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

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Message-Id: <1499245637-28870-2-git-send-email-geert@linux-m68k.org>
Date:   Wed,  5 Jul 2017 11:07:17 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-mmc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: [PATCH 2/2] mmc: block: Let MMC_IOC_MULTI_CMD return zero again for zero entries

With gcc 4.1.2:

    drivers/mmc/core/block.c: In function ‘mmc_blk_ioctl_cmd_issue’:
    drivers/mmc/core/block.c:630: warning: ‘ioc_err’ 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@...ux-m68k.org>
---
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,
 			   sizeof(num_of_cmds)))
 		return -EFAULT;
 
+	if (!num_of_cmds)
+		return 0;
+
 	if (num_of_cmds > MMC_IOC_MAX_CMDS)
 		return -EINVAL;
 
-- 
2.7.4