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Date:	Wed, 18 Mar 2015 13:27:24 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Paul Moore <pmoore@...hat.com>, Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: avoid recopying filename in getname_flags

On Fri, Mar 13, 2015 at 09:45:59AM -0400, Paul Moore wrote:
> 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.

Thank you for your reminder ;-)

After learning several changes on that branch, I get a question for
commit ca160d0 "kill struct filename.separate".
   
I think the two following situations explains how that commit works.
(`iname` and `name` are fields in struct filename)

Not separate:
   |---PATH_MAX bytes by names_cachep ------|
   |--struct filename--|---space for name---|
                       ^
                       iname/name

name->iname == name->name is true
 
Separate:
   |--by kzalloc ------|      |---PATH_MAX bytes by names_cachep---|
   |--struct filename--|......|---space for name-------------------|
                       ^      ^
                       iname  name

name->iname == name->name is false

However, I think of a third situation, which we were unlucky, that
the bytes allocated by kzalloc and the bytes allocated by names_cachep
somehow become continous, like the following situation:

Separate:
   |--by kzalloc ------|---PATH_MAX bytes by names_cachep ------|
   |--struct filename--|---space for name-----------------------|
                       ^      
                       iname/name 

In this situation, the struct and the name are separate but 
name->iname == name->name is true

Since struct filename is small, so kzalloc will call kmem_cache_alloc
actually. As I don't know much about kmem_cache allocators, my question
is "Can the 'unlucky' situation happen now?" If the answer is no, can it
happen in the future considering there may be new kmem_cache allocating
algorithms?
   
Thanks and Best Regards,
Boqun Feng

> 
> > On Wed, Feb 25, 2015 at 8:31 PM, Boqun Feng <boqun.feng@...il.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@...iv.linux.org.uk>
> > > Cc: Paul Moore <pmoore@...hat.com>
> > > Signed-off-by: Boqun Feng <boqun.feng@...il.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
> 

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