Re: [PATCH] MODSIGN: Change default key details [ver #2]

[Linus Torvalds]

On Tue, May 5, 2015 at 7:33 AM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> +define filechk_x509_list
>> +     echo $(X509_CERTIFICATES)
>> +endef
>>  targets += $(obj)/.x509.list
>> -$(obj)/.x509.list:
>> -     @echo $(X509_CERTIFICATES) >$@
>> +$(obj)/.x509.list: Makefile FORCE
>> +     $(call filechk,x509_list)
>
> How does that actually work?

So the Makefile dependency is entirely fake, it's just that the
"filechk" macro wants to have an input. The FORCE means that it ends
up being done every time, regardless of whether the Makefile has
changed or not.

See scripts/Kbuild.include for details on the "filechk" macro, bit it
basically takes another make macro (called "filechk_xyz"), runs that
macro as a shell script and outputs it to a temporary file:

        $(filechk_$(1)) < $< > $@.tmp;

(that "$<" is also why the rule wants a some input, in our case
"Makefile"). It then does a move-if-changed of the temporary file to
the target.

So the end result is that we run that "filechk_x509_list" script,
compare the output to the old target, and update the target iff it is
different. That would seem to be exactly what we want.

That said, as mentioned, the whole "X509_CERTIFICATES" thing is
unstable, and ends up being "./signing_key.x509" or "signing_key.x509"
depending on whether that file existed or not. That needs fixing, so
that we get stable output. So some filtering required.

                      Linus
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