Re: read-only partitions: does the policy apply to all metadata ?

[Robert Hancock <hancockrwd@gmail.com>]

porte64@free.fr wrote:
> Hello,
> 
> When a partition is mounted as read-only, does the kernel
> really prevent ANY byte to be written to it, including
> any metadata ?
> 
> (i am thinking here about file attributes -- last access time,
> as well as meta information about filesystem/blocks
> description etc ...).

As far as I know any of the visible attributes (last access, etc.) 
cannot be modified. However, mounting read-only may still write to the 
filesystem if the block device is writable, for example with ext3/ext4 
if the file system was not unmounted cleanly last time and the journal 
needs to be replayed.

> 
> And is the policy implemented in every filesystem type driver
> or at some abstraction level (as if the write() and like
> system calls were designed to return an error).
> 
> Unfortunately, on common (all?) hard drives, there seems
> to be no switch to set the device microcode in read-only
> mode.
> 
> By the way, this makes me also thing about memory cards
> with locks: is it a real protection or it is just a setting
> which tells the kernel that it *SHOULD* mount the device
> read-only ?

Normally the card reader indicates to the OS that the device is 
read-only, and then the kernel will mount it only read-only. However, 
note that some crappy SD card readers don't implement the write protect 
switch detection and will allow writing to a card marked read-only.

> 
> Please include my private address if you answer; i posted to the
> list because in some other forums we did not come up
> with a clear answer, so this is my last chance to get a
> definite done. However i didn't subscribe to the list as
> i have realized i am not able to help unfortunately:
> operating systems are the most complex thing ever design by humans !
> 
> Best regards
> Phil