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.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/dissect/dissect.c b/src/dissect/dissect.c
index 06564e94b1..e5264d09df 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;