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file-posix: Make auto-read-only dynamic 

Until now, with auto-read-only=on we tried to open the file read-write
first and if that failed, read-only was tried. This is actually not good
enough for libvirt, which gives QEMU SELinux permissions for read-write
only as soon as it actually intends to write to the image. So we need to
be able to switch between read-only and read-write at runtime.

This patch makes auto-read-only dynamic, i.e. the file is opened
read-only as long as no user of the node has requested write
permissions, but it is automatically reopened read-write as soon as the
first writer is attached. Conversely, if the last writer goes away, the
file is reopened read-only again.

bs->read_only is no longer set for auto-read-only=on files even if the
file descriptor is opened read-only because it will be transparently
upgraded as soon as a writer is attached. This changes the output of
qemu-iotests 232.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2019-03-01 22:15:11 +01:00
parent 6ceabe6f77
commit 23dece19da
2 changed files with 23 additions and 25 deletions
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36
block/file-posix.c
View File

@ -376,13 +376,21 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
}
}
static void raw_parse_flags(int bdrv_flags, int *open_flags)
static void raw_parse_flags(int bdrv_flags, int *open_flags, bool has_writers)
{
bool read_write = false;
assert(open_flags != NULL);
*open_flags |= O_BINARY;
*open_flags &= ~O_ACCMODE;
if (bdrv_flags & BDRV_O_RDWR) {
if (bdrv_flags & BDRV_O_AUTO_RDONLY) {
read_write = has_writers;
} else if (bdrv_flags & BDRV_O_RDWR) {
read_write = true;
}
if (read_write) {
*open_flags |= O_RDWR;
} else {
*open_flags |= O_RDONLY;
@ -518,24 +526,12 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
false);
s->open_flags = open_flags;
raw_parse_flags(bdrv_flags, &s->open_flags);
raw_parse_flags(bdrv_flags, &s->open_flags, false);
s->fd = -1;
fd = qemu_open(filename, s->open_flags, 0644);
ret = fd < 0 ? -errno : 0;
if (ret == -EACCES || ret == -EROFS) {
/* Try to degrade to read-only, but if it doesn't work, still use the
* normal error message. */
if (bdrv_apply_auto_read_only(bs, NULL, NULL) == 0) {
bdrv_flags &= ~BDRV_O_RDWR;
raw_parse_flags(bdrv_flags, &s->open_flags);
assert(!(s->open_flags & O_CREAT));
fd = qemu_open(filename, s->open_flags);
ret = fd < 0 ? -errno : 0;
}
}
if (ret < 0) {
error_setg_errno(errp, -ret, "Could not open '%s'", filename);
if (ret == -EROFS) {
@ -846,12 +842,14 @@ static int raw_handle_perm_lock(BlockDriverState *bs,
}
static int raw_reconfigure_getfd(BlockDriverState *bs, int flags,
int *open_flags, bool force_dup,
int *open_flags, uint64_t perm, bool force_dup,
Error **errp)
{
BDRVRawState *s = bs->opaque;
int fd = -1;
int ret;
bool has_writers = perm &
(BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED | BLK_PERM_RESIZE);
int fcntl_flags = O_APPEND | O_NONBLOCK;
#ifdef O_NOATIME
fcntl_flags |= O_NOATIME;
@ -862,7 +860,7 @@ static int raw_reconfigure_getfd(BlockDriverState *bs, int flags,
*open_flags |= O_NONBLOCK;
}
raw_parse_flags(flags, open_flags);
raw_parse_flags(flags, open_flags, has_writers);
#ifdef O_ASYNC
/* Not all operating systems have O_ASYNC, and those that don't
@ -942,7 +940,7 @@ static int raw_reopen_prepare(BDRVReopenState *state,
qemu_opts_to_qdict(opts, state->options);
rs->fd = raw_reconfigure_getfd(state->bs, state->flags, &rs->open_flags,
true, &local_err);
state->perm, true, &local_err);
if (local_err) {
error_propagate(errp, local_err);
ret = -1;
@ -2720,7 +2718,7 @@ static int raw_check_perm(BlockDriverState *bs, uint64_t perm, uint64_t shared,
s->perm_change_fd = rs->fd;
} else {
/* We may need a new fd if auto-read-only switches the mode */
ret = raw_reconfigure_getfd(bs, bs->open_flags, &open_flags,
ret = raw_reconfigure_getfd(bs, bs->open_flags, &open_flags, perm,
false, errp);
if (ret < 0) {
return ret;

12
tests/qemu-iotests/232.out
View File

@ -22,12 +22,12 @@ NODE_NAME: TEST_DIR/t.IMGFMT (file, read-only)
NODE_NAME: TEST_DIR/t.IMGFMT (file, read-only)
QEMU_PROG: -drive driver=file,file=TEST_DIR/t.IMGFMT,if=none,read-only=off,auto-read-only=off: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
NODE_NAME: TEST_DIR/t.IMGFMT (file, read-only)
NODE_NAME: TEST_DIR/t.IMGFMT (file, read-only)
NODE_NAME: TEST_DIR/t.IMGFMT (file)
NODE_NAME: TEST_DIR/t.IMGFMT (file)
QEMU_PROG: -drive driver=file,file=TEST_DIR/t.IMGFMT,if=none,auto-read-only=off: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
NODE_NAME: TEST_DIR/t.IMGFMT (file, read-only)
NODE_NAME: TEST_DIR/t.IMGFMT (file, read-only)
NODE_NAME: TEST_DIR/t.IMGFMT (file)
NODE_NAME: TEST_DIR/t.IMGFMT (file)
=== -blockdev with read-write image: read-only/auto-read-only combinations ===
@ -50,11 +50,11 @@ node0: TEST_DIR/t.IMGFMT (file, read-only)
node0: TEST_DIR/t.IMGFMT (file, read-only)
QEMU_PROG: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=node0,read-only=off,auto-read-only=off: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
node0: TEST_DIR/t.IMGFMT (file, read-only)
node0: TEST_DIR/t.IMGFMT (file)
QEMU_PROG: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=node0,read-only=off: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
QEMU_PROG: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=node0,auto-read-only=off: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
node0: TEST_DIR/t.IMGFMT (file, read-only)
node0: TEST_DIR/t.IMGFMT (file)
QEMU_PROG: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=node0: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
=== Try commit to backing file with auto-read-only ===