dm-verity: don't make read-only-rootfs sound like a requirement

Adding to your local.conf right out of the gate:

  EXTRA_IMAGE_FEATURES = "read-only-rootfs"

while you are trying to sort out other things can be just another
complication to an already steep learning curve.

For example, I found simply enabling this with systemd caused:

  systemd[1]: Failed to fork off sandboxing environment for executing generators: Protocol error
  [!!!!!!] Failed to start up manager.
  systemd[1]: Freezing execution.

While I'd like to get to the root cause of that, it doesn't change that
things boot fine w/o adding to EXTRA_IMAGE_FEATURES, even though the
rootfs is still read-only courtesy of dm-verity.

Reword things so as to make it clear it isn't strictly a hard requirement
and hence can be delayed as people work through their implementation.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/docs/dm-verity.txt b/docs/dm-verity.txt
index ce18395..c2dce73 100644
--- a/docs/dm-verity.txt
+++ b/docs/dm-verity.txt
@@ -53,11 +53,18 @@ conf/local.conf and conf/bblayers.conf from the oe-init-build-env
 Firstly, you need the meta-security layer to conf/bblayers.conf along with
 the dependencies it has -- see the top level meta-security README for that.
 
-Next, assuming you'll be using dm-verity for validation of your rootfs,
-you'll need to enable read-only rootfs support in your local.conf with:
+Note that if you are using dm-verity for your rootfs, then it enforces a
+read-only mount right at the kernel level, so be prepared for issues such
+as failed creation of temporary files and similar.
+
+Yocto does support additional checks and changes via setting:
 
 EXTRA_IMAGE_FEATURES = "read-only-rootfs"
 
+...but since read-only is enforced at the kernel level already, using
+this feature isn't a hard requirement.  It may be best to delay/defer
+making use of this until after you've established basic booting.
+
 For more details, see the associated documentation:
 
 https://docs.yoctoproject.org/dev/dev-manual/read-only-rootfs.html