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loop-util: track CLOCK_MONOTONIC timestamp immediately before attaching a loopback device

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This is similar to the preceding work to store the uevent seqnum, but
this stores the CLOCK_MONOTONIC timestamp.

Why? This allows to validate udev database entries, to determine if they
were created *after* we attached the device.

The uevent seqnum logic allows us to validate uevent, and the timestamp
database entries, hence together we should be able to validate both
sources of truth for us.

(note that this is all racy, just a bit less racy, since we cannot
atomically attach loopback devices and get the timestamp for it, the
same way we can't get the uevent seqnum. Thus is shortens the race
window, but doesn#t close it).
This commit is contained in:
Lennart Poettering
2021-04-20 17:20:38 +02:00
parent 8626b43be4
commit 8ede1e86b2
2 changed files with 16 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/shared/loop-util.c
+14 -2
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@@ -132,12 +132,14 @@ static int loop_configure(
int nr,
const struct loop_config *c,
bool *try_loop_configure,
uint64_t *ret_seqnum_not_before) {
uint64_t *ret_seqnum_not_before,
usec_t *ret_timestamp_not_before) {
_cleanup_(sd_device_unrefp) sd_device *d = NULL;
_cleanup_free_ char *sysname = NULL;
_cleanup_close_ int lock_fd = -1;
uint64_t seqnum;
usec_t timestamp;
int r;
assert(fd >= 0);
@@ -195,6 +197,7 @@ static int loop_configure(
r = get_current_uevent_seqnum(&seqnum);
if (r < 0)
return r;
timestamp = now(CLOCK_MONOTONIC);
if (ioctl(fd, LOOP_CONFIGURE, c) < 0) {
/* Do fallback only if LOOP_CONFIGURE is not supported, propagate all other
@@ -255,6 +258,8 @@ static int loop_configure(
if (ret_seqnum_not_before)
*ret_seqnum_not_before = seqnum;
if (ret_timestamp_not_before)
*ret_timestamp_not_before = timestamp;
return 0;
}
@@ -264,6 +269,7 @@ static int loop_configure(
r = get_current_uevent_seqnum(&seqnum);
if (r < 0)
return r;
timestamp = now(CLOCK_MONOTONIC);
/* Since kernel commit 5db470e229e22b7eda6e23b5566e532c96fb5bc3 (kernel v5.0) the LOOP_SET_STATUS64
* ioctl can return EAGAIN in case we change the lo_offset field, if someone else is accessing the
@@ -294,6 +300,8 @@ static int loop_configure(
if (ret_seqnum_not_before)
*ret_seqnum_not_before = seqnum;
if (ret_timestamp_not_before)
*ret_timestamp_not_before = timestamp;
return 0;
@@ -353,6 +361,7 @@ int loop_device_make(
struct loop_config config;
LoopDevice *d = NULL;
uint64_t seqnum = UINT64_MAX;
usec_t timestamp = USEC_INFINITY;
struct stat st;
int nr = -1, r;
@@ -396,6 +405,7 @@ int loop_device_make(
.relinquished = true, /* It's not allocated by us, don't destroy it when this object is freed */
.devno = st.st_rdev,
.uevent_seqnum_not_before = UINT64_MAX,
.timestamp_not_before = USEC_INFINITY,
};
*ret = d;
@@ -443,7 +453,7 @@ int loop_device_make(
if (!IN_SET(errno, ENOENT, ENXIO))
return -errno;
} else {
r = loop_configure(loop, nr, &config, &try_loop_configure, &seqnum);
r = loop_configure(loop, nr, &config, &try_loop_configure, &seqnum, &timestamp);
if (r >= 0) {
loop_with_fd = TAKE_FD(loop);
break;
@@ -481,6 +491,7 @@ int loop_device_make(
.nr = nr,
.devno = st.st_rdev,
.uevent_seqnum_not_before = seqnum,
.timestamp_not_before = timestamp,
};
*ret = d;
@@ -617,6 +628,7 @@ int loop_device_open(const char *loop_path, int open_flags, LoopDevice **ret) {
.relinquished = true, /* It's not ours, don't try to destroy it when this object is freed */
.devno = st.st_dev,
.uevent_seqnum_not_before = UINT64_MAX,
.timestamp_not_before = USEC_INFINITY,
};
*ret = d;