loop: use LOOP_CONFIGURE instead of LOOP_SET_FD
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
Achilleas Koutsou
parent
04eab998b7
commit
da11ef4eb0
3 changed files with 108 additions and 52 deletions
140
osbuild/loop.py
140
osbuild/loop.py
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@ -52,6 +52,15 @@ class LoopInfo(ctypes.Structure):
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self.lo_inode == info.st_ino)
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class LoopConfig(ctypes.Structure):
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_fields_ = [
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('fd', ctypes.c_uint32),
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('block_size', ctypes.c_uint32),
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('info', LoopInfo),
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('__reserved', ctypes.c_uint64*8),
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]
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class Loop:
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"""Loopback device
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@ -60,8 +69,8 @@ class Loop:
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Methods
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-------
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set_fd(fd)
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Bind a file descriptor to the loopback device
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loop_configure(fd)
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Bind a file descriptor to the loopback device and set properties of the loopback device
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clear_fd()
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Unbind the file descriptor from the loopback device
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change_fd(fd)
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@ -89,6 +98,7 @@ class Loop:
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LOOP_SET_CAPACITY = 0x4C07
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LOOP_SET_DIRECT_IO = 0x4C08
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LOOP_SET_BLOCK_SIZE = 0x4C09
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LOOP_CONFIGURE = 0x4C0A
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def __init__(self, minor, dir_fd=None):
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"""
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@ -182,18 +192,9 @@ class Loop:
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linux.ioctl_blockdev_flushbuf(self.fd)
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def set_fd(self, fd):
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"""Bind a file descriptor to the loopback device
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The loopback device must be unbound. The backing file must be
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either a regular file or a block device. If the backing file is
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itself a loopback device, then a cycle must not be created. If
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the backing file is opened read-only, then the resulting
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loopback device will be read-only too.
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Parameters
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----------
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fd : int
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the file descriptor to bind
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"""
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Deprecated, use configure instead.
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TODO delete this after image-info gets updated.
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"""
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fcntl.ioctl(self.fd, self.LOOP_SET_FD, fd)
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@ -315,6 +316,24 @@ class Loop:
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# it is bound, check if it is bound by `fd`
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return loop_info.is_bound_to(file_info)
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def _config_info(self, info, offset, sizelimit, autoclear, partscan):
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# pylint: disable=attribute-defined-outside-init
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if offset:
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info.lo_offset = offset
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if sizelimit:
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info.lo_sizelimit = sizelimit
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if autoclear is not None:
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if autoclear:
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info.lo_flags |= self.LO_FLAGS_AUTOCLEAR
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else:
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info.lo_flags &= ~self.LO_FLAGS_AUTOCLEAR
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if partscan is not None:
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if partscan:
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info.lo_flags |= self.LO_FLAGS_PARTSCAN
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else:
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info.lo_flags &= ~self.LO_FLAGS_PARTSCAN
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return info
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def set_status(self, offset=None, sizelimit=None, autoclear=None, partscan=None):
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"""Set properties of the loopback device
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@ -353,24 +372,66 @@ class Loop:
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unchanged (default is None)
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"""
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# pylint: disable=attribute-defined-outside-init
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info = self.get_status()
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if offset:
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info.lo_offset = offset
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if sizelimit:
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info.lo_sizelimit = sizelimit
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if autoclear is not None:
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if autoclear:
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info.lo_flags |= self.LO_FLAGS_AUTOCLEAR
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else:
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info.lo_flags &= ~self.LO_FLAGS_AUTOCLEAR
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if partscan is not None:
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if partscan:
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info.lo_flags |= self.LO_FLAGS_PARTSCAN
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else:
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info.lo_flags &= ~self.LO_FLAGS_PARTSCAN
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info = self._config_info(self.get_status(), offset, sizelimit, autoclear, partscan)
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fcntl.ioctl(self.fd, self.LOOP_SET_STATUS64, info)
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def configure(self, fd: int, offset=None, sizelimit=None, autoclear=None, partscan=None):
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"""
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Configure the loopback device
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Bind and configure in a single operation a file descriptor to the
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loopback device.
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Only supported for kenel >= 5.8
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The loopback device must be unbound. The backing file must be
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either a regular file or a block device. If the backing file is
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itself a loopback device, then a cycle must not be created. If
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the backing file is opened read-only, then the resulting
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loopback device will be read-only too.
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The properties will be cleared once the device is unbound, but preserved
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by changing the backing file descriptor.
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Note that this operation is not atomic: All the current properties
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are read out, the ones specified in this function call are modified,
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and then they are written back. For this reason, concurrent
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modification of the properties must be avoided.
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Setting sizelimit means the size of the loopback device is taken
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to be the max of the size of the backing file and the limit. A
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limit of 0 is taken to mean unlimited.
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Enabling autoclear has the same effect as calling clear_fd().
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When partscan is first enabled, the partition table of the
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device is scanned, and new blockdevices potentially added for
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the partitions.
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Parameters
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----------
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fd : int
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the file descriptor to bind
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offset : int, optional
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The offset in bytes from the start of the backing file, or
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None to leave unchanged (default is None)
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sizelimit : int, optional
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The max size in bytes to make the loopback device, or None
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to leave unchanged (default is None)
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autoclear : bool, optional
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Whether or not to enable autoclear, or None to leave unchanged
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(default is None)
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partscan : bool, optional
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Whether or not to enable partition scanning, or None to leave
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unchanged (default is None)
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"""
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# pylint: disable=attribute-defined-outside-init
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config = LoopConfig()
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config.fd = fd
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# Previous implementation was not configuring the block size.
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# Keep same behavior here by setting the value to 0.
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config.block_size = 0
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config.info = self._config_info(LoopInfo(), offset, sizelimit, autoclear, partscan)
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fcntl.ioctl(self.fd, self.LOOP_CONFIGURE, config)
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def get_status(self) -> LoopInfo:
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"""Get properties of the loopback device
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@ -598,21 +659,18 @@ class LoopControl:
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continue
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try:
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lo.set_fd(fd)
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except OSError as e:
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lo.close()
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if e.errno == errno.EBUSY:
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continue
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raise e
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# `set_status` returns EBUSY when the pages from the
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# previously bound file have not been fully cleared yet.
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try:
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lo.set_status(**kwargs)
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lo.configure(fd, **kwargs)
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except BlockingIOError:
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lo.clear_fd()
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lo.close()
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continue
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except OSError as e:
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lo.close()
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# `loop_configure` returns EBUSY when the pages from the
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# previously bound file have not been fully cleared yet.
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if e.errno == errno.EBUSY:
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continue
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raise e
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break
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return lo
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