[PATCH v2] gpiolib: Fix line event handling in syscall compatible mode

Andy Shevchenko
2020-09-14T14:38:46Z

The introduced line even handling ABI in the commit

61f922db7221 ("gpio: userspace ABI for reading GPIO line events")

missed the fact that 64-bit kernel may serve for 32-bit applications.
In such case the very first check in the lineevent_read() will fail
due to alignment differences.

To workaround this introduce lineeven_to_user() helper which returns actual
size of the structure and copies its content to user if asked.

Fixes: 61f922db7221 ("gpio: userspace ABI for reading GPIO line events")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
v2: moved to just calculate size (Arnd, Kent), added good comment (Arnd)
drivers/gpio/gpiolib-cdev.c | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index e6c9b78adfc2..95af4a470f1e 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -423,6 +423,21 @@ static __poll_t lineevent_poll(struct file *file,
return events;
}

+static ssize_t lineevent_get_size(void)
+{
+#ifdef __x86_64__
+ /* i386 has no padding after 'id' */
+ if (in_ia32_syscall()) {
+ struct compat_gpioeevent_data {
+ compat_u64 timestamp;
+ u32 id;
+ };
+
+ return sizeof(struct compat_gpioeevent_data);
+ }
+#endif
+ return sizeof(struct gpioevent_data);
+}

static ssize_t lineevent_read(struct file *file,
char __user *buf,
@@ -432,9 +447,20 @@ static ssize_t lineevent_read(struct file *file,
struct lineevent_state *le = file->private_data;
struct gpioevent_data ge;
ssize_t bytes_read = 0;
+ ssize_t ge_size;
int ret;

- if (count < sizeof(ge))
+ /*
+ * When compatible system call is being used the struct gpioevent_data,
+ * in case of at least ia32, has different size due to the alignment
+ * differences. Because we have first member 64 bits followed by one of
+ * 32 bits there is no gap between them. The only problematic is the
+ * padding at the end of the data structure. Hence, we calculate the
+ * actual sizeof() and pass this as an argument to copy_to_user() to
+ * drop unneeded bytes from the output.
+ */
+ ge_size = lineevent_get_size();
+ if (count < ge_size)
return -EINVAL;

do {
@@ -470,10 +496,10 @@ static ssize_t lineevent_read(struct file *file,
break;
}

- if (copy_to_user(buf + bytes_read, &ge, sizeof(ge)))
+ if (copy_to_user(buf + bytes_read, &ge, ge_size))
return -EFAULT;
- bytes_read += sizeof(ge);
- } while (count >= bytes_read + sizeof(ge));
+ bytes_read += ge_size;
+ } while (count >= bytes_read + ge_size);

return bytes_read;
}
--
2.28.0