On 08/01/2012 11:48 AM, Karel Zak wrote:
> On Wed, Aug 01, 2012 at 11:11:41AM +0200, Niels de Vos wrote:
>> it seems that some of the options given in /etc/fstab are not overloaded by
>> options given on the mount-commandline.
>
> Well, all depends on kernel (and FS driver) how mount options are
> evaluated. We have no any strict rules for 'source' and 'mount
> options' in the mount(2) syscall.
>
> The generic convention (!= rule) is that the option overwrites an
> option previously specified in the mount options string:
>
> aaa=x,bbb,aaa=y -> aaa should be 'y'
>
> This is reason why mount(8) only appends the options from command line
> to the options string from fstab.
>
> Anyway, depends on FS driver how the options string will be
> interpreted -- for example btrfs supports duplicate device= mount
> option.
Ah, okay, if there are valid use-cases for options to be listed multiple
times, it won't be possible to overload the options from /etc/fstab by the
options from the commandline.
>> Some options are not allowed to be
>> passed multiple times (like the SElinux context options) and mounting will fail
>> if options are present in both /etc/fstab and on the commandline.
>
> This is stupid SELinux disadvantage and SELinux should be fixed.
Yeah, sounds like it. I may have a look on how the kernel can handle the
SElinux options better.
>> Is there a reason for not overloading the options from /etc/fstab by the
>> options given on the commandline?
>>
>> The patch below changes the behaviour of mount so that options on the
>> commandline replace options given in /etc/fstab.
>
> I'd like to avoid that mount(8) tries to understand filesystem specific
> options. The command mount(8) has no clue about the options (non-VFS
> options). For example:
>
> fstab:
> /dev/sda /mnt btrfs device=/dev/sdb 0 0
>
> command line:
>
> mount /mnt -o device=/dev/sdc,device=/dev/sdd
>
>
> so the final mount options string:
>
> device=/dev/sdb,device=/dev/sdc,device=/dev/sdd
>
> .. and this all is pretty valid for btrfs.
Nice example!
> I'd like to implement for the next release 2.23 a new command line
> options to specify a way how compose the final options string in the
> mount(8). It's already in our Documentation/TODO file, see
> --options-mode, probably something like:
>
> --options-mode={ignore,append,prepend,replace,dedup}
>
> or so.
Hmm, that sounds like it'll solve the problem, unless you'r combining
SElinux contexts and btrsf...
Thanks for your detailed response,
Niels
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