Re: [linux 2.6 patch] ext2 support for hurd extensions via xattr interface

[Michael Banck]

On Thu, Feb 12, 2004 at 11:19:42PM -0800, Roland McGrath wrote:
> Hi folks.  I've just whipped up the following patch, which is against the
> current Linux 2.6 source tree.  This provides a means to fetch and store
> the extra data stored in nodes on ext2 filesystems with "creator_os" set to
> EXT2_OS_HURD, i.e. from `mke2fs -o hurd' or filesystems created natively on
> the Hurd.  

Just curious, would it not make sense to have this for ext3, too? After
all, one can mount ext3 filesystems as ext2 on GNU/Hurd (AFAIK), so
extracting passive translators on a (incidently) ext3 file system for
later use on GNU/Hurd might make sense (although I admit this might not
be a common operation). Linux-2.6 has separate config options for
extended attributes for ext2 and ext3, so I assume the latter is not
supported right now. Is there a fundamental problem with ext3, or did
you just not do it for ext2?

> It also makes it possible to back up and restore a Hurdish ext2
> filesystem from a GNU/Linux system, using any tools that understand
> the extended attributes.  (Off hand the only such thing I'm aware of
> is `star', which I have not tried using myself.) 

I tried using star, but I get errors when I try to extract /gnu/servers.
I am not sure whether this is expected to work at all, though, as I get
EOPNOTSUPP.

I used 'star artype=exustar -xattr -c -v -f servers.tar /gnu/servers' as
command-line for tarring up the translators and 'star artype=exustar
-xattr -x -v -f servers.tar' for extracting. Tarring up seems to work
fine, but subsequent extracting yields error messages like this:

star: Operation not supported. Cannot setxattr for 'gnu/servers/exec'.

strace:

[...]
lstat64("gnu/servers/exec", 0xbfffd440) = -1 ENOENT (No such file or directory)
write(1, "x gnu/servers/exec 0 bytes, 0 ta"..., 42) = 42
open("gnu/servers/exec", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 4
fcntl64(4, F_GETFL)                     = 0x8001 (flags O_WRONLY|O_LARGEFILE)
fstat64(4, {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 
0x40975000
_llseek(4, 0, [0], SEEK_CUR)            = 0
munmap(0x40975000, 4096)                = 0
fcntl64(4, F_GETFD)                     = 0
fcntl64(4, F_GETFD)                     = 0
fsync(4)                                = 0
close(4)                                = 0
utimes("gnu/servers/exec", {1057965161, 0}) = 0
chmod("gnu/servers/exec", 0644)         = 0
setxattr("gnu/servers/exec", "gnu.translator", 0x808c110, 11, ) = -1 EOPNOTSUPP 
(Operation not supported)
write(2, "star: Operation not supported. C"..., 71star: Operation not 
supported. Cannot setxattr for 'gnu/servers/exec'.
) = 71
utimes("gnu/servers/exec", {1057965161, 0}) = 0
[...]

I then tried to debug star (you have to add '-g' to a read-only file in
RULES/foo-linux-cc, d'oh) and libattr, and it seems either the functions
arguments get corrupted when calling setxattr() or my mundane gdb skills
are at their limit here.

This is when tarring up the translator:

Breakpoint 7, get_xattr (info=0xbfffd4a0) at xattr.c:57
57      {
66              free_xattr(&static_xattr);
$65 = {f_tcb = 0x40171a00, f_name = 0xbfffd4a0 "/gnu/servers/exec", 
  f_namelen = 19, f_lname = 0xbfffa420 "", f_lnamelen = 0, f_uname = 0x0, 
  f_umaxlen = 0, f_gname = 0x0, f_gmaxlen = 0, f_dir = 0x808c100 "", 
  f_dirinos = 0x0, f_dirlen = 20, f_dirents = 4, f_dev = 782, f_ino = 57353, 
  f_nlink = 1, f_mode = 420, f_uid = 0, f_gid = 0, f_size = 0, f_rsize = 0, 
  f_contoffset = 137573730948, f_flags = 0, f_xflags = 7, f_xftype = 1, 
  f_rxftype = 1, f_filetype = 1, f_typeflag = 0 '\0', f_type = 32768, 
  f_rdev = 0, f_rdevmaj = 0, f_rdevmin = 0, f_atime = 1057965161, f_ansec = 0, 
  f_mtime = 1057965161, f_mnsec = 0, f_ctime = 1057965535, f_cnsec = 0, 
  f_fflags = 0, f_acl_access = 0xbffff7f0 "", 
  f_acl_default = 0x40171000 "././@PaxHeader", f_xattr = 0x808c0f0}
[...]
$74 = {name = 0x808c0f0 "gnu.translator", value = 0x808c040, value_len = 11}
[...]
$79 = 0x808c040 "/hurd/exec"

And this is when extracting it:

Breakpoint 3, set_xattr (info=0x1) at xattr.c:164
164     {
168             if (info->f_xattr == NULL)
$134 = {f_tcb = 0xbffff560, f_name = 0xbfffe551 "gnu/servers/exec", 
  f_namelen = 0, f_lname = 0xbfffd540 "", f_lnamelen = 0, 
  f_uname = 0xbffff669 "root", f_umaxlen = 32, f_gname = 0xbffff689 "nobody", 
  f_gmaxlen = 32, f_dir = 0x0, f_dirinos = 0x0, f_dirlen = 0, f_dirents = 0, 
  f_dev = 0, f_ino = 0, f_nlink = 0, f_mode = 420, f_uid = 0, f_gid = 0, 
  f_size = 0, f_rsize = 0, f_contoffset = 0, f_flags = 0, f_xflags = 524295, 
  f_xftype = 1, f_rxftype = 1, f_filetype = 1, f_typeflag = 48 '0', 
  f_type = 32768, f_rdev = 0, f_rdevmaj = 0, f_rdevmin = 0, 
  f_atime = 1057965161, f_ansec = 0, f_mtime = 1057965161, f_mnsec = 0, 
  f_ctime = 1057965535, f_cnsec = 0, f_fflags = 0, f_acl_access = 0x0, 
  f_acl_default = 0x0, f_xattr = 0x808c0b0}
$135 = {name = 0x808c0d0 "gnu.translator", value = 0x808c100, value_len = 11}
$136 = 0x808c100 "/hurd/exec"
171             for (xap = info->f_xattr; xap->name != NULL; xap++) {
172                     if (setxattr(info->f_name, xap->name, xap->value,
173                         xap->value_len, 0) != 0) {
setxattr (path=0xbfffe551 "gnu/servers/exec", 
    name=0xbfffe551 "gnu/servers/exec", value=0xbfffe551, size=3221218641, 
    flags=-1073748655) at syscalls.c:213
213             return SYSCALL(__NR_setxattr, path, name, value, size, flags);
$137 = 0xbfffe551 "gnu/servers/exec"
$138 = 0xbfffe551 "gnu/servers/exec"
$139 = 0xbfffe551 "gnu/servers/exec"
214     }
set_xattr (info=0xbffff820) at xattr.c:174
174                             if (!errhidden(E_SETXATTR, info->f_name)) {
175                                     errmsg("Cannot setxattr for '%s'.\n",
176                                                             info->f_name);
177                                     xstats.s_setxattr++;

Anyway, one problem is that star (or whoever else) seems to cut off after
the first "\0":

$144 = (star_xattr_t *) 0x808c0b0
$145 = {name = 0x808c0d0 "gnu.translator", value = 0x808c100, value_len = 72}
$146 = 0x808c100 "/hurd/pfinet"

>       bash$ getfattr -Rh -d -m '.*' /gnu/servers getfattr: Removing
>       leading '/' from absolute path names # file:
>       gnu/servers/socket/1 gnu.translator="/hurd/pflocal\000"

[...]

This worked well for me.


Sorry if I might have totally missed the point,

Michael