[PATCH v2] loopdev: sync capacity after setting it

Jeff Mahoney
2013-03-27T20:45:08Z

I recently tried to mount an hfsplus file system from an image file with
a partition table by using the loop offset and sizelimit options to specify
the location of the file system.

hfsplus stores some metadata at a set offset from the end of the partition,
so it's sensitive to the device size reported by the kernel.

It worked with this:
# losetup -r -o 32k --sizelimit=102400000 <loopdev> <imagefile>
# mount -r -t hfsplus <loopdev> <mountpoint>

But failed with this:
# mount -r -oloop,offset=32k,sizelimit=102400000 <imagefile> <mountpoint>

# losetup -a
/dev/loop0: [0089]:2 (<imagefile>), offset 32768, sizelimit 102400000
/dev/loop1: [0089]:2 (<imagefile>), offset 32768, sizelimit 102400000

/proc/partitions shows the correct number of blocks to match the sizelimit.

But if I set a breakpoint in mount before the mount syscall, I could see:
# blockdev --getsize64 /dev/loop[01]
102400000
102432768

The kernel loop driver will set the gendisk capacity of the device at
LOOP_SET_STATUS64 but won't sync it to the block device until one of two
conditions are met: All open file descriptors referring to the device are
closed (and it will sync when re-opened) or if the LOOP_SET_CAPACITY ioctl
is called to sync it. Since mount opens the device and passes it directly
to the mount syscall after LOOP_SET_STATUS64 without closing and reopening
it, the sizelimit argument is effectively ignroed. The capacity needs to
be synced immediately for it to work as expected.

This patch adds the LOOP_SET_CAPACITY call to loopctx_setup_device since
the device isn't yet released to the user, so it's safe to sync the capacity
immediately.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
---
include/loopdev.h | 1
lib/Makefile.am | 4 +-
lib/blkdev.c | 2 -
lib/loopdev.c | 89 +++++++++++++++++++++++++++++++++++++++++++++--
libmount/src/Makefile.am | 1
mount/mount.8 | 16 ++++++++
sys-utils/Makefile.am | 3 +
sys-utils/losetup.c | 19 ++--------
8 files changed, 114 insertions(+), 21 deletions(-)

--- a/include/loopdev.h
+++ b/include/loopdev.h
@@ -160,6 +160,7 @@ extern int loopcxt_next(struct loopdev_c

extern int loopcxt_setup_device(struct loopdev_cxt *lc);
extern int loopcxt_delete_device(struct loopdev_cxt *lc);
+extern int loopcxt_set_capacity(struct loopdev_cxt *lc);

int loopcxt_set_offset(struct loopdev_cxt *lc, uint64_t offset);
int loopcxt_set_sizelimit(struct loopdev_cxt *lc, uint64_t sizelimit);
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -13,6 +13,7 @@ noinst_PROGRAMS += test_sysfs test_loopd
endif

test_blkdev_SOURCES = blkdev.c
+test_blkdev_CFLAGS = -DTEST_PROGRAM_BLKDEV
test_ismounted_SOURCES = ismounted.c
test_wholedisk_SOURCES = wholedisk.c
test_mangle_SOURCES = mangle.c
@@ -31,7 +32,8 @@ test_sysfs_CFLAGS = -DTEST_PROGRAM_SYSFS
test_loopdev_SOURCES = loopdev.c \
$(test_sysfs_SOURCES) \
$(top_srcdir)/lib/linux_version.c \
- $(top_srcdir)/lib/canonicalize.c
+ $(top_srcdir)/lib/canonicalize.c \
+ $(top_srcdir)/lib/blkdev.c

test_loopdev_CFLAGS = -DTEST_PROGRAM_LOOPDEV
endif
--- a/lib/blkdev.c
+++ b/lib/blkdev.c
@@ -263,7 +263,7 @@ int blkdev_is_cdrom(int fd)
#endif
}

-#ifdef TEST_PROGRAM
+#ifdef TEST_PROGRAM_BLKDEV
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
--- a/lib/loopdev.c
+++ b/lib/loopdev.c
@@ -37,6 +37,7 @@
#include "pathnames.h"
#include "loopdev.h"
#include "canonicalize.h"
+#include "blkdev.h"

#define CONFIG_LOOPDEV_DEBUG

@@ -968,6 +969,66 @@ int loopcxt_set_backing_file(struct loop
}

/*
+ * In kernels prior to v3.9, if the offset or sizelimit options
+ * are used, the block device's size won't be synced automatically.
+ * blockdev --getsize64 and filesystems will use the backing
+ * file size until the block device has been re-opened or the
+ * LOOP_SET_CAPACITY ioctl is called to sync the sizes.
+ *
+ * Since mount -oloop uses the LO_FLAGS_AUTOCLEAR option and passes
+ * the open file descriptor to the mount system call, we need to use
+ * the ioctl. Calling losetup directly doesn't have this problem since
+ * it closes the device when it exits and whatever consumes the device
+ * next will re-open it, causing the resync.
+ */
+static int
+loopcxt_check_size(struct loopdev_cxt *lc, int file_fd)
+{
+ unsigned long long size, expected_size;
+ int dev_fd;
+ struct stat st;
+ int retry = 2;
+
+ if (!lc->info.lo_offset && !lc->info.lo_sizelimit)
+ return 0;
+
+ if (fstat(file_fd, &st))
+ return -errno;
+
+ expected_size = st.st_size;
+
+ if (lc->info.lo_offset > 0)
+ expected_size -= lc->info.lo_offset;
+
+ if (lc->info.lo_sizelimit > 0 && lc->info.lo_sizelimit < expected_size)
+ expected_size = lc->info.lo_sizelimit;
+
+ dev_fd = loopcxt_get_fd(lc);
+ if (dev_fd < 0)
+ return -errno;
+
+ if (blkdev_get_size(dev_fd, &size))
+ return -errno;
+
+ if (expected_size != size) {
+ if (loopcxt_set_capacity(lc)) {
+ /* ioctl not available */
+ if (errno == ENOTTY || errno == EINVAL)
+ errno = ERANGE;
+ return -errno;
+ }
+
+ if (blkdev_get_size(dev_fd, &size))
+ return -errno;
+
+ if (expected_size != size)
+ return -ERANGE;
+ }
+
+ return 0;
+}
+
+/*
* @cl: context
*
* Associate the current device (see loopcxt_{set,get}_device()) with
@@ -1045,9 +1106,6 @@ int loopcxt_setup_device(struct loopdev_

DBG(lc, loopdev_debug("setup: LOOP_SET_FD: OK"));

- close(file_fd);
- file_fd = -1;
-
if (ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info)) {
DBG(lc, loopdev_debug("LOOP_SET_STATUS64 failed: %m"));
goto err;
@@ -1055,6 +1113,12 @@ int loopcxt_setup_device(struct loopdev_

DBG(lc, loopdev_debug("setup: LOOP_SET_STATUS64: OK"));

+ if ((rc = loopcxt_check_size(lc, file_fd)))
+ goto err;
+
+ close(file_fd);
+ file_fd = -1;
+
memset(&lc->info, 0, sizeof(lc->info));
lc->has_info = 0;
lc->info_failed = 0;
@@ -1071,6 +1135,25 @@ err:
return rc;
}

+int loopcxt_set_capacity(struct loopdev_cxt *lc)
+{
+ int fd = loopcxt_get_fd(lc);
+ int rc;
+
+ if (fd < 0)
+ return -EINVAL;
+
+ /* Kernels prior to v2.6.30 don't support this ioctl */
+ if (ioctl(fd, LOOP_SET_CAPACITY, 0) < 0) {
+ int rc = -errno;
+ DBG(lc, loopdev_debug("LOOP_SET_CAPACITY failed: %m"));
+ return rc;
+ }
+
+ DBG(lc, loopdev_debug("capacity set"));
+ return 0;
+}
+
int loopcxt_delete_device(struct loopdev_cxt *lc)
{
int fd = loopcxt_get_fd(lc);
--- a/libmount/src/Makefile.am
+++ b/libmount/src/Makefile.am
@@ -23,6 +23,7 @@ libmount_la_SOURCES = mountP.h version.c
$(top_srcdir)/lib/strutils.c \
$(top_srcdir)/lib/env.c \
$(top_srcdir)/lib/loopdev.c \
+ $(top_srcdir)/lib/blkdev.c \
$(top_srcdir)/lib/sysfs.c \
$(top_srcdir)/lib/linux_version.c

--- a/mount/mount.8
+++ b/mount/mount.8
@@ -2844,6 +2844,22 @@ and
.BR ioctl
families of functions) may lead to inconsistent result due to the lack of
consistency check in kernel even if noac is used.
+.PP
+The
+.B loop
+option with the
+.B offset
+or
+.B sizelimit
+options used may fail when using older kernels if the
+.B mount
+command can't confirm that the size of the block device has been configured
+as requested. This situation can be worked around by using
+the
+.B losetup
+command manually before calling
+.B mount
+with the configured loop device.
.SH HISTORY
A
.B mount
--- a/sys-utils/Makefile.am
+++ b/sys-utils/Makefile.am
@@ -34,7 +34,8 @@ losetup_SOURCES = losetup.c \
$(top_srcdir)/lib/loopdev.c \
$(top_srcdir)/lib/canonicalize.c \
$(top_srcdir)/lib/xgetpass.c \
- $(top_srcdir)/lib/strutils.c
+ $(top_srcdir)/lib/strutils.c \
+ $(top_srcdir)/lib/blkdev.c

if HAVE_STATIC_LOSETUP
bin_PROGRAMS += losetup.static
--- a/sys-utils/losetup.c
+++ b/sys-utils/losetup.c
@@ -107,20 +107,6 @@ static int show_all_loops(struct loopdev
return 0;
}

-static int set_capacity(struct loopdev_cxt *lc)
-{
- int fd = loopcxt_get_fd(lc);
-
- if (fd < 0)
- warn(_("%s: open failed"), loopcxt_get_device(lc));
- else if (ioctl(fd, LOOP_SET_CAPACITY) != 0)
- warnx(_("%s: set capacity failed"), loopcxt_get_device(lc));
- else
- return 0;
-
- return -1;
-}
-
static int delete_loop(struct loopdev_cxt *lc)
{
if (loopcxt_delete_device(lc))
@@ -394,7 +380,10 @@ int main(int argc, char **argv)
warn(_("%s"), loopcxt_get_device(&lc));
break;
case A_SET_CAPACITY:
- res = set_capacity(&lc);
+ res = loopcxt_set_capacity(&lc);
+ if (res)
+ warn(_("%s: set capacity failed"),
+ loopcxt_get_device(&lc));
break;
default:
usage(stderr);

--
Jeff Mahoney
SUSE Labs