i2c-dev: Add support for I2C_M_RECV_LEN

As the bus driver side implementation of I2C_M_RECV_LEN is heavily
tied to SMBus, we can't support received length over 32 bytes, but
let's at least support that.

In practice, the caller will have to setup a buffer large enough to
cover the case where received length byte has value 32, so minimum
32 + 1 = 33 bytes, possibly more if there is a fixed number of bytes
added for the specific slave (for example a checksum.)

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>

1 files changed, 26 insertions, 4 deletions

diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c
index 45048323b75e..5ec2261574ec 100644
--- a/drivers/i2c/i2c-dev.c
+++ b/drivers/i2c/i2c-dev.c
@@ -265,19 +265,41 @@ static noinline int i2cdev_ioctl_rdrw(struct i2c_client *client,
 
         res = 0;
         for (i = 0; i < rdwr_arg.nmsgs; i++) {
-                /* Limit the size of the message to a sane amount;
-                 * and don't let length change either. */
-                if ((rdwr_pa[i].len > 8192) ||
-                    (rdwr_pa[i].flags & I2C_M_RECV_LEN)) {
+                /* Limit the size of the message to a sane amount */
+                if (rdwr_pa[i].len > 8192) {
                         res = -EINVAL;
                         break;
                 }
+
                 data_ptrs[i] = (u8 __user *)rdwr_pa[i].buf;
                 rdwr_pa[i].buf = memdup_user(data_ptrs[i], rdwr_pa[i].len);
                 if (IS_ERR(rdwr_pa[i].buf)) {
                         res = PTR_ERR(rdwr_pa[i].buf);
                         break;
                 }
+
+                /*
+                 * If the message length is received from the slave (similar
+                 * to SMBus block read), we must ensure that the buffer will
+                 * be large enough to cope with a message length of
+                 * I2C_SMBUS_BLOCK_MAX as this is the maximum underlying bus
+                 * drivers allow. The first byte in the buffer must be
+                 * pre-filled with the number of extra bytes, which must be
+                 * at least one to hold the message length, but can be
+                 * greater (for example to account for a checksum byte at
+                 * the end of the message.)
+                 */
+                if (rdwr_pa[i].flags & I2C_M_RECV_LEN) {
+                        if (!(rdwr_pa[i].flags & I2C_M_RD) ||
+                            rdwr_pa[i].buf[0] < 1 ||
+                            rdwr_pa[i].len < rdwr_pa[i].buf[0] +
+                                             I2C_SMBUS_BLOCK_MAX) {
+                                res = -EINVAL;
+                                break;
+                        }
+
+                        rdwr_pa[i].len = rdwr_pa[i].buf[0];
+                }
         }
         if (res < 0) {
                 int j;