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Date:   Wed, 18 Jul 2018 21:04:11 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] call_with_creds()
Message-ID: <20180718200411.GW30522@ZenIV.linux.org.uk>
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On Wed, Jul 18, 2018 at 12:53:48PM -0700, Linus Torvalds wrote:
> On Wed, Jul 18, 2018 at 12:46 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Huh?  Nevermind ->write(), what about open()?
> 
> What about open?
> 
> At open time, file->f_cred is the same as current_cred().

int cachefiles_write_page(struct fscache_storage *op, struct page *page)
{
...
        file = dentry_open(&path, O_RDWR | O_LARGEFILE, cache->cache_cred);


int ecryptfs_privileged_open(struct file **lower_file,
                             struct dentry *lower_dentry,
                             struct vfsmount *lower_mnt,
                             const struct cred *cred)
...
        (*lower_file) = dentry_open(&req.path, flags, cred);


/* Derived from fs/exec.c:flush_old_files. */
static inline void flush_unauthorized_files(const struct cred *cred,
                                            struct files_struct *files)
...
        devnull = dentry_open(&selinux_null, O_RDWR, cred);

(granted, here we don't care much, /dev/null being what it is)

In mainline:
struct file *filp_clone_open(struct file *oldfile)
{
...
        retval = vfs_open(&oldfile->f_path, file, oldfile->f_cred);