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loop: use LOOP_CONFIGURE instead of LOOP_SET_FD

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LOOP_CONFIGURE allows to atomically configure the decive when opening
it. This avoid the possibility of a race condition where between set_fd
and set_status some operations are already accepted by the loopback
device. See https://lwn.net/Articles/820408/

This feature was included in the linux kernel 5.8 however it is safe to
not include any kind of fallback to the previous method as @obudai
points out that:

LOOP_CONFIGURE was backported into RHEL 8 kernel in RHEL 8.4 as a part
of https://bugzilla.redhat.com/show_bug.cgi?id=1881760 (block layer:
update to upstream v5.8).

Since RHEL 8.4 is currently the oldest supported release that we support
running osbuild on, it might be just fine implementing this without the
fallback.

From a centos stream 8 container:
kernel-4.18.0-448.el8.x86_64
- loop: Fix missing discard support when using LOOP_CONFIGURE (Ming Lei) [1997338]
- [block] loop: Set correct device size when using LOOP_CONFIGURE (Ming Lei) [1881760]
- [block] loop: unset GENHD_FL_NO_PART_SCAN on LOOP_CONFIGURE (Ming Lei) [1881760]
- [block] loop: Add LOOP_CONFIGURE ioctl (Ming Lei) [1881760]
This commit is contained in:
Thomas Lavocat 2023-03-03 14:11:50 +01:00 • committed by Achilleas Koutsou
parent 04eab998b7
commit da11ef4eb0
3 changed files with 108 additions and 52 deletions
Download patch file Download diff file Expand all files Collapse all files

140
osbuild/loop.py
View file

@ -52,6 +52,15 @@ class LoopInfo(ctypes.Structure):
self.lo_inode == info.st_ino)
class LoopConfig(ctypes.Structure):
_fields_ = [
('fd', ctypes.c_uint32),
('block_size', ctypes.c_uint32),
('info', LoopInfo),
('__reserved', ctypes.c_uint64*8),
]
class Loop:
"""Loopback device
@ -60,8 +69,8 @@ class Loop:
Methods
-------
set_fd(fd)
Bind a file descriptor to the loopback device
loop_configure(fd)
Bind a file descriptor to the loopback device and set properties of the loopback device
clear_fd()
Unbind the file descriptor from the loopback device
change_fd(fd)
@ -89,6 +98,7 @@ class Loop:
LOOP_SET_CAPACITY = 0x4C07
LOOP_SET_DIRECT_IO = 0x4C08
LOOP_SET_BLOCK_SIZE = 0x4C09
LOOP_CONFIGURE = 0x4C0A
def __init__(self, minor, dir_fd=None):
"""
@ -182,18 +192,9 @@ class Loop:
linux.ioctl_blockdev_flushbuf(self.fd)
def set_fd(self, fd):
"""Bind a file descriptor to the loopback device
The loopback device must be unbound. The backing file must be
either a regular file or a block device. If the backing file is
itself a loopback device, then a cycle must not be created. If
the backing file is opened read-only, then the resulting
loopback device will be read-only too.
Parameters
----------
fd : int
the file descriptor to bind
"""
Deprecated, use configure instead.
TODO delete this after image-info gets updated.
"""
fcntl.ioctl(self.fd, self.LOOP_SET_FD, fd)
@ -315,6 +316,24 @@ class Loop:
# it is bound, check if it is bound by `fd`
return loop_info.is_bound_to(file_info)
def _config_info(self, info, offset, sizelimit, autoclear, partscan):
# pylint: disable=attribute-defined-outside-init
if offset:
info.lo_offset = offset
if sizelimit:
info.lo_sizelimit = sizelimit
if autoclear is not None:
if autoclear:
info.lo_flags |= self.LO_FLAGS_AUTOCLEAR
else:
info.lo_flags &= ~self.LO_FLAGS_AUTOCLEAR
if partscan is not None:
if partscan:
info.lo_flags |= self.LO_FLAGS_PARTSCAN
else:
info.lo_flags &= ~self.LO_FLAGS_PARTSCAN
return info
def set_status(self, offset=None, sizelimit=None, autoclear=None, partscan=None):
"""Set properties of the loopback device
@ -353,24 +372,66 @@ class Loop:
unchanged (default is None)
"""
# pylint: disable=attribute-defined-outside-init
info = self.get_status()
if offset:
info.lo_offset = offset
if sizelimit:
info.lo_sizelimit = sizelimit
if autoclear is not None:
if autoclear:
info.lo_flags |= self.LO_FLAGS_AUTOCLEAR
else:
info.lo_flags &= ~self.LO_FLAGS_AUTOCLEAR
if partscan is not None:
if partscan:
info.lo_flags |= self.LO_FLAGS_PARTSCAN
else:
info.lo_flags &= ~self.LO_FLAGS_PARTSCAN
info = self._config_info(self.get_status(), offset, sizelimit, autoclear, partscan)
fcntl.ioctl(self.fd, self.LOOP_SET_STATUS64, info)
def configure(self, fd: int, offset=None, sizelimit=None, autoclear=None, partscan=None):
"""
Configure the loopback device
Bind and configure in a single operation a file descriptor to the
loopback device.
Only supported for kenel >= 5.8
The loopback device must be unbound. The backing file must be
either a regular file or a block device. If the backing file is
itself a loopback device, then a cycle must not be created. If
the backing file is opened read-only, then the resulting
loopback device will be read-only too.
The properties will be cleared once the device is unbound, but preserved
by changing the backing file descriptor.
Note that this operation is not atomic: All the current properties
are read out, the ones specified in this function call are modified,
and then they are written back. For this reason, concurrent
modification of the properties must be avoided.
Setting sizelimit means the size of the loopback device is taken
to be the max of the size of the backing file and the limit. A
limit of 0 is taken to mean unlimited.
Enabling autoclear has the same effect as calling clear_fd().
When partscan is first enabled, the partition table of the
device is scanned, and new blockdevices potentially added for
the partitions.
Parameters
----------
fd : int
the file descriptor to bind
offset : int, optional
The offset in bytes from the start of the backing file, or
None to leave unchanged (default is None)
sizelimit : int, optional
The max size in bytes to make the loopback device, or None
to leave unchanged (default is None)
autoclear : bool, optional
Whether or not to enable autoclear, or None to leave unchanged
(default is None)
partscan : bool, optional
Whether or not to enable partition scanning, or None to leave
unchanged (default is None)
"""
# pylint: disable=attribute-defined-outside-init
config = LoopConfig()
config.fd = fd
# Previous implementation was not configuring the block size.
# Keep same behavior here by setting the value to 0.
config.block_size = 0
config.info = self._config_info(LoopInfo(), offset, sizelimit, autoclear, partscan)
fcntl.ioctl(self.fd, self.LOOP_CONFIGURE, config)
def get_status(self) -> LoopInfo:
"""Get properties of the loopback device
@ -598,21 +659,18 @@ class LoopControl:
continue
try:
lo.set_fd(fd)
except OSError as e:
lo.close()
if e.errno == errno.EBUSY:
continue
raise e
# `set_status` returns EBUSY when the pages from the
# previously bound file have not been fully cleared yet.
try:
lo.set_status(**kwargs)
lo.configure(fd, **kwargs)
except BlockingIOError:
lo.clear_fd()
lo.close()
continue
except OSError as e:
lo.close()
# `loop_configure` returns EBUSY when the pages from the
# previously bound file have not been fully cleared yet.
if e.errno == errno.EBUSY:
continue
raise e
break
return lo

18
osbuild/remoteloop.py
View file

@ -48,20 +48,18 @@ class LoopServer(api.BaseAPI):
# file descriptor to it is racy, so we must use a retry loop
lo = loop.Loop(self.ctl.get_unbound())
try:
lo.set_fd(fd)
except OSError as e:
lo.close()
if e.errno == errno.EBUSY:
continue
raise e
# `set_status` returns EBUSY when the pages from the previously
# bound file have not been fully cleared yet.
try:
lo.set_status(offset=offset, sizelimit=sizelimit, autoclear=True)
lo.configure(fd, offset=offset, sizelimit=sizelimit, autoclear=True)
except BlockingIOError:
lo.clear_fd()
lo.close()
continue
except OSError as e:
lo.close()
# `configure` returns EBUSY when the pages from the previously
# bound file have not been fully cleared yet.
if e.errno == errno.EBUSY:
continue
raise e
break
lo.mknod(dir_fd)

2
test/run/test_devices.py
View file

@ -67,7 +67,7 @@ def test_loopback_basic(tmpdir):
with pytest.raises(OSError):
with open(testfile, "wb") as f:
f.truncate(1)
lo.set_fd(f.fileno())
lo.configure(f.fileno())
lo.close()