Re: [PATCH v2] hfsplus: don't store special "osx" xattr prefix on-disk

[Thomas Hebb <tommyhebb@gmail.com>]

This version of the commit is rebased on linux-next, as Andrew requested.
The conflicting changes were just refactoring, so this version ought to
be functionally identical to the first.

It does change the osx-namespace functions to call __hfsplus_?etxattr()
directly instead of going through the refactored hfsplus_?etxattr()
functions, which allocate space for a copy of the name with a prepended
namespace. Since the osx attributes aren't stored with a namespace, we
save ourselves a copy by bypassing the functions, but we also introduce
the potential for future bugs if the hfsplus_?etattr() functions are
later changed to do something that all namespaces need.

If the extra copy is inconsequential (or if there's a reason it's needed
that I'm unaware of), I'm happy to revise the patch again to call
hfsplus_?etattr() with an empty namespace parameter, instead of bypassing
them altogether.

Thanks,
-Thomas

On Sat, 2015-04-04 at 19:08 -0400, Thomas Hebb wrote:
> On Mac OS X, HFS+ extended attributes are not namespaced. Since we want
> to be compatible with OS X filesystems and yet still support the Linux
> namespacing system, the hfsplus driver implements a special "osx"
> namespace that is reported for any attribute that is not namespaced
> on-disk. However, the current code for getting and setting these
> unprefixed attributes is broken.
> 
> hfsplus_osx_setattr() and hfsplus_osx_getattr() are passed names that
> have already had their "osx." prefixes stripped by the generic functions.
> The functions first, quite correctly, check those names to make sure
> that they aren't prefixed with a known namespace, which would allow
> namespace access restrictions to be bypassed. However, the functions
> then prepend "osx." to the name they're given before passing it on to
> hfsplus_getattr() and hfsplus_setattr(). Not only does this cause the
> "osx." prefix to be stored on-disk, defeating its purpose, it also breaks
> the check for the special "com.apple.FinderInfo" attribute, which is
> reported for all files, and as a consequence makes some userspace
> applications (e.g. GNU patch) fail even when extended attributes are not
> otherwise in use.
> 
> There are five commits which have touched this particular code:
> 
>   127e5f5ae51e ("hfsplus: rework functionality of getting, setting and deleting of extended attributes")
>   b168fff72109 ("hfsplus: use xattr handlers for removexattr")
>   bf29e886b242 ("hfsplus: correct usage of HFSPLUS_ATTR_MAX_STRLEN for non-English attributes")
>   fcacbd95e121 ("fs/hfsplus: move xattr_name allocation in hfsplus_getxattr()")
>   ec1bbd346f18 ("fs/hfsplus: move xattr_name allocation in hfsplus_setxattr()")
> 
> The first commit creates the functions to begin with. The namespace is
> prepended by the original code, which I believe was correct at the time,
> since hfsplus_?etattr() stripped the prefix if found. The second commit
> removes this behavior from hfsplus_?etattr() and appears to have been
> intended to also remove the prefixing from hfsplus_osx_?etattr(). However,
> what it actually does is remove a necessary strncpy() call completely,
> breaking the osx namespace entirely. The third commit re-adds the
> strncpy() call as it was originally, but doesn't mention it in its
> commit message. The final two commits refactor the code and don't
> affect its functionality.
> 
> This commit does what b168fff attempted to do (prevent the prefix from
> being added), but does it properly, instead of passing in an empty buffer
> (which is what b168fff actually did).
> 
> Fixes: b168fff72109 ("hfsplus: use xattr handlers for removexattr")
> Cc: stable@vger.kernel.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Hin-Tak Leung <htl10@users.sourceforge.net>
> Cc: Sergei Antonov <saproj@gmail.com>
> Cc: Anton Altaparmakov <anton@tuxera.com>
> Cc: Fabian Frederick <fabf@skynet.be>
> Cc: Christian Kujau <lists@nerdbynature.de>
> Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
> ---
>  fs/hfsplus/xattr.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/hfsplus/xattr.c b/fs/hfsplus/xattr.c
> index 8d62de0..89f262d 100644
> --- a/fs/hfsplus/xattr.c
> +++ b/fs/hfsplus/xattr.c
> @@ -862,8 +862,13 @@ static int hfsplus_osx_getxattr(struct dentry *dentry, const char *name,
>         if (is_known_namespace(name))
>                 return -EOPNOTSUPP;
>  
> -       return hfsplus_getxattr(dentry, name, buffer, size,
> -                               XATTR_MAC_OSX_PREFIX, XATTR_MAC_OSX_PREFIX_LEN);
> +       /*
> +        * osx is the namespace we use to indicate an unprefixed
> +        * attribute on the filesystem (like the ones that OS X
> +        * creates), so we pass the name through unmodified (after
> +        * ensuring it doesn't conflict with another namespace).
> +        */
> +       return __hfsplus_getxattr(dentry->d_inode, name, buffer, size);
>  }
>  
>  static int hfsplus_osx_setxattr(struct dentry *dentry, const char *name,
> @@ -879,9 +884,13 @@ static int hfsplus_osx_setxattr(struct dentry *dentry, const char *name,
>         if (is_known_namespace(name))
>                 return -EOPNOTSUPP;
>  
> -       return hfsplus_setxattr(dentry, name, buffer, size, flags,
> -                               XATTR_MAC_OSX_PREFIX,
> -                               XATTR_MAC_OSX_PREFIX_LEN);
> +       /*
> +        * osx is the namespace we use to indicate an unprefixed
> +        * attribute on the filesystem (like the ones that OS X
> +        * creates), so we pass the name through unmodified (after
> +        * ensuring it doesn't conflict with another namespace).
> +        */
> +       return __hfsplus_setxattr(dentry->d_inode, name, buffer, size, flags);
>  }
>  
>  static size_t hfsplus_osx_listxattr(struct dentry *dentry, char *list,