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SubjectRe: [PATCH -V7 5/9] vfs: Add freadlink syscall
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?

NeilBrown


From: NeilBrown <neilb@suse.de>

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@suse.de>


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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