#3 A patch to add lo_change_fd support

[David Eduardo Gomez Noguera]

This patch intends to add support for the lo_change_fd
ioctl call, used among other by RedHat based
installation cd's

I have tested it in what I think was success, but would
like some feedback.

mail is
tsukebumi at gmail .com

[Jari Ruusu]

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I looked at your patch. It is a good start but it
is far from complete.

Device backed handling is completely missing. This
is not a trivial thing to do.

get_loop_size() is not entirely correct, but is
easy to fix.

gpf_mask restoring/setting needs to handle the -1
case as well as __GFP_HIGH case.

File backed code uses file->f_op->read()/write()
so file backed loop_change_fd() code should check
for ->read() instead of ->sendfile().

Spanish language comments need to be translated to
english.

[David Eduardo Gomez Noguera]

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I am just porting a patch that was used some time before.

The point of it is to be able to switch between two file
backed loops.

This is necesary in the installation process since the
images used in the installation process mounted with a loop
device, reside on the cd. It is done that way because they
don't have a proper filesystem where they could move the
image before mounting it.
They copy it to one filesystem right after it has been
created, and to be able to switch cds, they need this ioctl
command.
So I think I should return an error if I detect a device
backed loop? (LOOP_CHANGE_FD is intended for file backed
loops only, don't know if somebody would find an instance
where she would need it otherwise)

I am working on the rest at the time. the comments were
there just to help me understand how the loop-AES code
works. I translated the relevant comments to english, and
removed the rest.

Thank you very much for your pointers. I really appreciate that.
This is my first take on some meaningfull community driven,
not to mention kernel, code.

[David Eduardo Gomez Noguera]

original patch for loop.c from http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.4-rc1/2.6.4-rc1-mm1/broken-out/LOOP_CHANGE_FD.patch

[David Eduardo Gomez Noguera]

revision of the patch. still missing some corrections pointed out by jariruusu

[Jari Ruusu]

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If this LOOP_CHANGE_FD ioctl can be limited to file
backed loops only, then your patch can be merged.

I will merge your second patch version, and fix up the
remaining loose ends. If I don't encounter any problems
in testing, then this functionality will be present in
next version of loop-AES.

One more thing; Can you post link to Red Hat source
code that calls this LOOP_CHANGE_FD ioctl? Usage is
obvious from kernel module source, but I would like to
see the userspace code that calls it.

Thanks for your patch porting work.

[David Eduardo Gomez Noguera]

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It is part of <a
href="http://download.fedora.redhat.com/pub/fedora/linux/core/development/SRPMS/">anaconda</a>.

It is part python and C.
The relevant files are isys/isys.py and isys/isys.c, called
from image.py

----isys.py
def lochangefd(device, file):
loop = os.open(device, os.O_RDONLY)
targ = os.open(file, os.O_RDONLY)
try:
_isys.lochangefd(loop, targ)
finally:
os.close(loop)
os.close(targ)
----isys.c
static PyObject * doLoChangeFd(PyObject * s, PyObject * args) {
int loopfd;
int targfd;

if (!PyArg_ParseTuple(args, "ii", &loopfd, &targfd))
return NULL;
if (ioctl(loopfd, LOOP_CHANGE_FD, targfd)) {
PyErr_SetFromErrno(PyExc_SystemError);
return NULL;
}

Py_INCREF(Py_None);
return Py_None;
}
----image.py
def systemUnmounted(self):
if self.loopbackFile:
isys.makeDevInode("loop0", "/tmp/loop")
isys.lochangefd("/tmp/loop",
"%s/%s/base/stage2.img" %
(self.tree, productPath))
self.loopbackFile = None

def systemMounted(self, fsset, chroot):
self.loopbackFile = "%s%s%s" % (chroot,

fsset.filesystemSpace(chroot)[0][0],
"/rhinstall-stage2.img")

try:
iutil.copyFile("%s/%s/base/stage2.img" %
(self.tree, productPath),
self.loopbackFile,
(self.progressWindow, _("Copying
File"),
_("Transferring install image to
hard drive...")))
except Exception, e:
log("error transferring stage2.img: %s" %(e,))
self.messageWindow(_("Error"),
_("An error occurred transferring the
install image "
"to your hard drive. You are probably
out of disk "
"space."))
os.unlink(self.loopbackFile)
return 1

isys.makeDevInode("loop0", "/tmp/loop")
isys.lochangefd("/tmp/loop", self.loopbackFile)

[David Eduardo Gomez Noguera]

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I just noted this piece of code where change_fd will return
-EINVAL unless the file is a regular file or a block device

error = -EINVAL;
if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode)){
goto out_putf;
}
Now I am not sure how this must work.
Then is the Device backing handling missing? I though it
should have only checked whether it was regular file only,
but the original code checks both.

[Jari Ruusu]

Merged version of loop-change-fd patch

[Jari Ruusu]

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I uploaded new edited version of the patch.

David, any chance of you testing this?

[David Eduardo Gomez Noguera]

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It worked allright in the installation CD.

Sorry if it took some time.

[Jari Ruusu]

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OK. Thanks for testing.

[Jari Ruusu]

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loop-AES-v3.0d includes this patch.