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loop-util: LOOP_CLR_FD is async, don't retry to reuse a device right after issuing it

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When we fall back to classic LOOP_SET_FD logic in case LOOP_CONFIGURE
didn't work we issue LOOP_CLR_FD first. But that call turns out to be
potentially async in the kernel: if something else (let's say
udev/blkid) is accessing the device the ioctl just sets the autoclear
flag and exits. Hence quite often the LOOP_SET_FD will subsequently
fail. Let's avoid the trouble, and immediately exit with EBUSY if
LOOP_CONFIGURE fails, and but remember that LOOP_CONFIGURE is not
available so that on the next iteration we go directly for LOOP_SET_FD
instead.
This commit is contained in:
Lennart Poettering 2020-09-25 15:22:48 +02:00
parent 738f29cb53
commit 95c5009248
1 changed files with 62 additions and 43 deletions
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105
src/shared/loop-util.c
View file

@ -33,60 +33,78 @@ static void cleanup_clear_loop_close(int *fd) {
(void) safe_close(*fd);
}
static int loop_configure(int fd, const struct loop_config *c) {
static int loop_configure(
int fd,
const struct loop_config *c,
bool *try_loop_configure) {
_cleanup_close_ int lock_fd = -1;
int r;
assert(fd >= 0);
assert(c);
assert(try_loop_configure);
if (ioctl(fd, LOOP_CONFIGURE, c) < 0) {
/* Do fallback only if LOOP_CONFIGURE is not supported, propagate all other errors. Note that
* the kernel is weird: non-existing ioctls currently return EINVAL rather than ENOTTY on
* loopback block devices. They should fix that in the kernel, but in the meantime we accept
* both here. */
if (!ERRNO_IS_NOT_SUPPORTED(errno) && errno != EINVAL)
return -errno;
} else {
bool good = true;
if (*try_loop_configure) {
if (ioctl(fd, LOOP_CONFIGURE, c) < 0) {
/* Do fallback only if LOOP_CONFIGURE is not supported, propagate all other
* errors. Note that the kernel is weird: non-existing ioctls currently return EINVAL
* rather than ENOTTY on loopback block devices. They should fix that in the kernel,
* but in the meantime we accept both here. */
if (!ERRNO_IS_NOT_SUPPORTED(errno) && errno != EINVAL)
return -errno;
if (c->info.lo_sizelimit != 0) {
/* Kernel 5.8 vanilla doesn't properly propagate the size limit into the block
* device. If it's used, let's immediately check if it had the desired effect
* hence. And if not use classic LOOP_SET_STATUS64. */
uint64_t z;
*try_loop_configure = false;
} else {
bool good = true;
if (ioctl(fd, BLKGETSIZE64, &z) < 0) {
r = -errno;
if (c->info.lo_sizelimit != 0) {
/* Kernel 5.8 vanilla doesn't properly propagate the size limit into the
* block device. If it's used, let's immediately check if it had the desired
* effect hence. And if not use classic LOOP_SET_STATUS64. */
uint64_t z;
if (ioctl(fd, BLKGETSIZE64, &z) < 0) {
r = -errno;
goto fail;
}
if (z != c->info.lo_sizelimit) {
log_debug("LOOP_CONFIGURE is broken, doesn't honour .lo_sizelimit. Falling back to LOOP_SET_STATUS64.");
good = false;
}
}
if (FLAGS_SET(c->info.lo_flags, LO_FLAGS_PARTSCAN)) {
/* Kernel 5.8 vanilla doesn't properly propagate the partition scanning flag
* into the block device. Let's hence verify if things work correctly here
* before returning. */
r = blockdev_partscan_enabled(fd);
if (r < 0)
goto fail;
if (r == 0) {
log_debug("LOOP_CONFIGURE is broken, doesn't honour LO_FLAGS_PARTSCAN. Falling back to LOOP_SET_STATUS64.");
good = false;
}
}
if (!good) {
/* LOOP_CONFIGURE doesn't work. Remember that. */
*try_loop_configure = false;
/* We return EBUSY here instead of retrying immediately with LOOP_SET_FD,
* because LOOP_CLR_FD is async: if the operation cannot be executed right
* away it just sets the autoclear flag on the device. This means there's a
* good chance we cannot actually reuse the loopback device right-away. Hence
* let's assume it's busy, avoid the trouble and let the calling loop call us
* again with a new, likely unused device. */
r = -EBUSY;
goto fail;
}
if (z != c->info.lo_sizelimit) {
log_debug("LOOP_CONFIGURE is broken, doesn't honour .lo_sizelimit. Falling back to LOOP_SET_STATUS64.");
good = false;
}
}
if (FLAGS_SET(c->info.lo_flags, LO_FLAGS_PARTSCAN)) {
/* Kernel 5.8 vanilla doesn't properly propagate the partition scanning flag into the
* block device. Let's hence verify if things work correctly here before
* returning. */
r = blockdev_partscan_enabled(fd);
if (r < 0)
goto fail;
if (r == 0) {
log_debug("LOOP_CONFIGURE is broken, doesn't honour LO_FLAGS_PARTSCAN. Falling back to LOOP_SET_STATUS64.");
good = false;
}
}
if (good)
return 0;
/* Otherwise, undo the attachment and use the old APIs */
if (ioctl(fd, LOOP_CLR_FD) < 0)
return -errno;
}
}
/* Since kernel commit 5db470e229e22b7eda6e23b5566e532c96fb5bc3 (kernel v5.0) the LOOP_SET_STATUS64
@ -143,6 +161,7 @@ int loop_device_make(
LoopDevice **ret) {
_cleanup_free_ char *loopdev = NULL;
bool try_loop_configure = true;
struct loop_config config;
LoopDevice *d = NULL;
struct stat st;
@ -233,7 +252,7 @@ int loop_device_make(
if (!IN_SET(errno, ENOENT, ENXIO))
return -errno;
} else {
r = loop_configure(loop, &config);
r = loop_configure(loop, &config, &try_loop_configure);
if (r >= 0) {
loop_with_fd = TAKE_FD(loop);
break;