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

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On Jan 12, 2021, at 8:53 AM, KP Singh [off-list ref] wrote:

On Tue, Jan 12, 2021 at 5:32 PM Yonghong Song [off-list ref] wrote:

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On 1/11/21 3:45 PM, Song Liu wrote:

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On Jan 11, 2021, at 1:58 PM, Martin Lau [off-list ref] wrote:

On Mon, Jan 11, 2021 at 10:35:43PM +0100, KP Singh wrote:

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

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

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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

[...]

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+#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.

Instead of whitelist, can refcount_inc_not_zero(&tsk->usage) be used?

I think we can put refcount_inc_not_zero() in bpf_task_storage_get, like:

diff --git i/kernel/bpf/bpf_task_storage.c w/kernel/bpf/bpf_task_storage.c
index f654b56907b69..93d01b0a010e6 100644
--- i/kernel/bpf/bpf_task_storage.c
+++ w/kernel/bpf/bpf_task_storage.c

@@ -216,6 +216,9 @@ BPF_CALL_4(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
          * by an RCU read-side critical section.
          */
         if (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) {
+               if (!refcount_inc_not_zero(&task->usage))
+                       return -EBUSY;
+
                 sdata = bpf_local_storage_update(
                         task, (struct bpf_local_storage_map *)map, value,
                         BPF_NOEXIST);

But where shall we add the refcount_dec()? IIUC, we cannot add it to
__put_task_struct().

Maybe put_task_struct()?

Yeah, something like, or if you find a more elegant alternative :)

--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h

@@ -107,13 +107,20 @@ extern void __put_task_struct(struct task_struct *t);

static inline void put_task_struct(struct task_struct *t)
{
-       if (refcount_dec_and_test(&t->usage))
+
+       if (rcu_access_pointer(t->bpf_storage)) {
+               if (refcount_sub_and_test(2, &t->usage))
+                       __put_task_struct(t);
+       } else if (refcount_dec_and_test(&t->usage))
                __put_task_struct(t);
}

static inline void put_task_struct_many(struct task_struct *t, int nr)
{
-       if (refcount_sub_and_test(nr, &t->usage))
+       if (rcu_access_pointer(t->bpf_storage)) {
+               if (refcount_sub_and_test(nr + 1, &t->usage))
+                       __put_task_struct(t);
+       } else if (refcount_sub_and_test(nr, &t->usage))
                __put_task_struct(t);
}

It is not ideal to leak bpf_storage here. How about we only add the
following:

diff --git i/kernel/bpf/bpf_task_storage.c w/kernel/bpf/bpf_task_storage.c
index f654b56907b69..2811b9fc47233 100644
--- i/kernel/bpf/bpf_task_storage.c
+++ w/kernel/bpf/bpf_task_storage.c

@@ -216,6 +216,10 @@ BPF_CALL_4(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
          * by an RCU read-side critical section.
          */
         if (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) {
+               /* the task_struct is being freed, fail over*/
+               if (!refcount_read(&task->usage))
+                       return -EBUSY;