Re: [Y2038] [RFC 02/15] vfs: Change all structures to support 64 bit time

[Arnd Bergmann]

On Friday 15 January 2016 13:49:37 Dave Chinner wrote:
> On Thu, Jan 14, 2016 at 11:46:16PM +0100, Arnd Bergmann wrote:
> > On Friday 15 January 2016 08:00:01 Dave Chinner wrote:
> > > On Thu, Jan 14, 2016 at 05:53:21PM +0100, Arnd Bergmann wrote:
> > > > On Thursday 14 January 2016 08:04:36 Dave Chinner wrote:
> > 
> > Yes, that is the obvious case, and I guess works for at least half the
> > file systems when they always assign righthand side and lefthand
> > side of the time stamps using the external types or helpers like
> > CURRENT_TIME and current_fs_time().
> > 
> > However, there are a couple of file systems that need a bit more refactoring
> > before we can do this, e.g. in ntfs_truncate:
> 
> Sure, and nfs is a pain because of all it's internal use of
> timespecs, too.

lustre is probably the worst.

> But we have these timespec_to_timespec64 helper
> functions, and that's what we should use in these cases where the
> filesystem cannot support full 64 bit timestamps internally. In
> those cases, they'll be telling the superblock this at mount time
> things like current_fs_time() won't be returning then a timestamp
> that is out of range for a 32 bit timestamp....

I'm not worried about the runtime problems here, just how to
get a series of patches that are each doing one reasonable thing
at a time.

> >         if (!IS_NOCMTIME(VFS_I(base_ni)) && !IS_RDONLY(VFS_I(base_ni))) {
> >                 struct timespec now = current_fs_time(VFS_I(base_ni)->i_sb);
> >                 int sync_it = 0;
> > 
> >                 if (!timespec_equal(&VFS_I(base_ni)->i_mtime, &now) ||
> >                     !timespec_equal(&VFS_I(base_ni)->i_ctime, &now))
> >                         sync_it = 1;
> >                 VFS_I(base_ni)->i_mtime = now;
> >                 VFS_I(base_ni)->i_ctime = now;
> > 	}
> > 
> > The type of the local variable must match the return code of
> > current_fs_time(), so if we change over i_mtime and current_fs_time
> > globally, this either has to be rewritten first to avoid the use of
> > local variables, or it needs temporary conversion helpers, or
> > it has to be changed in the same patch. None of those is particularly
> > appealing. There are a few dozen such things in various file systems.
> 
> it gets rewritten to:
> 
> 		struct timespec now;
> 
> 		now = timespec64_to_timespec(current_fs_time(VFS_I(base_ni)->i_sb));
> ....
> 
> and the valid timestamp range for ntfs is set to 32bit timestamps.
> This then leaves it up to the filesystem developers to make
> the ntfs filesystem code 64 bit timestamp clean if the on disk
> format is ever changed to support 64 bit times.
> 
> Same goes for NFS, and any of the other filesystems that use struct
> timespec internally for time representation.

That is what I meant in my previous mail about approach c) being ugly
because it requires sprinkling lots of timespec64_to_timespec() and
timespec_to_timespec64() in the initial patch in order to atomically
change the types in inode/iattr/kstat/... without introducing build
regressions.

It's a rather horrible patch, and quite likely to cause conflicts with
other patches that introduce another use of those structures in the
merge window.

> > Having a global sysctl knob or
> > a compile-time option is better than having each file system
> > implementor take a guess at what users might prefer, if we can't
> > come up with a behavior (e.g. clamp all the time, or error out
> > all the time) that everybody agrees is always correct.
> 
> filesystem implementors will use the helper funtions that are
> provided for this. If they don't (like all the current use of
> CURRENT_TIME), then that's a bug that needs fixing.

Ok, then we are in total agreement here: the policy remains
to be decided by common code, but the implementation can differ
per file system.

> i.e. we need
> a timespec_clamp() function, similar to timespec_trunc(), and y2038k
> compliant filesystems and syscalls need to use them....

I was thinking we end up with a single function that does both
clamp() and trunk(), but that's an implementation detail.
> > > > Let me clarify what my idea is here: I want a global kernel option
> > > > that disables all code that has known y2038 issues. If anyone tries
> > > > to build an embedded system with support beyond 2038, that should
> > > > disable all of those things, including file systems, drivers and
> > > > system calls, so we can reasonably assume that everything that works
> > > > today with that kernel build will keep working in the future and
> > > > not break in random ways.
> > > 
> > > It's not that black and white when it comes to filesystems. y2038k
> > > support is determined by the on-disk structure of the filesystem
> > > being mounted, and that is determined at mount time.  When the
> > > filesystem mounts and sets it's valid timestamp ranges the VFS
> > > will need to decide as to whether the filesystem is allowed to
> > > continue mounting or not.
> > 
> > Some file systems are always broken around 2038 (e.g. HFS in 2040),
> > so if we can't fix them, I want to be able to turn them off
> > in Kconfig along with the 32-bit time_t syscalls.
> 
> That can be done with kconfig depends rules - it has nothing to do
> with this patch set.

kconfig dependencies is what I meant for the simple cases where a
file system is known to always be broken, we just need a small
modification for the cases you mentioned below.

> > ext2/3/4, xfs and ocfs2 (maybe one or two more, I'd have to check)
> > currently behave in a consistent manner across 32-bit and 64-bit
> > architectures by allowing a range between 1902 and 2037, and we
> > obviously don't have a choice there but to keep that current
> > behavior, and extend the time format in one way or another to
> > store additional bits for the epoch.
> 
> That's a filesystem implementation problem, not a generic inode
> timestamp problem. i.e. this is handled when the filesystem converts
> the inode timestamp from a timespec64 in the struct inode to
> whatever format it stores the timestamp on disk. That conversion
> does not change just because the VFS inode moves from a timespec to
> a timespec64. Again, those on-disk format changes to support beyond
> the current epoch are outside the scope of this patchset, because
> they are not affected by the timestamp format the VFS choses to use.

Fine with me, we can have another series to add the Kconfig dependencies
and modify the file systems that need this.

	Arnd