[PATCH 1/1] loop device: report mount failure in case of set_loop failure

Denys Vlasenko vda.linux at googlemail.com
Wed Dec 9 19:53:19 UTC 2020


On Wed, Dec 9, 2020 at 10:19 AM Thomas De Schampheleire
<patrickdepinguin at gmail.com> wrote:
> From: Philippe Belet <philippe.belet at nokia.com>
>
> When mounting, in parallel, multiple loop devices (squashfs for the
> submitter's case), the following behavior can be observed:
>
> stat64(/path/to/image, {st_mode=S_IFREG|0644, st_size=4096, ...}) = 0
> openat(AT_FDCWD, /path/to/image, O_RDWR|O_LARGEFILE) = 3
> openat(AT_FDCWD, /dev/loop-control, O_RDWR|O_LARGEFILE|O_CLOEXEC) = 4
> ioctl(4, LOOP_CTL_GET_FREE)             = 12
> close(4)                                = 0
> openat(AT_FDCWD, /dev/loop12, O_RDWR|O_LARGEFILE) = 4
> ioctl(4, LOOP_GET_STATUS64, {lo_offset=0, lo_number=12, lo_flags=LO_FLAGS_AUTOCLEAR, lo_file_name=/path/to/image, ...}) = 0
> close(4)                                = 0
> close(3)                                = 0
> write(2, "mount: can't setup loop device\n", 31mount: can't setup loop device
> ) = 31
> exit_group(0)                           = ?
> +++ exited with 0 +++
>
> The ioctl LOOP_CTL_GET_FREE has resulted in the same result for
> a competing mount process. The subsequent ioctl LOOP_GET_STATUS64
> fails, having succeeded for the competing mount process. Next we
> see a series of other system calls.
>
> The mount code checks the errno value of the last close()
> syscall

No it does not. errno is not cleared to zero on successful syscalls,
it's left untouched.

Does just this change:

-                               return errno;
+                               return loopfd;

fix the problem?


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