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    SubjectRe: [PATCH -V7 5/9] vfs: Add freadlink syscall
    On Thu, 13 May 2010 11:55:56 +0530
    "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

    > On Thu, 13 May 2010 11:43:51 +1000, Neil Brown <neilb@suse.de> wrote:
    > > On Wed, 12 May 2010 21:20:40 +0530
    > > "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
    > >
    > > > This enables to use open-by-handle and then get the link target
    > > > details of a symlink using the fd returned by handle
    > > >
    > >
    > >
    > > I find it very frustrating that a new syscall seems to be needed here.
    > > We have 'readlinkat', and it should be enough.
    > > How: the 'dfd' has to be a 'directory', and the path name as to be non-empty.
    > >
    > > The following patch allows 'path' to be NULL and in that case 'dfd' to be a
    > > non-directory. This allows readlinkat and faccessat (and probably others)
    > > to be used on an fd with not following path name.
    > >
    > > What do people think of this alternative?
    > >
    >
    > I will add this in the next iteration and drop the freadlink syscall.
    >

    I wouldn't be quite that hasty. It was only a proposal. It might not be
    appropriate to change all those syscalls..

    An alternative that I don't think is a nice, but is a lot 'safer' is
    to just change readlinkat to accept a NULL path:


    Signed-off-by: NeilBrown <neilb@suse.de>
    diff --git a/fs/stat.c b/fs/stat.c
    index c4ecd52..85d0856 100644
    --- a/fs/stat.c
    +++ b/fs/stat.c
    @@ -284,26 +284,40 @@ SYSCALL_DEFINE2(newfstat, unsigned int, fd, struct stat __user *, statbuf)
    SYSCALL_DEFINE4(readlinkat, int, dfd, const char __user *, pathname,
    char __user *, buf, int, bufsiz)
    {
    - struct path path;
    + struct path path, *pp;
    + struct file *file = NULL;
    int error;

    if (bufsiz <= 0)
    return -EINVAL;

    - error = user_path_at(dfd, pathname, 0, &path);
    + if (filename == NULL && dfd != AT_FDCWD) {
    + struct file *file = fget(dfd);
    +
    + if (file)
    + pp = &file->f_path;
    + else
    + error = -EBADF;
    + } else {
    + error = user_path_at(dfd, pathname, 0, &path);
    + pp = &path;
    + }
    if (!error) {
    - struct inode *inode = path.dentry->d_inode;
    + struct inode *inode = pp->dentry->d_inode;

    error = -EINVAL;
    if (inode->i_op->readlink) {
    - error = security_inode_readlink(path.dentry);
    + error = security_inode_readlink(pp->dentry);
    if (!error) {
    - touch_atime(path.mnt, path.dentry);
    - error = inode->i_op->readlink(path.dentry,
    + touch_atime(pp->mnt, pp->dentry);
    + error = inode->i_op->readlink(pp->dentry,
    buf, bufsiz);
    }
    }
    - path_put(&path);
    + if (file)
    + fput(file);
    + else
    + path_put(&path);
    }
    return error;
    }

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