[PATCH 2/2] mmc: block: Let MMC_IOC_MULTI_CMD return zero again for zero entries

From: Geert Uytterhoeven
Date: Wed Jul 05 2017 - 05:07:41 EST

With gcc 4.1.2:

drivers/mmc/core/block.c: In function âmmc_blk_ioctl_cmd_issueâ:
drivers/mmc/core/block.c:630: warning: âioc_errâ may be used uninitialized in this function

Indeed, if mq_rq->ioc_count is zero, an uninitialized value will be
stored in mq_rq->drv_op_result and passed to blk_end_request_all().

Can mq_rq->ioc_count be zero?
- mmc_blk_ioctl_cmd() sets ioc_count to 1, so this is safe,
- mmc_blk_ioctl_multi_cmd() obtains ioc_count from user space in
response to the MMC_IOC_MULTI_CMD ioctl, and does allow zero.

To avoid returning an uninitialized value, and as it is pointless to do
all this work when the MMC_IOC_MULTI_CMD ioctl is used with zero
entries, check for this early in mmc_blk_ioctl_multi_cmd(), and return
zero, like was returned before.

Fixes: 3ecd8cf23f88d5df ("mmc: block: move multi-ioctl() to use block layer")
Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Should mmc_blk_ioctl_multi_cmd() return -EINVAL instead for this case?
That would change behavior as seen from userspace, though.

Feel free to fold in the previous patch if deemed appropriate.
drivers/mmc/core/block.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
index 9472e89af3efa37e..b236d345d53291f0 100644
--- a/drivers/mmc/core/block.c
+++ b/drivers/mmc/core/block.c
@@ -637,6 +637,9 @@ static int mmc_blk_ioctl_multi_cmd(struct block_device *bdev,
return -EFAULT;

+ if (!num_of_cmds)
+ return 0;
if (num_of_cmds > MMC_IOC_MAX_CMDS)
return -EINVAL;