From 759aaedc5ce8fb42c99fbccb374f35f063194a4c Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Wed, 4 Oct 2017 17:40:46 +0200
Subject: [PATCH] dissect: when we invoke dissection on a loop device with
 partscan help the user

This adds some simply detection logic for cases where dissection is
invoked on an externally created loop device, and partitions have been
detected on it, but partition scanning so far was off. If this is
detected we now print a brief message indicating what the issue is,
instead of failing with a useless EINVAL message the kernel passed to
us.
---
 src/dissect/dissect.c      |  4 ++++
 src/nspawn/nspawn.c        |  4 ++++
 src/shared/dissect-image.c | 31 ++++++++++++++++++++++---------
 3 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/src/dissect/dissect.c b/src/dissect/dissect.c
index 06564e9..e5264d0 100644
--- a/src/dissect/dissect.c
+++ b/src/dissect/dissect.c
@@ -216,6 +216,10 @@ int main(int argc, char *argv[]) {
                 log_error_errno(r, "No suitable root partition found in image %s.", arg_image);
                 goto finish;
         }
+        if (r == -EPROTONOSUPPORT) {
+                log_error_errno(r, "Device %s is loopback block device with partition scanning turned off, please turn it on.", arg_image);
+                goto finish;
+        }
         if (r < 0) {
                 log_error_errno(r, "Failed to dissect image: %m");
                 goto finish;
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index d1e3855..cbe98a3 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -3883,6 +3883,10 @@ int main(int argc, char *argv[]) {
                         log_error_errno(r, "--image= is not supported, compiled without blkid support.");
                         goto finish;
                 }
+                if (r == -EPROTONOSUPPORT) {
+                        log_error_errno(r, "Device is loopback block device with partition scanning turned off, please turn it on.");
+                        goto finish;
+                }
                 if (r < 0) {
                         log_error_errno(r, "Failed to dissect image: %m");
                         goto finish;
diff --git a/src/shared/dissect-image.c b/src/shared/dissect-image.c
index 4b59f2c..de9510d 100644
--- a/src/shared/dissect-image.c
+++ b/src/shared/dissect-image.c
@@ -266,18 +266,34 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                         return -EIO;
                 }
                 if (n < z + 1) {
-                        unsigned j;
+                        unsigned j = 0;
 
                         /* The kernel has probed fewer partitions than blkid? Maybe the kernel prober is still running
                          * or it got EBUSY because udev already opened the device. Let's reprobe the device, which is a
                          * synchronous call that waits until probing is complete. */
 
-                        for (j = 0; j < 20; j++) {
+                        for (;;) {
+                                if (j++ > 20)
+                                        return -EBUSY;
 
-                                r = ioctl(fd, BLKRRPART, 0);
-                                if (r < 0)
+                                if (ioctl(fd, BLKRRPART, 0) < 0) {
                                         r = -errno;
-                                if (r >= 0 || r != -EBUSY)
+
+                                        if (r == -EINVAL) {
+                                                struct loop_info64 info;
+
+                                                /* If we are running on a loop device that has partition scanning off,
+                                                 * return an explicit recognizable error about this, so that callers
+                                                 * can generate a proper message explaining the situation. */
+
+                                                if (ioctl(fd, LOOP_GET_STATUS64, &info) >= 0 && (info.lo_flags & LO_FLAGS_PARTSCAN) == 0) {
+                                                        log_debug("Device is loop device and partition scanning is off!");
+                                                        return -EPROTONOSUPPORT;
+                                                }
+                                        }
+                                        if (r != -EBUSY)
+                                                return r;
+                                } else
                                         break;
 
                                 /* If something else has the device open, such as an udev rule, the ioctl will return
@@ -286,11 +302,8 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                                  *
                                  * This is really something they should fix in the kernel! */
 
-                                usleep(50 * USEC_PER_MSEC);
+                                (void) usleep(50 * USEC_PER_MSEC);
                         }
-
-                        if (r < 0)
-                                return r;
                 }
 
                 e = udev_enumerate_unref(e);
-- 
2.7.4