Re: [PATCH] vfs: avoid recopying filename in getname_flags

[Paul Moore <pmoore@redhat.com>]

On Monday, March 09, 2015 04:24:32 PM Boqun Feng wrote:
> Ping.
> Any opinion?

You might want to look at some of the recent changes to Al's vfs.git#for-next 
branch; at the very least it looks like your patch should be rebased against 
those changes.

> On Wed, Feb 25, 2015 at 8:31 PM, Boqun Feng <boqun.feng@gmail.com> wrote:
> > In the current implementation of getname_flags, filename in the
> > user-space will be recopied if it takes more space that
> > EMBEDDED_NAME_MAX, however, at this moment, EMBEDDED_NAME_MAX bytes of
> > the filename are already copied into kernel space, the only reason why
> > the recopy is needed is that "kname" needs to be relocated.
> > 
> > And the recopy can be avoided if we change the memory layout of the
> > "names_cache" allocation. By putting the struct "filename" at the tail
> > of the allocation instead of the head, relocation of kname is avoided.
> > 
> > Once putting the struct at the tail, each byte in the user space will be
> > copied only one time, so the recopy is avoided and code is more clear.
> > 
> > Of course, other functions aware of the layout of the names_cache
> > allocation, i.e., getname_kernel and putname also need to be modified to
> > adapt to the new layout.
> > 
> > This patch is based on v4.0-rc1.
> > 
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Paul Moore <pmoore@redhat.com>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> > 
> >  fs/namei.c | 50 +++++++++++++++++++++++++++++++-------------------
> >  1 file changed, 31 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index c83145a..3be372b 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -133,21 +133,20 @@ getname_flags(const char __user *filename, int
> > flags, int *empty)> 
> >         if (result)
> >         
> >                 return result;
> > 
> > -       result = __getname();
> > -       if (unlikely(!result))
> > +       kname = __getname();
> > +       if (unlikely(!kname))
> > 
> >                 return ERR_PTR(-ENOMEM);
> > 
> > -       result->refcnt = 1;
> > 
> >         /*
> >         
> >          * First, try to embed the struct filename inside the names_cache
> >          * allocation
> >          */
> > 
> > -       kname = (char *)result + sizeof(*result);
> > +       result = (struct filename *)(kname + EMBEDDED_NAME_MAX);
> > 
> >         result->name = kname;
> >         result->separate = false;
> > 
> > +       result->refcnt = 1;
> > 
> >         max = EMBEDDED_NAME_MAX;
> > 
> > -recopy:
> >         len = strncpy_from_user(kname, filename, max);
> >         if (unlikely(len < 0)) {
> >         
> >                 err = ERR_PTR(len);
> > 
> > @@ -157,23 +156,34 @@ recopy:
> >         /*
> >         
> >          * Uh-oh. We have a name that's approaching PATH_MAX. Allocate a
> >          * separate struct filename so we can dedicate the entire
> > 
> > -        * names_cache allocation for the pathname, and re-do the copy
> > from
> > +        * names_cache allocation for the pathname, and continue the copy
> > from> 
> >          * userland.
> >          */
> > 
> > -       if (len == EMBEDDED_NAME_MAX && max == EMBEDDED_NAME_MAX) {
> > -               kname = (char *)result;
> > -
> > +       if (len == EMBEDDED_NAME_MAX) {
> > 
> >                 result = kzalloc(sizeof(*result), GFP_KERNEL);
> >                 if (!result) {
> >                 
> >                         err = ERR_PTR(-ENOMEM);
> > 
> > -                       result = (struct filename *)kname;
> > +                       result = (struct filename *)(kname +
> > EMBEDDED_NAME_MAX);> 
> >                         goto error;
> >                 
> >                 }
> >                 result->name = kname;
> >                 result->separate = true;
> >                 result->refcnt = 1;
> > 
> > -               max = PATH_MAX;
> > -               goto recopy;
> > +               max = PATH_MAX - EMBEDDED_NAME_MAX;
> > +               /* we can't simply add the number of rest chars we copy to
> > len, +                * since strncpy_from_user may return negative to
> > indicate +                * something is wrong, so we do the addition
> > later, after +                * strncpy_from_user succeeds, and we know
> > we already copy +                * EMBEDDED_NAME_MAX chars.
> > +                */
> > +               len = strncpy_from_user(kname + EMBEDDED_NAME_MAX,
> > +                               filename + EMBEDDED_NAME_MAX, max);
> > +
> > +               if (unlikely(len < 0)) {
> > +                       err = ERR_PTR(len);
> > +                       goto error;
> > +               }
> > +               len += EMBEDDED_NAME_MAX;
> > 
> >         }
> >         
> >         /* The empty path is special. */
> > 
> > @@ -209,28 +219,30 @@ struct filename *
> > 
> >  getname_kernel(const char * filename)
> >  {
> >  
> >         struct filename *result;
> > 
> > +       char *kname;
> > 
> >         int len = strlen(filename) + 1;
> > 
> > -       result = __getname();
> > -       if (unlikely(!result))
> > +       kname = __getname();
> > +       if (unlikely(!kname))
> > 
> >                 return ERR_PTR(-ENOMEM);
> >         
> >         if (len <= EMBEDDED_NAME_MAX) {
> > 
> > -               result->name = (char *)(result) + sizeof(*result);
> > +               result = (struct filename *)(kname + EMBEDDED_NAME_MAX);
> > +               result->name = kname;
> > 
> >                 result->separate = false;
> >         
> >         } else if (len <= PATH_MAX) {
> >         
> >                 struct filename *tmp;
> >                 
> >                 tmp = kmalloc(sizeof(*tmp), GFP_KERNEL);
> >                 if (unlikely(!tmp)) {
> > 
> > -                       __putname(result);
> > +                       __putname(kname);
> > 
> >                         return ERR_PTR(-ENOMEM);
> >                 
> >                 }
> > 
> > -               tmp->name = (char *)result;
> > +               tmp->name = kname;
> > 
> >                 tmp->separate = true;
> >                 result = tmp;
> >         
> >         } else {
> > 
> > -               __putname(result);
> > +               __putname(kname);
> > 
> >                 return ERR_PTR(-ENAMETOOLONG);
> >         
> >         }
> >         memcpy((char *)result->name, filename, len);
> > 
> > @@ -253,7 +265,7 @@ void putname(struct filename *name)
> > 
> >                 __putname(name->name);
> >                 kfree(name);
> >         
> >         } else
> > 
> > -               __putname(name);
> > +               __putname(name->name);
> > 
> >  }
> >  
> >  static int check_acl(struct inode *inode, int mask)
> > 
> > --
> > 2.3.0

-- 
paul moore
security @ redhat