x/sys/unix: add *xattr functions on darwin.

[hirochachacha]

OS X has own *xattr implementations.

https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/setxattr.2.html#//apple_ref/doc/man/2/setxattr

Parameter is different, but we can make workaround for this.

[rfjakob]

Third-party implementations are already out: https://github.com/pkg/xattr/blob/master/xattr_darwin.go

[gopherbot]

Change https://golang.org/cl/113995 mentions this issue: unix: add {get,set,remove,list}xattr to darwin amd64

[rfjakob]

Nice! But what about the symlink-safe variants Lgetxattr / Lsetxattr / Llistxattr etc ? It's just a matter of passing the option XATTR_NOFOLLOW to the underlying syscall.

[gopherbot]

Change https://golang.org/cl/114198 mentions this issue: unix: add symlink-safe *xattr functions on darwin

[rfjakob]

Wow, awesome!

I noticed a test fails with EPERM on linux, i hit this recently, maybe related?

// https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/xattr.c?h=v4.17-rc5#n122 :
// In the user.* namespace, only regular files and directories can have
// extended attributes.

https://github.com/rfjakob/pkg-xattr/blob/LGet/xattr_test.go

[tklauser]

@rfjakob exactly, that's what I just found out too 😃

From http://man7.org/linux/man-pages/man7/xattr.7.html:

       The file permission bits of regular files and directories are
       interpreted differently from the file permission bits of special
       files and symbolic links.  For regular files and directories the file
       permission bits define access to the file's contents, while for
       device special files they define access to the device described by
       the special file.  The file permissions of symbolic links are not
       used in access checks.  These differences would allow users to
       consume filesystem resources in a way not controllable by disk quotas
       for group or world writable special files and directories.

       For this reason, extended user attributes are allowed only for
       regular files and directories, and access to extended user attributes
       is restricted to the owner and to users with appropriate capabilities
       for directories with the sticky bit set (see the chmod(1) manual page
       for an explanation of the sticky bit).

I'll just drop the corresponding tests from the CL for now.

[rfjakob]

Ok - if you want to keep the tests, what I have done in pkg/xattr is to operate on a broken symlink, and check that we get EPERM for Lgetxattr and ENOENT for Getxattr.

[tklauser]

@rfjakob nice, thanks. Will do that too for x/sys/unix then.