[bpf-next,v4,4/6] bpf/helpers: mark the callback of bpf_timer_set_sleepable_cb() as sleepable

Commit Message

Benjamin Tissoires March 15, 2024, 2:29 p.m. UTC

Now that we have bpf_timer_set_sleepable_cb() available and working, we
can tag the attached callback as sleepable, and let the verifier check
in the correct context the calls and kfuncs.

Signed-off-by: Benjamin Tissoires <[e-mail]>

---

changes in v4:
- use a function parameter to forward the sleepable information

new in v3 (split from v2 02/10)
---
 include/linux/bpf_verifier.h |  1 +
 kernel/bpf/verifier.c        | 13 ++++++++++---
 2 files changed, 11 insertions(+), 3 deletions(-)

Comments

Eduard Zingerman March 18, 2024, 11:54 p.m. UTC | #1

On Fri, 2024-03-15 at 15:29 +0100, Benjamin Tissoires wrote:
[...]

> @@ -5279,7 +5281,8 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
>  
>  static bool in_sleepable(struct bpf_verifier_env *env)
>  {
> -	return env->prog->sleepable;
> +	return env->prog->sleepable ||
> +	       (env->cur_state && env->cur_state->in_sleepable);
>  }

I was curious why 'env->cur_state &&' check was needed and found that
removing it caused an error in the following fragment:

static int do_misc_fixups(struct bpf_verifier_env *env)
{
		...
		if (is_storage_get_function(insn->imm)) {
			if (!in_sleepable(env) ||
			    env->insn_aux_data[i + delta].storage_get_func_atomic)
				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_ATOMIC);
			else
				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_KERNEL);
			...
		}
		...
}

When do_misc_fixups() is done env->cur_state is NULL.
Current implementation would use GFP_ATOMIC allocation even for
sleepable callbacks, where GFP_KERNEL is sufficient.
Is this is something we want to address?

>  
>  /* The non-sleepable programs and sleepable programs with explicit bpf_rcu_read_lock()

Benjamin Tissoires March 21, 2024, 4:09 p.m. UTC | #2

On Tue, Mar 19, 2024 at 12:54 AM Eduard Zingerman <[e-mail]> wrote:
>
> On Fri, 2024-03-15 at 15:29 +0100, Benjamin Tissoires wrote:
> [...]
>
> > @@ -5279,7 +5281,8 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
> >
> >  static bool in_sleepable(struct bpf_verifier_env *env)
> >  {
> > -     return env->prog->sleepable;
> > +     return env->prog->sleepable ||
> > +            (env->cur_state && env->cur_state->in_sleepable);
> >  }
>
> I was curious why 'env->cur_state &&' check was needed and found that
> removing it caused an error in the following fragment:
>
> static int do_misc_fixups(struct bpf_verifier_env *env)
> {
>                 ...
>                 if (is_storage_get_function(insn->imm)) {
>                         if (!in_sleepable(env) ||
>                             env->insn_aux_data[i + delta].storage_get_func_atomic)
>                                 insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_ATOMIC);
>                         else
>                                 insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_KERNEL);
>                         ...
>                 }
>                 ...
> }
>
> When do_misc_fixups() is done env->cur_state is NULL.
> Current implementation would use GFP_ATOMIC allocation even for
> sleepable callbacks, where GFP_KERNEL is sufficient.
> Is this is something we want to address?

I honestly have no idea of the impact there.

AFAICT, if env->cur_state is not set, we don't even know if the
callback will be sleepable or not, so if there is a small penalty,
then it's the safest option, no?

Cheers,
Benjamin

>
> >
> >  /* The non-sleepable programs and sleepable programs with explicit bpf_rcu_read_lock()
>

Patch

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 7cb1b75eee38..14e4ee67b694 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -426,6 +426,7 @@  struct bpf_verifier_state {
 	 * while they are still in use.
 	 */
 	bool used_as_loop_entry;
+	bool in_sleepable;
 
 	/* first and last insn idx of this verifier state */
 	u32 first_insn_idx;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 53f85e114a33..0be07da38f8a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1434,6 +1434,7 @@  static int copy_verifier_state(struct bpf_verifier_state *dst_state,
 	}
 	dst_state->speculative = src->speculative;
 	dst_state->active_rcu_lock = src->active_rcu_lock;
+	dst_state->in_sleepable = src->in_sleepable;
 	dst_state->curframe = src->curframe;
 	dst_state->active_lock.ptr = src->active_lock.ptr;
 	dst_state->active_lock.id = src->active_lock.id;
@@ -2407,7 +2408,7 @@  static void init_func_state(struct bpf_verifier_env *env,
 /* Similar to push_stack(), but for async callbacks */
 static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
 						int insn_idx, int prev_insn_idx,
-						int subprog)
+						int subprog, bool is_sleepable)
 {
 	struct bpf_verifier_stack_elem *elem;
 	struct bpf_func_state *frame;
@@ -2434,6 +2435,7 @@  static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
 	 * Initialize it similar to do_check_common().
 	 */
 	elem->st.branches = 1;
+	elem->st.in_sleepable = is_sleepable;
 	frame = kzalloc(sizeof(*frame), GFP_KERNEL);
 	if (!frame)
 		goto err;
@@ -5279,7 +5281,8 @@  static int map_kptr_match_type(struct bpf_verifier_env *env,
 
 static bool in_sleepable(struct bpf_verifier_env *env)
 {
-	return env->prog->sleepable;
+	return env->prog->sleepable ||
+	       (env->cur_state && env->cur_state->in_sleepable);
 }
 
 /* The non-sleepable programs and sleepable programs with explicit bpf_rcu_read_lock()
@@ -9493,7 +9496,8 @@  static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins
 		/* there is no real recursion here. timer callbacks are async */
 		env->subprog_info[subprog].is_async_cb = true;
 		async_cb = push_async_cb(env, env->subprog_info[subprog].start,
-					 insn_idx, subprog);
+					 insn_idx, subprog,
+					 is_bpf_timer_set_sleepable_cb_impl_kfunc(insn->imm));
 		if (!async_cb)
 			return -EFAULT;
 		callee = async_cb->frame[0];
@@ -16937,6 +16941,9 @@  static bool states_equal(struct bpf_verifier_env *env,
 	if (old->active_rcu_lock != cur->active_rcu_lock)
 		return false;
 
+	if (old->in_sleepable != cur->in_sleepable)
+		return false;
+
 	/* for states to be equal callsites have to be the same
 	 * and all frame states need to be equivalent
 	 */