usblib: fix race & delay waiting for perms on open
in usb_device_open, if permission is denied for a USB
device node, the current code retries read-only, then sleeps
for a second before retrying.
If the permission was changed to +rw between the two file opens,
the device could be inadvertently opened read-only.
Also, change the polling interval to 100ms. 1s is a long wait
for a function on the critical path of a user interaction.
Bug: 68337205
Bug: 68782236
Test: with debug messages enabled, connected & disconnected
a USB audio headset. Saw the writeable file descriptor
returned.
Change-Id: I06048cc2c09bf6ed1abada5d12b5559be768fbaf
diff --git a/libusbhost/usbhost.c b/libusbhost/usbhost.c
index 44b878d..4d286bf 100644
--- a/libusbhost/usbhost.c
+++ b/libusbhost/usbhost.c
@@ -324,29 +324,39 @@
struct usb_device *usb_device_open(const char *dev_name)
{
- int fd, did_retry = 0, writeable = 1;
-
+ int fd, attempts, writeable = 1;
+ const int SLEEP_BETWEEN_ATTEMPTS_US = 100000; /* 100 ms */
+ const int64_t MAX_ATTEMPTS = 10; /* 1s */
D("usb_device_open %s\n", dev_name);
-retry:
- fd = open(dev_name, O_RDWR);
- if (fd < 0) {
- /* if we fail, see if have read-only access */
- fd = open(dev_name, O_RDONLY);
- D("usb_device_open open returned %d errno %d\n", fd, errno);
- if (fd < 0 && (errno == EACCES || errno == ENOENT) && !did_retry) {
- /* work around race condition between inotify and permissions management */
- sleep(1);
- did_retry = 1;
- goto retry;
+ /* Hack around waiting for permissions to be set on the USB device node.
+ * Should really be a timeout instead of attempt count, and should REALLY
+ * be triggered by the perm change via inotify rather than polling.
+ */
+ for (attempts = 0; attempts < MAX_ATTEMPTS; ++attempts) {
+ if (access(dev_name, R_OK | W_OK) == 0) {
+ writeable = 1;
+ break;
+ } else {
+ if (access(dev_name, R_OK) == 0) {
+ /* double check that write permission didn't just come along too! */
+ writeable = (access(dev_name, R_OK | W_OK) == 0);
+ break;
+ }
}
-
- if (fd < 0)
- return NULL;
- writeable = 0;
- D("[ usb open read-only %s fd = %d]\n", dev_name, fd);
+ /* not writeable or readable - sleep and try again. */
+ D("usb_device_open no access sleeping\n");
+ usleep(SLEEP_BETWEEN_ATTEMPTS_US);
}
+ if (writeable) {
+ fd = open(dev_name, O_RDWR);
+ } else {
+ fd = open(dev_name, O_RDONLY);
+ }
+ D("usb_device_open open returned %d writeable %d errno %d\n", fd, writeable, errno);
+ if (fd < 0) return NULL;
+
struct usb_device* result = usb_device_new(dev_name, fd);
if (result)
result->writeable = writeable;