On Fri, May 19 2017, Karel Zak wrote:
> On Fri, May 19, 2017 at 05:14:24PM +1000, NeilBrown wrote:
>> Suppose /foo and /bar are bind mounts to the same
>> filesystem which is currently mounted read-only,
>> and suppose the /etc/fstab contains
>>
>> /dev/sda1 /bar ext4 defaults 0 1
>> /bar /foo none bind 0 0
>>
>> Now if I want /foo to be writeable I might try:
>>
>> mount /foo -o remount,rw
>>
>> and would then be surprised that this doesn't work.
>>
>> What is happening is that because only one path has been given, mount
>> needs to find the other and goes looking in /etc/fstab.
>
> Frankly remount & fstab sucks.
:-)
>
> The problem is that we want to read fstab because remount works as
> "replace all old options with new options", so if you don't read
> fstab then all options unspecified on command line will be removed.
>
> Let's imagine your command line is "-o remount,ro", but you have also
> "noexec" in your fstab. If you don't read fstab then "noexec" will be
> removed by the remount operation. This is unexpected by many users.
> They want to change only specified options (ro->rw, etc).
>
> It would be possible to read /proc/self/mountinfo on remount (to get
> old options), but unfortunately this is not backwardly compatible,
> because we have users who call
>
> mount /mnt -o remount
>
> (yes, without another options) and they assume that all options will
> be reset according to fstab.
I can see how the need to support that usage make it difficult to ignore
fstab.
>
>
> *** See man mount: ***
>
> The remount functionality follows the standard way the mount command
> works with options from fstab. This means that mount does not read
> fstab (or mtab) only when both device and dir are specified.
>
> mount -o remount,rw /dev/foo /dir
>
> After this call all old mount options are replaced and arbitrary stuff
> from fstab (or mtab) is ignored, except the loop= option which is
> internally generated and maintained by the mount command.
oooh... it just occurred to me that for a mount, I can make it
read/write by:
mount -o remount,rw none /mount/point
providing I know that no other flags are needed. I don't need to give
the correct device path, do I?
>
> mount -o remount,rw /dir
>
> After this call, mount reads fstab and merges these options with the
> options from the command line (-o). If no mountpoint is found in
> fstab, then a remount with unspecified source is allowed.
BTW, if fstab contains just
/bar /foo none bind 0 0
Then both
mount -o remount,rw /bar
and
mount -o remount,rw /foo
will try to remount /foo. This is also surprising....
Normally you would have an entry for "/bar" listed earlier, and that
would be used for "mount -o remount,rw /bar". So it probably isn't a
real problem.
>
>> If finds an appropriate line and parses out the options.
>> Then the mount system call used is
>>
>> mount( "/bar", "/foo", ..., MS_REMOUNT | MS_BIND ,....)
>>
>> This changes the per-mountpoint ro flag to rw, but doesn't change the
>> filesystem itself. This can be seen in /proc/self/mountinfo. There
>> are two r[wo] flags, and they are different.
>>
>> Had I run:
>>
>> mount /foo -o remount,bind,rw
>>
>> I would have expected this. But as I didn't explicitly ask for "bind",
>> it is confusing.
>>
>> I think it might be good to ignore "bind" in /etc/fstab when "remount"
>> is used.
>
> Another exception :-) I'll think about it, maybe it's not so bad idea
> because "remount,bind" is very special.
>
>> However.... when "remount" is used, the "device" is ignored, so there
>> isn't a lot of point hunting through /etc/fstab to find it.
>
> Yes, but then you have to specify all options on command line,
> unspecified will be removed by remount op.
>
> We have in TODO for years
>
> - add options to control fstab/mtab mount options usage, something like:
>
> --options-mode={ignore,append,prepend,replace} MNT_OMODE_{IGNORE, ...}
> --options-source={fstab,mtab,disable} MNT_OMODE_{FSTAB,MTAB,NOTAB}
> --options-source-force MNT_OMODE_FORCE
>
>
> all this already supported by libmount, but no exported to the
> mount(8) command line.
>
> Maybe I should implement it :)
Maybe. I'm currently focused on the default behaviour being a bit
confusing and this wouldn't help with that.
While the functionality that these flags provide seems sensible,
I do wonder if it would actually get used.
>
>> If mount is given just one path and the "remount" option, then maybe
>> it shouldn't try to find an /etc/fstab entry at all?
>> I guess you might want to remount a device??
>> Is
>> mount -o remount,rw /dev/sda1
>> allowed? In that case, don't look through /etc/fstab if the path
>> is a directory.
>>
>> I haven't provided a patch because I'm not 100% sure what the best
>> approach would be.
>> Does anyone have opinions on what mount should or shouldn't do when
>> remounting and only one path is given?
>>
>> You could possibly argue that the current behaviour is correct
>> as /foo is listed as bind mount. That doesn't stop is being surprising.
>> Also
>> mount /bar /foo -o remount,rw
>>
>> doesn't pick up the "bind" flag, so the filesystem gets remounted.
>> So I think there is definitely something wrong.
>
> That's question... "remount,bind" is really special and maybe "bind"
> from fstab should be really ignored in this case.
I think that is probably the best way forward, at least in the short
term.
I might try to dig into the code and see how easy this would be.
Thanks for your detailed response!
NeilBrown
>
> Karel
>
> --
> Karel Zak <kzak@xxxxxxxxxx>
> http://karelzak.blogspot.com
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