On Thu, 11 Mar 2021 12:04:19 +0000, David Howells said:

> 	EXTRACT_CERTS   /usr/src/linux-next/"certs/signing_key.pem"
>
> but I don't know why.  There are some odd quotes in your line also which may
> be related to the problem.  The relevant config line looks the same:
>
> 	CONFIG_MODULE_SIG_KEY="certs/signing_key.pem"

Aha.  I figured it out.

If you have a *totally* clean source tree, 'make -O' works for all users.
If you have in the past done a build in the tree, and then done a 'make mrproper'
to clean it out so 'make -O' doesn't complain, it fails because it
finds an *old* certs/signing_key.pem in /usr/src/linux-next and tries to
put the new generated files in the same directory.

So the root cause was: 'make mrproper doesn't clean certs/' out enough,
and this chunk of certs/Makefile

# If CONFIG_MODULE_SIG_KEY isn't a PKCS#11 URI, depend on it
ifeq ($(patsubst pkcs11:%,%,$(firstword $(MODULE_SIG_KEY_FILENAME))),$(firstword $(MODULE_SIG_KEY_FILENAME)))
X509_DEP := $(MODULE_SIG_KEY_SRCPREFIX)$(MODULE_SIG_KEY_FILENAME)
endif

MODULE_SIG_KEY_SRCPREFIX was where my /usr/src/linux-next was coming from...

I admit not being sure how (or if) this should be fixed