bpf: Disable migration when freeing stashed local kptr using obj drop

When a local kptr is stashed in a map and freed when the map goes away,
currently an error like the below appears:

[ 39.195695] BUG: using smp_processor_id() in preemptible [00000000] code: kworker/u32:15/2875
[ 39.196549] caller is bpf_mem_free+0x56/0xc0
[ 39.196958] CPU: 15 PID: 2875 Comm: kworker/u32:15 Tainted: G O 6.2.0-13016-g22df776a9a86 #4477
[ 39.197897] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
[ 39.198949] Workqueue: events_unbound bpf_map_free_deferred
[ 39.199470] Call Trace:
[ 39.199703] <TASK>
[ 39.199911] dump_stack_lvl+0x60/0x70
[ 39.200267] check_preemption_disabled+0xbf/0xe0
[ 39.200704] bpf_mem_free+0x56/0xc0
[ 39.201032] ? bpf_obj_new_impl+0xa0/0xa0
[ 39.201430] bpf_obj_free_fields+0x1cd/0x200
[ 39.201838] array_map_free+0xad/0x220
[ 39.202193] ? finish_task_switch+0xe5/0x3c0
[ 39.202614] bpf_map_free_deferred+0xea/0x210
[ 39.203006] ? lockdep_hardirqs_on_prepare+0xe/0x220
[ 39.203460] process_one_work+0x64f/0xbe0
[ 39.203822] ? pwq_dec_nr_in_flight+0x110/0x110
[ 39.204264] ? do_raw_spin_lock+0x107/0x1c0
[ 39.204662] ? lockdep_hardirqs_on_prepare+0xe/0x220
[ 39.205107] worker_thread+0x74/0x7a0
[ 39.205451] ? process_one_work+0xbe0/0xbe0
[ 39.205818] kthread+0x171/0x1a0
[ 39.206111] ? kthread_complete_and_exit+0x20/0x20
[ 39.206552] ret_from_fork+0x1f/0x30
[ 39.206886] </TASK>

This happens because the call to __bpf_obj_drop_impl I added in the patch
adding support for stashing local kptrs doesn't disable migration. Prior
to that patch, __bpf_obj_drop_impl logic only ran when called by a BPF
progarm, whereas now it can be called from map free path, so it's
necessary to explicitly disable migration.

Also, refactor a bit to just call __bpf_obj_drop_impl directly instead
of bothering w/ dtor union and setting pointer-to-obj_drop.

Fixes: c8e187540914 ("bpf: Support __kptr to local kptrs")
Reported-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Dave Marchevsky <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/bpf.h

···
 };
 
 typedef void (*btf_dtor_kfunc_t)(void *);
-typedef void (*btf_dtor_obj_drop)(void *, const struct btf_record *);
 
 struct btf_field_kptr {
 	struct btf *btf;
 	struct module *module;
-	union {
-		/* dtor used if btf_is_kernel(btf), otherwise the type
-		 * is program-allocated and obj_drop is used
-		 */
-		btf_dtor_kfunc_t dtor;
-		btf_dtor_obj_drop obj_drop;
-	};
+	/* dtor used if btf_is_kernel(btf), otherwise the type is
+	 * program-allocated, dtor is NULL,  and __bpf_obj_drop_impl is used
+	 */
+	btf_dtor_kfunc_t dtor;
 	u32 btf_id;
 };
 

kernel/bpf/btf.c

···
 	return -EINVAL;
 }
 
-extern void __bpf_obj_drop_impl(void *p, const struct btf_record *rec);
-
 static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
 			  struct btf_field_info *info)
 {
···
 		/* Type exists only in program BTF. Assume that it's a MEM_ALLOC
 		 * kptr allocated via bpf_obj_new
 		 */
-		field->kptr.dtor = (void *)&__bpf_obj_drop_impl;
+		field->kptr.dtor = NULL;
 		id = info->kptr.type_id;
 		kptr_btf = (struct btf *)btf;
 		btf_get(kptr_btf);

kernel/bpf/syscall.c

···
 	bpf_timer_cancel_and_free(obj + rec->timer_off);
 }
 
+extern void __bpf_obj_drop_impl(void *p, const struct btf_record *rec);
+
 void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
 {
 	const struct btf_field *fields;
···
 				pointee_struct_meta = btf_find_struct_meta(field->kptr.btf,
 									   field->kptr.btf_id);
 				WARN_ON_ONCE(!pointee_struct_meta);
-				field->kptr.obj_drop(xchgd_field, pointee_struct_meta ?
-								  pointee_struct_meta->record :
-								  NULL);
+				migrate_disable();
+				__bpf_obj_drop_impl(xchgd_field, pointee_struct_meta ?
+								 pointee_struct_meta->record :
+								 NULL);
+				migrate_enable();
 			} else {
 				field->kptr.dtor(xchgd_field);
 			}