(..., BPF_LOCAL_STORAGE_GET_F_CREATE),
new task storage will be created after bpf_task_storage_free().

I recalled there was an earlier discussion with KP and KP mentioned
BPF_LSM will not be called with a task that is going away.
It seems enabling bpf task storage in bpf tracing will break
this assumption and needs to be addressed?

For tracing programs, I think we will need an allow list where
task local storage can be used.