Re: [RFC PATCH] fs: Add user_file_or_path_at and use it for truncate

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Aug 27, 2013 at 12:32 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Aug 27, 2013 at 12:16 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> This is an experiment to see if we can get nice semantics for all syscalls
>> that either follow symlinks or allow AT_EMPTY_PATH without jumping through
>> enormous hoops.  This converts truncate (although you can't tell using
>> truncate from coreutils, because it actually uses open + ftruncate).
>
> So this seems *way* too complex. I'd much rather see "nd->flags" get a
> LOOKUP_READONLY flag, for example, that gets set by
> proc_pid_follow_link() when it hits a read-only file descriptor (and
> gets cleared by other lookups).
>
> Wouldn't that be *much* more straightforward?

It would if it works.  It certainly would for truncate, setxattr, etc.

There are funny cases, though.  For example, execing an O_WRONLY fd
should probably fail, as should opening an O_WRONLY fd as O_RDWR.
O_APPEND is also funny.  flink will (I suspect) always want to be a
bit special.

There are also O_PATH fds, and I'm not sure what the semantics of
O_PATH fds are or should be when they refer to something other than a
directory.

The benefit of my approach is that it's really obvious that
truncate("/proc/self/fd/N") and ftruncate(N) do exactly the same
thing.  The downside is that the namei code is a bit gross and there
was more rearranging of ftruncate than I would have liked.  (I also
lost the benefit of fget_light, but I could fix that by passing a
struct fd around instead of a struct file *.)

--Andy