linux-kernel - Re: build_path_from_dentry_optional_prefix() may schedule from invalid context

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Message-ID: <20191209003413.GY4203@ZenIV.linux.org.uk>
Date:   Mon, 9 Dec 2019 00:34:13 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Pavel Shilovsky <piastryyy@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steve French <stfrench@...rosoft.com>,
        Ronnie Sahlberg <lsahlber@...hat.com>,
        linux-cifs <linux-cifs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: build_path_from_dentry_optional_prefix() may schedule from
 invalid context

On Mon, Sep 30, 2019 at 10:32:16AM -0700, Pavel Shilovsky wrote:
> сб, 21 сент. 2019 г. в 15:38, Al Viro <viro@...iv.linux.org.uk>:

> > IOW, kindly lose that nonsense.  More importantly, why bother
> > with that kmalloc()?  Just __getname() in the very beginning
> > and __putname() on failure (and for freeing the result afterwards).
> >
> > What's more, you are open-coding dentry_path_raw(), badly.
> > The only differences are
> >         * use of dirsep instead of '/' and
> >         * a prefix slapped in the beginning.
> >
> > I'm fairly sure that
> >         char *buf = __getname();
> >         char *s;
> >
> >         *to_free = NULL;
> >         if (unlikely(!buf))
> >                 return NULL;
> >
> >         s = dentry_path_raw(dentry, buf, PATH_MAX);
> >         if (IS_ERR(s) || s < buf + prefix_len)
> >                 __putname(buf);
> >                 return NULL; // assuming that you don't care about details
> >         }
> >
> >         if (dirsep != '/') {
> >                 char *p = s;
> >                 while ((p = strchr(p, '/')) != NULL)
> >                         *p++ = dirsep;
> >         }
> >
> >         s -= prefix_len;
> >         memcpy(s, prefix, prefix_len);
> >
> >         *to_free = buf;
> >         return s;
> >
> > would end up being faster, not to mention much easier to understand.
> > With the caller expected to pass &to_free among the arguments and
> > __putname() it once it's done.
> >
> > Or just do __getname() in the caller and pass it to the function -
> > in that case freeing (in all cases) would be up to the caller.
> 
> Thanks for pointing this out. Someone should look at this closely and
> clean it up.

Could you take a look through vfs.git#misc.cifs?