binder: enable CONFIG_ANDROID_BINDER_DEVICES=""

Several users have tried to only rely on binderfs to provide binder devices
and set CONFIG_ANDROID_BINDER_DEVICES="" empty. This is a great use-case of
binderfs and one that was always intended to work. However, this is
currently not possible since setting CONFIG_ANDROID_BINDER_DEVICES="" emtpy
will simply panic the kernel:

kobject: (00000000028c2f79): attempted to be registered with empty name!
WARNING: CPU: 7 PID: 1703 at lib/kobject.c:228 kobject_add_internal+0x288/0x2b0
Modules linked in: binder_linux(+) bridge stp llc ipmi_ssif gpio_ich dcdbas coretemp kvm_intel kvm irqbypass serio_raw input_leds lpc_ich i5100_edac mac_hid ipmi_si ipmi_devintf ipmi_msghandler sch_fq_codel ib_i
CPU: 7 PID: 1703 Comm: modprobe Not tainted 5.0.0-rc2-brauner-binderfs torvalds#263
Hardware name: Dell      DCS XS24-SC2          /XS24-SC2              , BIOS S59_3C20 04/07/2011
RIP: 0010:kobject_add_internal+0x288/0x2b0
Code: 12 95 48 c7 c7 78 63 3b 95 e8 77 35 71 ff e9 91 fe ff ff 0f 0b eb a7 0f 0b eb 9a 48 89 de 48 c7 c7 00 63 3b 95 e8 f8 95 6a ff <0f> 0b 41 bc ea ff ff ff e9 6d fe ff ff 41 bc fe ff ff ff e9 62 fe
RSP: 0018:ffff973f84237a30 EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff8b53e2472010 RCX: 0000000000000006
RDX: 0000000000000007 RSI: 0000000000000086 RDI: ffff8b53edbd63a0
RBP: ffff973f84237a60 R08: 0000000000000342 R09: 0000000000000004
R10: ffff973f84237af0 R11: 0000000000000001 R12: 0000000000000000
R13: ffff8b53e9f1a1e0 R14: 00000000e9f1a1e0 R15: 0000000000a00037
FS:  00007fbac36f7540(0000) GS:ffff8b53edbc0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fbac364cfa7 CR3: 00000004a6d48000 CR4: 00000000000406e0
Call Trace:
 kobject_add+0x71/0xd0
 ? _cond_resched+0x19/0x40
 ? mutex_lock+0x12/0x40
 device_add+0x12e/0x6b0
 device_create_groups_vargs+0xe4/0xf0
 device_create_with_groups+0x3f/0x60
 ? _cond_resched+0x19/0x40
 misc_register+0x140/0x180
 binder_init+0x1ed/0x2d4 [binder_linux]
 ? trace_event_define_fields_binder_transaction_fd_send+0x8e/0x8e [binder_linux]
 do_one_initcall+0x4a/0x1c9
 ? _cond_resched+0x19/0x40
 ? kmem_cache_alloc_trace+0x151/0x1c0
 do_init_module+0x5f/0x216
 load_module+0x223d/0x2b20
 __do_sys_finit_module+0xfc/0x120
 ? __do_sys_finit_module+0xfc/0x120
 __x64_sys_finit_module+0x1a/0x20
 do_syscall_64+0x5a/0x120
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7fbac3202839
Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 1f f6 2c 00 f7 d8 64 89 01 48
RSP: 002b:00007ffd1494a908 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 000055b629ebec60 RCX: 00007fbac3202839
RDX: 0000000000000000 RSI: 000055b629c20d2e RDI: 0000000000000003
RBP: 000055b629c20d2e R08: 0000000000000000 R09: 000055b629ec2310
R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000000
R13: 000055b629ebed70 R14: 0000000000040000 R15: 000055b629ebec60

So check for the empty string since strsep() will otherwise return the
emtpy string which will cause kobject_add_internal() to panic when trying
to add a kobject with an emtpy name.

Signed-off-by: Christian Brauner <[email protected]>

drivers/android/binder.c

@@ -5898,21 +5898,23 @@ static int __init binder_init(void)
 				    &transaction_log_fops);
 	}
 
-	/*
-	 * Copy the module_parameter string, because we don't want to
-	 * tokenize it in-place.
-	 */
-	device_names = kstrdup(binder_devices_param, GFP_KERNEL);
-	if (!device_names) {
-		ret = -ENOMEM;
-		goto err_alloc_device_names_failed;
-	}
+	if (strcmp(binder_devices_param, "") != 0) {
+		/*
+		* Copy the module_parameter string, because we don't want to
+		* tokenize it in-place.
+		 */
+		device_names = kstrdup(binder_devices_param, GFP_KERNEL);
+		if (!device_names) {
+			ret = -ENOMEM;
+			goto err_alloc_device_names_failed;
+		}
 
-	device_tmp = device_names;
-	while ((device_name = strsep(&device_tmp, ","))) {
-		ret = init_binder_device(device_name);
-		if (ret)
-			goto err_init_binder_device_failed;
+		device_tmp = device_names;
+		while ((device_name = strsep(&device_tmp, ","))) {
+			ret = init_binder_device(device_name);
+			if (ret)
+				goto err_init_binder_device_failed;
+		}
 	}
 
 	ret = init_binderfs();