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

From: Boqun Feng
Date: Wed Mar 18 2015 - 01:27:23 EST

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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@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > > Cc: Paul Moore <pmoore@xxxxxxxxxx>
> > > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > ---
> > >
> > > 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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