Re: [PATCH] Make file struct available to fchmod FS handlers.

[Nikolaus Rath <Nikolaus@rath.org>]

On Nov 03 2016, Al Viro <viro@ZenIV.linux.org.uk> wrote:
> On Wed, Nov 02, 2016 at 11:53:40PM +0100, Jose Lopes wrote:
>> Syscall 'ftruncate' makes the 'file' struct available to filesystem
>> handlers. This makes it possible, e.g., for filesystems, such as,
>> FUSE, to access the file handle associated with the file descriptor
>> that was passed to 'ftruncate'. In the specific case of FUSE, this
>> also makes it possible for (userspace) FUSE-based filesystems to
>> distinguish between calls to 'truncate' and 'ftruncate'.
>
> Why FUSE is such a precious snowflake that it needs to make that distinction,
> unlike all other filesystems?

FUSE filesystems are often used as an extra layer on top of another
filesystem (that stores the actual data). If the user opens a file (in
the fuse filesystem), deletes it, and then truncates it, FUSE currently
cannot do the same operation in the underlying filesystem: it receives
a truncate() call with the inode, but there is no syscall that allows
truncation for an inode. If FUSE had access to the file handle, it can
use that to store a file descriptor for the file on the underlying
filesystem and use ftruncate.

Best,
-Nikolaus

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