Re: [PATCH PING] VFS: mount must return EACCES, not EROFS

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 3 Jul 2014 18:29:19 +0200 Philippe De Muyter <phdm@macqel.be> wrote:

> On Wed, Jul 02, 2014 at 12:46:51PM -0700, Andrew Morton wrote:
> > On Fri, 27 Jun 2014 10:20:58 +0200 Philippe De Muyter <phdm@macqel.be> wrote:
> > 
> > > Currently, the initial mount of the root file system by the linux
> > > kernel fails with a cryptic message instead of being retried with
> > > the MS_RDONLY flag set,  when the device is read-only and the
> > > combination of block driver and filesystem driver yields EROFS.
> > > 
> > > I do not know if POSIX mandates that mount(2) must fail with EACCES, nor
> > > if linux aims to strict compliance with POSIX on that point.  Consensus
> > > amongst the messages that I have read so far seems to show that linux
> > > kernel hackers feel that EROFS is a more appropriate error code than
> > > EACCES in that case.
> > 
> > Isn't the core problem that "the combination of block driver and
> > filesystem driver yields EROFS"?  That the fs should instead be
> > returning EACCESS in this case?
> 
> Does POSIX or Linux mandate that it should ?
> 
> > 
> > What fs and block driver are we talking about here, anyway?
> 
> The problem happened to me with a f2fs filesystem on a sd-card that was
> accidentally write-protected and that was put in a SD-card slot (mmc block
> driver).
> 
> I retested using mount(8) with a similar now intentionnaly write-protected
> sd card in a usb reader (usb_storage driver ?) with vfat, f2fs and ext4
> filesystems with the following results :
> 
>   mywdesk:~ # strace -e mount mount /dev/sdb1 /mnt
>   mount("/dev/sdb1", "/mnt", "vfat", MS_MGC_VAL, NULL) = -1 EROFS (Read-only file system)
>   mount: /dev/sdb1 is write-protected, mounting read-only
>   mount("/dev/sdb1", "/mnt", "vfat", MS_MGC_VAL|MS_RDONLY, NULL) = 0
>   +++ exited with 0 +++
>   mywdesk:~ # umount /mnt
>   mywdesk:~ # strace -e mount mount -t f2fs /dev/sdb2 /mnt
>   mount("/dev/sdb2", "/mnt", "f2fs", MS_MGC_VAL, NULL) = -1 EROFS (Read-only file system)
>   mount: /dev/sdb2 is write-protected, mounting read-only
>   mount("/dev/sdb2", "/mnt", "f2fs", MS_MGC_VAL|MS_RDONLY, NULL) = 0
>   +++ exited with 0 +++
>   mywdesk:~ # umount /mnt
>   mywdesk:~ # strace -e mount mount /dev/sdb3 /mnt
>   mount("/dev/sdb3", "/mnt", "ext4", MS_MGC_VAL, NULL) = -1 EROFS (Read-only file system)
>   mount: /dev/sdb3 is write-protected, mounting read-only
>   mount("/dev/sdb3", "/mnt", "ext4", MS_MGC_VAL|MS_RDONLY, NULL) = 0
>   +++ exited with 0 +++
>   mywdesk:~ #
> 
> All three file-systems (vfat, f2fs & ext4) yield EROFS.
> 
> I also quickly grepped for occurences of EROFS under fs/, and found no check
> to replace EROFS by EACCES,
> while the same grep under drivers/{block,cdrom,ide,md,memstick, mtd,
> s390/block,scsi,usb} gives plenty of "return -EROFS;"
> 
> So, if no filesystem driver replaces EROFS by EACCES and many block drivers
> return EROFS, it seems to me that many combinations will yield EROFS.

hm.  I'm thinking that in an ideal world, those block drivers will
return -EACCES rather than -EROFS.  You open() a read-only device for
rw, you get "permission denied".

Al, speak to us.