linux-kernel - Re: [RFC PATCH] fs: Add user_file_or_path

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Date:	Tue, 27 Aug 2013 16:13:12 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, Ingo Molnar <mingo@...nel.org>,
	"security@...nel.org" <security@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [RFC PATCH] fs: Add user_file_or_path_at and use it for truncate

On Tue, Aug 27, 2013 at 4:08 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Aug 27, 2013 at 12:16:34PM -0700, Andy Lutomirski 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).
>>
>> The basic idea is that there's a new helper function
>> user_file_or_path_at.  It takes an fd and a path and, depending on
>> flags, the emptiness of the path, and whether path is a magic /proc
>> symlink (or a symlink to a magic /proc/symlink), it returns either a
>> struct path or a struct file *.
>
> No.

I'm curious what's wrong with the general concept.  (I agree that the
implementation is heinous.)

If I ever wanted to add a new *at syscall, I'd like to be able to do:

int sys_whateverat(int dfd, const char __user *name, int flags)
{
  resolve_the_thing(dfd, name, flags);
  if (it's a file) {
    check fmode;
  } else {
    inode_permission();
  }

  actually_do_something(inode);

  unreference_whatever_i_got();

  return ret;
}

thereby killing the fwhatever and whateverat birds with one stone.


>
>> +     path_get(&nd->path);
>> +     if (nd->flags & LOOKUP_FILE) {
>> +             if (nd->last_symlink_file)
>> +                     fput(nd->last_symlink_file);
>> +             nd->last_symlink_file = file;
>
> This is ugly (and costs quite a bit of overhead)
>
>> -static int proc_cwd_link(struct dentry *dentry, struct path *path)
>> +static int proc_cwd_link(struct dentry *dentry, struct file_or_path *link)
>
> ... and this is even more vile.  Vetoed, for being too ugly to live.

...phew.  I wasn't looking forward to testing and debugging my crap :)

--Andy
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