implicit dependency in x86/page_64.h:task_size_max

From: Ulrich Drepper
Date: Wed Sep 08 2021 - 11:06:03 EST

I don't have a 5.14 kernel installed, yet, but from the looks of it
this problem still persists.

Out-of-tree modules were broken by commit
025768a966a3dde8455de46d1f121a51bacb6a77 which introduced a dependency
on a CPU feature macro in the task_size_max implementation
(X86_FEATURE_LA57) without making sure the macro is defined. The
result is that the compiler generates an object file with an reference
to a symbol with this name. The resulting modpost invocation catches

An example for the problem exists with the blackmagic-io module. I
name it here so people can perhaps find this post.

The "solution" so far is to include <asm/cpufeatures.h> before
task_size_max is defined.

In userlevel headers we had the convention that any header should
#include all the other headers to satisfy its dependencies. If
arch/x86/asm/page_64.h would include <asm/cpufeatures.h> first there
would be no problem with the module above but I haven't checked
whether this creates other problems. And as I mentioned, I don't know
what the convention is.

Someone will know and perhaps we could get a patch installed with
either the missing #include added or

#ifndef X86_FEATURE_LA57
# error "you need to include <asm/cpufeatures.h> first"