Re: [PATCH] ovl: Allow mount options to be parsed on remount

["André Almeida" <andrealmeid@igalia.com>]

Hi Christian, Amir,

Thanks for the feedback :)

Em 21/05/2025 08:20, Christian Brauner escreveu:
> On Wed, May 21, 2025 at 12:35:57PM +0200, Amir Goldstein wrote:
>> On Wed, May 21, 2025 at 8:45 AM André Almeida <andrealmeid@igalia.com> wrote:
>>>

[...]

>>
>> I see the test generic/623 failure - this test needs to be fixed for overlay
>> or not run on overlayfs.
>>
>> I do not see those other 5 failures although before running the test I did:
>> export LIBMOUNT_FORCE_MOUNT2=always
>>
>> Not sure what I am doing differently.
>>

I have created a smaller reproducer for this, have a look:

  mkdir -p ovl/lower ovl/upper ovl/merge ovl/work ovl/mnt
  sudo mount -t overlay overlay -o lowerdir=ovl/lower,upperdir=ovl/ 
upper,workdir=ovl/work ovl/mnt
  sudo mount ovl/mnt -o remount,ro

And this returns:

  mount: /tmp/ovl/mnt: fsconfig() failed: overlay: No changes allowed in 
  reconfigure.
        dmesg(1) may have more information after failed mount system call.

However, when I use mount like this:

  sudo mount -t overlay overlay -o remount,ro ovl/mnt

mount succeeds. Having a look at strace, I found out that the first 
mount command tries to set lowerdir to "ovl/lower" again, which will to 
return -EINVAL from ovl_parse_param():

    fspick(3, "", FSPICK_NO_AUTOMOUNT|FSPICK_EMPTY_PATH) = 4
    fsconfig(4, FSCONFIG_SET_STRING, "lowerdir", "/tmp/ovl/lower", 0) = 
-1 EINVAL (Invalid argument)

Now, the second mount command sets just the "ro" flag, which will return 
after vfs_parse_sb_flag(), before getting to ovl_parse_param():

    fspick(3, "", FSPICK_NO_AUTOMOUNT|FSPICK_EMPTY_PATH) = 4
    fsconfig(4, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0

After applying my patch and running the first mount command again, we 
can set that this flag is set only after setting all the strings:

    fsconfig(4, FSCONFIG_SET_STRING, "lowerdir", "/tmp/ovl/lower", 0) = 0
    fsconfig(4, FSCONFIG_SET_STRING, "upperdir", "/tmp/ovl/upper", 0) = 0
    fsconfig(4, FSCONFIG_SET_STRING, "workdir", "/tmp/ovl/work", 0) = 0
    fsconfig(4, FSCONFIG_SET_STRING, "uuid", "on", 0) = 0
    fsconfig(4, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0

I understood that the patch that I proposed is wrong, and now I wonder 
if the kernel needs to be fixed at all, or if the bug is how mount is 
using fsconfig() in the first mount command?

Thanks,
	André