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usblib: fix race & delay waiting for perms on open

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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
This commit is contained in:
Andrew Chant 2017-11-01 17:32:35 -07:00
parent dbef1eeb45
commit 3af9e40c66
1 changed files with 28 additions and 18 deletions
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46
libusbhost/usbhost.c
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@ -324,29 +324,39 @@ void usb_host_run(struct usb_host_context *context,
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;