On Thu, 13 May 2010 11:43:51 +1000, Neil Brown <neilb@xxxxxxx> wrote:
> On Wed, 12 May 2010 21:20:40 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> 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.
> NeilBrown
>
>
> From: NeilBrown <neilb@xxxxxxx>
>
> Allow '*at' syscalls to have a NULL path name, thus using the 'fd' directly.
>
> Move the 'is a directory' test from path_init to the start of link_path_walk
> (which is always called after a successful path_init).
> Remove the file_permission(MAY_EXEC) test from path_init as such a test
> already exists in link_path_walk as exec_permission()
> Allow user_path_at to use a NULL resulting in the dfd being the target object.
>
> This effectively provides an 'f*' version for all syscalls with a '*at'
> version. In particular: link, access, chmod, chown, stat, readlink, utimes
>
> Some of those already have f* versions, but link, access, readlink all
> benefit directly, and futimes code can be tidied up.
>
> NULL is only allowed if 'dfd' is not ATFD_CWD to reduce possible
> backward-compatibility issues.
>
> openat is not effected by this change, even in non-O_CREAT instances. Maybe
> it should.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
>
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 48e60a1..b9b091e 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -824,6 +824,11 @@ static int link_path_walk(const char *name, struct nameidata *nd)
> goto return_reval;
>
> inode = nd->path.dentry->d_inode;
> +
> + err = -ENOTDIR;
> + if (!S_ISDIR(inode->i_mode))
> + goto return_err;
> +
> if (nd->depth)
> lookup_flags = LOOKUP_FOLLOW | (nd->flags & LOOKUP_CONTINUE);
>
> @@ -1030,14 +1035,6 @@ static int path_init(int dfd, const char *name, unsigned int flags, struct namei
>
> dentry = file->f_path.dentry;
>
> - retval = -ENOTDIR;
> - if (!S_ISDIR(dentry->d_inode->i_mode))
> - goto fput_fail;
> -
> - retval = file_permission(file, MAY_EXEC);
> - if (retval)
> - goto fput_fail;
> -
> nd->path = file->f_path;
> path_get(&file->f_path);
>
> @@ -1241,17 +1238,22 @@ int user_path_at(int dfd, const char __user *name, unsigned flags,
> struct path *path)
> {
> struct nameidata nd;
> - char *tmp = getname(name);
> - int err = PTR_ERR(tmp);
> - if (!IS_ERR(tmp)) {
> -
> - BUG_ON(flags & LOOKUP_PARENT);
> + char *tmp;
> + int err;
>
> - err = do_path_lookup(dfd, tmp, flags, &nd);
> - putname(tmp);
> - if (!err)
> - *path = nd.path;
> + BUG_ON(flags & LOOKUP_PARENT);
> + if (name == NULL && dfd >= 0)
> + err = do_path_lookup(dfd, "", flags, &nd);
> + else {
> + tmp = getname(name);
> + err = PTR_ERR(tmp);
> + if (!IS_ERR(tmp)) {
> + err = do_path_lookup(dfd, tmp, flags, &nd);
> + putname(tmp);
> + }
> }
> + if (!err)
> + *path = nd.path;
> return err;
> }
>
> --
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