[PATCH v2] bpf: test_run: Use migrate_enable()/disable() universally

[Sahil Chandna <chandna.linuxkernel@gmail.com>]

The timer context can safely use migrate_disable()/migrate_enable()
universally instead of conditional preemption or migration disabling.
Previously, the timer was initialized in NO_PREEMPT mode by default,
which disabled preemption and forced execution in atomic context.
This caused issues on PREEMPT_RT configurations when invoking
spin_lock_bh() — a sleeping lock — leading to the following warning:

BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6107, name: syz.0.17
preempt_count: 1, expected: 0
RCU nest depth: 1, expected: 1
Preemption disabled at:
[<ffffffff891fce58>] bpf_test_timer_enter+0xf8/0x140 net/bpf/test_run.c:42

Reported-by: syzbot+1f1fbecb9413cdbfbef8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1f1fbecb9413cdbfbef8
Tested-by: syzbot+1f1fbecb9413cdbfbef8@syzkaller.appspotmail.com
Signed-off-by: Sahil Chandna <chandna.linuxkernel@gmail.com>

---
Link to v1: https://lore.kernel.org/all/20251006054320.159321-1-chandna.linuxkernel@gmail.com/

Changes since v1:
- Dropped `enum { NO_PREEMPT, NO_MIGRATE } mode` from `struct bpf_test_timer`.
- Removed all conditional preempt/migrate disable logic.
- Unified timer handling to use `migrate_disable()` / `migrate_enable()` universally.

Testing:
- Reproduced syzbot bug locally using the provided reproducer.
- Observed `BUG: sleeping function called from invalid context` on v1.
- Confirmed bug disappears after applying this patch.
- Validated normal functionality of `bpf_prog_test_run_*` helpers with C
  reproducer.
---
 net/bpf/test_run.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index dfb03ee0bb62..b23bc93e738e 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -29,7 +29,6 @@
 #include <trace/events/bpf_test_run.h>
 
 struct bpf_test_timer {
-	enum { NO_PREEMPT, NO_MIGRATE } mode;
 	u32 i;
 	u64 time_start, time_spent;
 };
@@ -38,10 +37,7 @@ static void bpf_test_timer_enter(struct bpf_test_timer *t)
 	__acquires(rcu)
 {
 	rcu_read_lock();
-	if (t->mode == NO_PREEMPT)
-		preempt_disable();
-	else
-		migrate_disable();
+	migrate_disable();
 
 	t->time_start = ktime_get_ns();
 }
@@ -50,11 +46,7 @@ static void bpf_test_timer_leave(struct bpf_test_timer *t)
 	__releases(rcu)
 {
 	t->time_start = 0;
-
-	if (t->mode == NO_PREEMPT)
-		preempt_enable();
-	else
-		migrate_enable();
+	migrate_enable();
 	rcu_read_unlock();
 }
 
@@ -374,7 +366,7 @@ static int bpf_test_run_xdp_live(struct bpf_prog *prog, struct xdp_buff *ctx,
 
 {
 	struct xdp_test_data xdp = { .batch_size = batch_size };
-	struct bpf_test_timer t = { .mode = NO_MIGRATE };
+	struct bpf_test_timer t;
 	int ret;
 
 	if (!repeat)
@@ -404,7 +396,7 @@ static int bpf_test_run(struct bpf_prog *prog, void *ctx, u32 repeat,
 	struct bpf_prog_array_item item = {.prog = prog};
 	struct bpf_run_ctx *old_ctx;
 	struct bpf_cg_run_ctx run_ctx;
-	struct bpf_test_timer t = { NO_MIGRATE };
+	struct bpf_test_timer t;
 	enum bpf_cgroup_storage_type stype;
 	int ret;
 
@@ -1377,7 +1369,7 @@ int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
 				     const union bpf_attr *kattr,
 				     union bpf_attr __user *uattr)
 {
-	struct bpf_test_timer t = { NO_PREEMPT };
+	struct bpf_test_timer t;
 	u32 size = kattr->test.data_size_in;
 	struct bpf_flow_dissector ctx = {};
 	u32 repeat = kattr->test.repeat;
@@ -1445,7 +1437,7 @@ int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
 int bpf_prog_test_run_sk_lookup(struct bpf_prog *prog, const union bpf_attr *kattr,
 				union bpf_attr __user *uattr)
 {
-	struct bpf_test_timer t = { NO_PREEMPT };
+	struct bpf_test_timer t;
 	struct bpf_prog_array *progs = NULL;
 	struct bpf_sk_lookup_kern ctx = {};
 	u32 repeat = kattr->test.repeat;
-- 
2.50.1