[PATCH bpf-next v4 3/6] bpf/helpers: introduce bpf_timer_set_sleepable_cb() kfunc

From: Benjamin Tissoires
Date: Fri Mar 15 2024 - 10:30:33 EST

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In this patch, bpf_timer_set_sleepable_cb() is functionally equivalent
to bpf_timer_set_callback(), to the exception that it enforces
the timer to be started with BPF_F_TIMER_SLEEPABLE.

But given that bpf_timer_set_callback() is a helper when
bpf_timer_set_sleepable_cb() is a kfunc, we need to teach the verifier
about its attached callback.
Marking that callback as sleepable will be done in a separate patch

Signed-off-by: Benjamin Tissoires <bentiss@xxxxxxxxxx>

---

changes in v4:
- added a new (ignored) argument to the kfunc so that we do not
need to wlak the stack

new in v3 (split from v2 02/10)
---
kernel/bpf/helpers.c | 46 +++++++++++++++++++++++++++++++++++++--
kernel/bpf/verifier.c | 60 +++++++++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 102 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 38de73a9df83..65c07c0df263 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1108,6 +1108,7 @@ struct bpf_hrtimer {
void *value;
struct rcu_head rcu;
spinlock_t sleepable_lock;
+ bool is_sleepable;
};

/* the actual struct hidden inside uapi struct bpf_timer */
@@ -1273,8 +1274,8 @@ static const struct bpf_func_proto bpf_timer_init_proto = {
.arg3_type = ARG_ANYTHING,
};

-BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callback_fn,
- struct bpf_prog_aux *, aux)
+static int __bpf_timer_set_callback(struct bpf_timer_kern *timer, void *callback_fn,
+ struct bpf_prog_aux *aux, bool is_sleepable)
{
struct bpf_prog *prev, *prog = aux->prog;
struct bpf_hrtimer *t;
@@ -1314,6 +1315,7 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
t->prog = prog;
}
rcu_assign_pointer(t->callback_fn, callback_fn);
+ t->is_sleepable = is_sleepable;
out:
if (t)
spin_unlock(&t->sleepable_lock);
@@ -1321,6 +1323,12 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
return ret;
}

+BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callback_fn,
+ struct bpf_prog_aux *, aux)
+{
+ return __bpf_timer_set_callback(timer, callback_fn, aux, false);
+}
+
static const struct bpf_func_proto bpf_timer_set_callback_proto = {
.func = bpf_timer_set_callback,
.gpl_only = true,
@@ -1350,6 +1358,11 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
goto out;
}

+ if (t->is_sleepable && !(flags & BPF_F_TIMER_SLEEPABLE)) {
+ ret = -EINVAL;
+ goto out;
+ }
+
if (flags & BPF_F_TIMER_ABS)
mode = HRTIMER_MODE_ABS_SOFT;
else
@@ -2627,6 +2640,34 @@ __bpf_kfunc void bpf_throw(u64 cookie)
WARN(1, "A call to BPF exception callback should never return\n");
}

+/**
+ * bpf_timer_set_sleepable_cb_impl() - Configure the timer to call %callback_fn
+ * static function in a sleepable context.
+ * @timer: The bpf_timer that needs to be configured
+ * @callback_fn: a static bpf function
+ *
+ * @returns %0 on success. %-EINVAL if %timer was not initialized with
+ * bpf_timer_init() earlier. %-EPERM if %timer is in a map that doesn't
+ * have any user references.
+ * The user space should either hold a file descriptor to a map with timers
+ * or pin such map in bpffs. When map is unpinned or file descriptor is
+ * closed all timers in the map will be cancelled and freed.
+ *
+ * This kfunc is equivalent to %bpf_timer_set_callback except that it tells
+ * the verifier that the target callback is run in a sleepable context.
+ */
+__bpf_kfunc int bpf_timer_set_sleepable_cb_impl(struct bpf_timer_kern *timer,
+ int (callback_fn)(void *map, int *key, struct bpf_timer *timer),
+ void *aux__ign)
+{
+ struct bpf_prog_aux *aux = (struct bpf_prog_aux *)aux__ign;
+
+ if (!aux)
+ return -EINVAL;
+
+ return __bpf_timer_set_callback(timer, (void *)callback_fn, aux, true);
+}
+
__bpf_kfunc_end_defs();

BTF_KFUNCS_START(generic_btf_ids)
@@ -2703,6 +2744,7 @@ BTF_ID_FLAGS(func, bpf_dynptr_is_null)
BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
BTF_ID_FLAGS(func, bpf_dynptr_size)
BTF_ID_FLAGS(func, bpf_dynptr_clone)
+BTF_ID_FLAGS(func, bpf_timer_set_sleepable_cb_impl)
BTF_KFUNCS_END(common_btf_ids)

static const struct btf_kfunc_id_set common_kfunc_set = {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1483ebc0ee73..53f85e114a33 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -501,8 +501,12 @@ static bool is_dynptr_ref_function(enum bpf_func_id func_id)
}

static bool is_sync_callback_calling_kfunc(u32 btf_id);
+static bool is_async_callback_calling_kfunc(u32 btf_id);
+static bool is_callback_calling_kfunc(u32 btf_id);
static bool is_bpf_throw_kfunc(struct bpf_insn *insn);

+static bool is_bpf_timer_set_sleepable_cb_impl_kfunc(u32 btf_id);
+
static bool is_sync_callback_calling_function(enum bpf_func_id func_id)
{
return func_id == BPF_FUNC_for_each_map_elem ||
@@ -530,7 +534,8 @@ static bool is_sync_callback_calling_insn(struct bpf_insn *insn)

static bool is_async_callback_calling_insn(struct bpf_insn *insn)
{
- return bpf_helper_call(insn) && is_async_callback_calling_function(insn->imm);
+ return (bpf_helper_call(insn) && is_async_callback_calling_function(insn->imm)) ||
+ (bpf_pseudo_kfunc_call(insn) && is_async_callback_calling_kfunc(insn->imm));
}

static bool is_may_goto_insn(struct bpf_insn *insn)
@@ -9471,7 +9476,7 @@ static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins
*/
env->subprog_info[subprog].is_cb = true;
if (bpf_pseudo_kfunc_call(insn) &&
- !is_sync_callback_calling_kfunc(insn->imm)) {
+ !is_callback_calling_kfunc(insn->imm)) {
verbose(env, "verifier bug: kfunc %s#%d not marked as callback-calling\n",
func_id_name(insn->imm), insn->imm);
return -EFAULT;
@@ -10981,6 +10986,7 @@ enum special_kfunc_type {
KF_bpf_percpu_obj_drop_impl,
KF_bpf_throw,
KF_bpf_iter_css_task_new,
+ KF_bpf_timer_set_sleepable_cb_impl,
};

BTF_SET_START(special_kfunc_set)
@@ -11007,6 +11013,7 @@ BTF_ID(func, bpf_throw)
#ifdef CONFIG_CGROUPS
BTF_ID(func, bpf_iter_css_task_new)
#endif
+BTF_ID(func, bpf_timer_set_sleepable_cb_impl)
BTF_SET_END(special_kfunc_set)

BTF_ID_LIST(special_kfunc_list)
@@ -11037,6 +11044,7 @@ BTF_ID(func, bpf_iter_css_task_new)
#else
BTF_ID_UNUSED
#endif
+BTF_ID(func, bpf_timer_set_sleepable_cb_impl)

static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta)
{
@@ -11365,12 +11373,28 @@ static bool is_sync_callback_calling_kfunc(u32 btf_id)
return btf_id == special_kfunc_list[KF_bpf_rbtree_add_impl];
}

+static bool is_async_callback_calling_kfunc(u32 btf_id)
+{
+ return btf_id == special_kfunc_list[KF_bpf_timer_set_sleepable_cb_impl];
+}
+
static bool is_bpf_throw_kfunc(struct bpf_insn *insn)
{
return bpf_pseudo_kfunc_call(insn) && insn->off == 0 &&
insn->imm == special_kfunc_list[KF_bpf_throw];
}

+static bool is_bpf_timer_set_sleepable_cb_impl_kfunc(u32 btf_id)
+{
+ return btf_id == special_kfunc_list[KF_bpf_timer_set_sleepable_cb_impl];
+}
+
+static bool is_callback_calling_kfunc(u32 btf_id)
+{
+ return is_sync_callback_calling_kfunc(btf_id) ||
+ is_async_callback_calling_kfunc(btf_id);
+}
+
static bool is_rbtree_lock_required_kfunc(u32 btf_id)
{
return is_bpf_rbtree_api_kfunc(btf_id);
@@ -12151,6 +12175,16 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
}
}

+ if (is_async_callback_calling_kfunc(meta.func_id)) {
+ err = push_callback_call(env, insn, insn_idx, meta.subprogno,
+ set_timer_callback_state);
+ if (err) {
+ verbose(env, "kfunc %s#%d failed callback verification\n",
+ func_name, meta.func_id);
+ return err;
+ }
+ }
+
rcu_lock = is_kfunc_bpf_rcu_read_lock(&meta);
rcu_unlock = is_kfunc_bpf_rcu_read_unlock(&meta);

@@ -19544,6 +19578,28 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
*cnt = 1;
+ } else if (is_bpf_timer_set_sleepable_cb_impl_kfunc(desc->func_id)) {
+ /* The verifier will process callback_fn as many times as necessary
+ * with different maps and the register states prepared by
+ * set_timer_callback_state will be accurate.
+ *
+ * The following use case is valid:
+ * map1 is shared by prog1, prog2, prog3.
+ * prog1 calls bpf_timer_init for some map1 elements
+ * prog2 calls bpf_timer_set_callback for some map1 elements.
+ * Those that were not bpf_timer_init-ed will return -EINVAL.
+ * prog3 calls bpf_timer_start for some map1 elements.
+ * Those that were not both bpf_timer_init-ed and
+ * bpf_timer_set_callback-ed will return -EINVAL.
+ */
+ struct bpf_insn ld_addrs[2] = {
+ BPF_LD_IMM64(BPF_REG_3, (long)env->prog->aux),
+ };
+
+ insn_buf[0] = ld_addrs[0];
+ insn_buf[1] = ld_addrs[1];
+ insn_buf[2] = *insn;
+ *cnt = 3;
}
return 0;
}

--
2.44.0

Next message: Benjamin Tissoires: "[PATCH bpf-next v4 4/6] bpf/helpers: mark the callback of bpf_timer_set_sleepable_cb() as sleepable"
Previous message: Benjamin Tissoires: "[PATCH bpf-next v4 2/6] bpf/verifier: add bpf_timer as a kfunc capable type"
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Next in thread: Benjamin Tissoires: "[PATCH bpf-next v4 4/6] bpf/helpers: mark the callback of bpf_timer_set_sleepable_cb() as sleepable"
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