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SubjectRe: [PATCH] vfs: avoid recopying filename in getname_flags
Date
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



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