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

[Theodore Ts'o <tytso@xxxxxxx>]

On Tue, Sep 22, 2015 at 05:12:42PM +0100, David Howells wrote:
> 
> Further, path-based truncate() makes no checks based on file-largeness, unlike
> ftruncate().

Right, but truncate in general is used to make files *smaller* so I'm
having trouble thinking of a scenario where a largefile-oblivious
program could get in trouble by truncating a file > 2TB to some
hard-coded length (normally zero).

> 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.

The size checks really were the primary points of O_LARGEFILE.  As I
recall the primary system calls where this really matters is open(2)
and stat(2) (since if st_size is too small to represent the size of
the file, then the user space program could get really confused).

Essentially O_LARGEFILE is an assertion that userspace can handle
handle 64-bit files, and won't get confused by system call interfaces
where off_t is 32-bit wide, because it will use the 64-bit variants.
So it's not at all surprising that the file systems and the VM in
general doesn't need to worry about the flag.

                                      - Ted