Re: [RFC v6 02/40] vfs: Add MAY_CREATE_FILE and MAY_CREATE_DIR permission flags

[bfields@fieldses.org (J. Bruce Fields)]

On Tue, Aug 04, 2015 at 01:53:00PM +0200, Andreas Gruenbacher wrote:
> Richacls distinguish between creating non-directories and directories. To
> support that, add an isdir parameter to may_create(). When checking
> inode_permission() for create permission, pass in an additional
> MAY_CREATE_FILE or MAY_CREATE_DIR mask flag.
> 
> To allow checking for delete *and* create access when replacing an existing
> file via vfs_rename(), add a replace parameter to may_delete().
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
>  fs/namei.c         | 42 ++++++++++++++++++++++++------------------
>  include/linux/fs.h |  2 ++
>  2 files changed, 26 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 8f3db24..3504d36 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -453,7 +453,8 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
>   * this, letting us set arbitrary permissions 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.

Why?  (Also: written as a simple list like that, it's a ambiguous how to
parse that comment: I think you mean that MAY_WRITE must be set whenever
MAY_APPEND, MAY_CREATE_FILE, or MAY_CREATE_DIR are set.)

--b.

>   */
>  int inode_permission(struct inode *inode, int mask)
>  {
> @@ -2522,10 +2523,11 @@ EXPORT_SYMBOL(__check_sticky);
>   * 10. We don't allow removal of NFS sillyrenamed files; it's handled by
>   *     nfs_async_unlink().
>   */
> -static int may_delete(struct inode *dir, struct dentry *victim, bool isdir)
> +static int may_delete(struct inode *dir, struct dentry *victim,
> +		      bool isdir, bool replace)
>  {
>  	struct inode *inode = d_backing_inode(victim);
> -	int error;
> +	int error, mask = MAY_WRITE | MAY_EXEC;
>  
>  	if (d_is_negative(victim))
>  		return -ENOENT;
> @@ -2534,7 +2536,9 @@ static int may_delete(struct inode *dir, struct dentry *victim, bool isdir)
>  	BUG_ON(victim->d_parent->d_inode != dir);
>  	audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);
>  
> -	error = inode_permission(dir, MAY_WRITE | MAY_EXEC);
> +	if (replace)
> +		mask |= isdir ? MAY_CREATE_DIR : MAY_CREATE_FILE;
> +	error = inode_permission(dir, mask);
>  	if (error)
>  		return error;
>  	if (IS_APPEND(dir))
> @@ -2565,14 +2569,16 @@ static int may_delete(struct inode *dir, struct dentry *victim, bool 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, bool isdir)
>  {
> +	int mask = isdir ? MAY_CREATE_DIR : MAY_CREATE_FILE;
> +
>  	audit_inode_child(dir, child, AUDIT_TYPE_CHILD_CREATE);
>  	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);
>  }
>  
>  /*
> @@ -2622,7 +2628,7 @@ EXPORT_SYMBOL(unlock_rename);
>  int vfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
>  		bool want_excl)
>  {
> -	int error = may_create(dir, dentry);
> +	int error = may_create(dir, dentry, false);
>  	if (error)
>  		return error;
>  
> @@ -3467,7 +3473,7 @@ EXPORT_SYMBOL(user_path_create);
>  
>  int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
>  {
> -	int error = may_create(dir, dentry);
> +	int error = may_create(dir, dentry, false);
>  
>  	if (error)
>  		return error;
> @@ -3559,7 +3565,7 @@ SYSCALL_DEFINE3(mknod, const char __user *, filename, umode_t, mode, unsigned, d
>  
>  int vfs_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
>  {
> -	int error = may_create(dir, dentry);
> +	int error = may_create(dir, dentry, true);
>  	unsigned max_links = dir->i_sb->s_max_links;
>  
>  	if (error)
> @@ -3640,7 +3646,7 @@ EXPORT_SYMBOL(dentry_unhash);
>  
>  int vfs_rmdir(struct inode *dir, struct dentry *dentry)
>  {
> -	int error = may_delete(dir, dentry, 1);
> +	int error = may_delete(dir, dentry, true, false);
>  
>  	if (error)
>  		return error;
> @@ -3762,7 +3768,7 @@ SYSCALL_DEFINE1(rmdir, const char __user *, pathname)
>  int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
>  {
>  	struct inode *target = dentry->d_inode;
> -	int error = may_delete(dir, dentry, 0);
> +	int error = may_delete(dir, dentry, false, false);
>  
>  	if (error)
>  		return error;
> @@ -3896,7 +3902,7 @@ 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, false);
>  
>  	if (error)
>  		return error;
> @@ -3979,7 +3985,7 @@ 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, false);
>  	if (error)
>  		return error;
>  
> @@ -4167,19 +4173,19 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
>  	if (source == target)
>  		return 0;
>  
> -	error = may_delete(old_dir, old_dentry, is_dir);
> +	error = may_delete(old_dir, old_dentry, is_dir, false);
>  	if (error)
>  		return error;
>  
>  	if (!target) {
> -		error = may_create(new_dir, new_dentry);
> +		error = may_create(new_dir, new_dentry, is_dir);
>  	} else {
>  		new_is_dir = d_is_dir(new_dentry);
>  
>  		if (!(flags & RENAME_EXCHANGE))
> -			error = may_delete(new_dir, new_dentry, is_dir);
> +			error = may_delete(new_dir, new_dentry, is_dir, true);
>  		else
> -			error = may_delete(new_dir, new_dentry, new_is_dir);
> +			error = may_delete(new_dir, new_dentry, new_is_dir, true);
>  	}
>  	if (error)
>  		return error;
> @@ -4442,7 +4448,7 @@ SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newna
>  
>  int vfs_whiteout(struct inode *dir, struct dentry *dentry)
>  {
> -	int error = may_create(dir, dentry);
> +	int error = may_create(dir, dentry, false);
>  	if (error)
>  		return error;
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 44e696e..9c44f27 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -81,6 +81,8 @@ typedef void (dax_iodone_t)(struct buffer_head *bh_map, int uptodate);
>  #define MAY_CHDIR		0x00000040
>  /* called from RCU mode, don't block */
>  #define MAY_NOT_BLOCK		0x00000080
> +#define MAY_CREATE_FILE		0x00000100
> +#define MAY_CREATE_DIR		0x00000200
>  
>  /*
>   * flags in file.f_mode.  Note that FMODE_READ and FMODE_WRITE must correspond
> -- 
> 2.5.0
> 
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