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Date: Sat, 21 Mar 2015 12:22:11 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Paul Moore <pmoore@...hat.com>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: avoid recopying filename in getname_flags
Hi Al,
Ping and sorry to bother you.
Could you please have a look at my question? Thank you!
Regards,
Boqun Feng
On Wed, Mar 18, 2015 at 01:27:24PM +0800, Boqun Feng wrote:
> 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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