Re: [RFC PATCH 1/2] VFS: Kill use of O_LARGEFILE inside the kernel

[David Howells]

Theodore Ts'o <tytso@xxxxxxx> wrote:

> So what this means is that on 32-bit systems, if we have a userspace
> program which isn't using the Largefile-enabled, and it opens a file
> which is larger than can be addressed with a 32-bit off_t, it can get
> surprised and possibly cause data loss.

Good point.  I was initially thinking that 32-bit userspace on a 64-bit system
would have O_LARGEFILE automatically enabled - but I guess it'll trap through
the compat entry points which avoid that.

That said, fanotify and xfs_open_by_handle() will both automatically set
O_LARGEFILE irrespectively of the 32-bitness of the original caller.

Further, path-based truncate() makes no checks based on file-largeness, unlike
ftruncate().

> Is this something we are willing to live with?  After all, there was a
> originally a really good reason for the O_LARGEFILE flag in the first
> place, and it was primarily about making sure that a non-LARGEFILE
> capable program would hard fail on the open, instead of after it had
> trashed the user's data.

Okay, that seems reasonable - but it still leaves truncate() dangling.  I'm
not sure there's a good answer to that, though.

> Was there a reason that motivated this change, other than just an
> clean up?

Overlayfs and one or two other places need to potentially apply O_LARGEFILE to
the things that they do on behalf of userspace - but other than suppressing
some size checks, it seems to be ignored by the filesystems and the VM.

I vaguely seem to remember that at one point there were still filesystems that
couldn't handle large files and would reject such opens - but they appear to
all have been fixed.

David
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