On Mon, 12 Jul 2010, Aneesh Kumar K.V wrote:
> The patch update may_open to allow handle based open on symlinks.
> The file handle based API use file descritor returned from open_by_handle_at
> to do different file system operations. To find the link target name we
> need to get a file descriptor on symlinks.
>
> We should be able to read the link target using file handle. The exact
> usecase is with respect to implementing READLINK operation on a
> userspace NFS server. The request contain the file handle and the
> response include target name.
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> ---
> fs/namei.c | 10 +++++++++-
> fs/open.c | 9 ++++++++-
> include/linux/fs.h | 1 +
> 3 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 4d590a3..a6a8093 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1456,7 +1456,15 @@ int may_open(struct path *path, int acc_mode, int flag)
>
> switch (inode->i_mode & S_IFMT) {
> case S_IFLNK:
> - return -ELOOP;
> + /*
> + * Allow only if acc_mode contain
> + * open link request and read request.
> + */
> + if (acc_mode != (MAY_OPEN_LINK | MAY_READ))
Why require MAY_READ?
Actually, open_by_handle() should be a good place to start supporting
O_NOACCESS from the start. I.e. neigher read, nor write access is
permitted on the file.
> + return -ELOOP;
> + if (flag != O_RDONLY)
> + return -ELOOP;
> + break;
> case S_IFDIR:
> if (acc_mode & MAY_WRITE)
> return -EISDIR;
> diff --git a/fs/open.c b/fs/open.c
> index df5d21e..afb089e 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -1268,8 +1268,15 @@ static long do_sys_open_by_handle(int mountdirfd,
> */
> if (open_flag & __O_SYNC)
> open_flag |= O_DSYNC;
> + /*
> + * Handle based API allow open on a symlink
> + */
> + if (S_ISLNK(path->dentry->d_inode->i_mode))
> + acc_mode = MAY_OPEN_LINK;
> + else
> + acc_mode = MAY_OPEN;
>
> - acc_mode = MAY_OPEN | ACC_MODE(open_flag);
> + acc_mode |= ACC_MODE(open_flag);
>
> /* O_TRUNC implies we need access checks for write permissions */
> if (open_flag & O_TRUNC)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index a458b4e..08afa72 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -53,6 +53,7 @@ struct inodes_stat_t {
> #define MAY_APPEND 8
> #define MAY_ACCESS 16
> #define MAY_OPEN 32
> +#define MAY_OPEN_LINK 64
>
> /*
> * flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond
> --
> 1.7.2.rc1
>
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