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    SubjectRe: [PATCH -V4 08/11] vfs: Add new file and directory create permission flags
    On Sat, 25 Sep 2010 00:46:03 +0530
    "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

    > On Fri, 24 Sep 2010 11:54:23 -0400, Jeff Layton <jlayton@redhat.com> wrote:
    > > On Fri, 24 Sep 2010 18:18:11 +0530
    > > "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
    > >
    > > > From: Andreas Gruenbacher <agruen@suse.de>
    > > >
    > > > Some permission models distinguish between the permission to create a
    > > > non-directory and a directory. Pass this information down to
    > > > inode_permission() as mask flags
    > > >
    > > > Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
    > > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    > > > ---
    > > > fs/namei.c | 21 ++++++++++++---------
    > > > include/linux/fs.h | 2 ++
    > > > 2 files changed, 14 insertions(+), 9 deletions(-)
    > > >
    > > > diff --git a/fs/namei.c b/fs/namei.c
    > > > index b0b8a71..ed786b2 100644
    > > > --- a/fs/namei.c
    > > > +++ b/fs/namei.c
    > > > @@ -253,7 +253,8 @@ int generic_permission(struct inode *inode, int mask,
    > > > * for filesystem access without changing the "normal" uids which
    > > > * are used for other things.
    > > > *
    > > > - * When checking for MAY_APPEND, MAY_WRITE must also be set in @mask.
    > > > + * When checking for MAY_APPEND, MAY_CREATE_FILE, MAY_CREATE_DIR,
    > > > + * MAY_WRITE must also be set in @mask.
    > > > */
    > > > int inode_permission(struct inode *inode, int mask)
    > > > {
    > > > @@ -1337,13 +1338,15 @@ static int may_delete(struct inode *dir,struct dentry *victim,int isdir)
    > > > * 3. We should have write and exec permissions on dir
    > > > * 4. We can't do it if dir is immutable (done in permission())
    > > > */
    > > > -static inline int may_create(struct inode *dir, struct dentry *child)
    > > > +static inline int may_create(struct inode *dir, struct dentry *child, int isdir)
    > > ^^^^^
    > > nit: maybe saner as a bool?
    > > > {
    > > > + int mask = isdir ? MAY_CREATE_DIR : MAY_CREATE_FILE;
    > > > +
    > > > if (child->d_inode)
    > > > return -EEXIST;
    > > > if (IS_DEADDIR(dir))
    > > > return -ENOENT;
    > > > - return inode_permission(dir, MAY_WRITE | MAY_EXEC);
    > > > + return inode_permission(dir, MAY_WRITE | MAY_EXEC | mask);
    > > > }
    > > >
    > > > /*
    > > > @@ -1391,7 +1394,7 @@ void unlock_rename(struct dentry *p1, struct dentry *p2)
    > > > int vfs_create(struct inode *dir, struct dentry *dentry, int mode,
    > > > struct nameidata *nd)
    > > > {
    > > > - int error = may_create(dir, dentry);
    > > > + int error = may_create(dir, dentry, 0);
    > > >
    > > > if (error)
    > > > return error;
    > > > @@ -1953,7 +1956,7 @@ EXPORT_SYMBOL_GPL(lookup_create);
    > > >
    > > > int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
    > > > {
    > > > - int error = may_create(dir, dentry);
    > > > + int error = may_create(dir, dentry, 0);
    > > >
    > > > if (error)
    > > > return error;
    > > > @@ -2057,7 +2060,7 @@ SYSCALL_DEFINE3(mknod, const char __user *, filename, int, mode, unsigned, dev)
    > > >
    > > > int vfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
    > > > {
    > > > - int error = may_create(dir, dentry);
    > > > + int error = may_create(dir, dentry, 1);
    > > >
    > > > if (error)
    > > > return error;
    > > > @@ -2342,7 +2345,7 @@ SYSCALL_DEFINE1(unlink, const char __user *, pathname)
    > > >
    > > > int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
    > > > {
    > > > - int error = may_create(dir, dentry);
    > > > + int error = may_create(dir, dentry, 0);
    > > >
    > > > if (error)
    > > > return error;
    > > > @@ -2415,7 +2418,7 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
    > > > if (!inode)
    > > > return -ENOENT;
    > > >
    > > > - error = may_create(dir, new_dentry);
    > > > + error = may_create(dir, new_dentry, S_ISDIR(inode->i_mode));
    > >
    > > ^^^^ this is a little
    > > scary, but even if it's
    > > a directory, it'll get
    > > kicked out in a later
    > > check. Would it be
    > > clearer to move up the
    > > S_ISDIR() check in this
    > > function and then pass
    > > this in as false?
    >
    > Can you elaborate on this ?
    >
    > -aneesh
    >

    Hardlinked directories are a no-no, of course. So when I first saw this
    patch, it gave me pause. There's a later check in vfs_link though that
    explicitly rejects hardlinking directories, so the above is harmless.
    It may be more efficient to go ahead and return error if the target
    is a directory however and bypass the permission check.

    OTOH, maybe there's good reason to do it this way or I'm just being
    excessively nitpicky.

    --
    Jeff Layton <jlayton@redhat.com>


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