linux-kernel - Re: [PATCH v3 1/6] selftests/mm: mseal, self_elf: fix missing __NR

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Date: Tue, 18 Jun 2024 14:29:32 -0700
From: John Hubbard <jhubbard@...dia.com>
To: David Hildenbrand <david@...hat.com>, Andrew Morton
	<akpm@...ux-foundation.org>, Jeff Xu <jeffxu@...omium.org>, Shuah Khan
	<shuah@...nel.org>
CC: Andrei Vagin <avagin@...gle.com>, Axel Rasmussen
	<axelrasmussen@...gle.com>, Christian Brauner <brauner@...nel.org>, Kees Cook
	<kees@...nel.org>, Kent Overstreet <kent.overstreet@...ux.dev>, "Liam R .
 Howlett" <Liam.Howlett@...cle.com>, Muhammad Usama Anjum
	<usama.anjum@...labora.com>, Peter Xu <peterx@...hat.com>, Rich Felker
	<dalias@...c.org>, <linux-mm@...ck.org>, <linux-kselftest@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/6] selftests/mm: mseal, self_elf: fix missing
 __NR_mseal

On 6/18/24 1:54 PM, David Hildenbrand wrote:
> On 18.06.24 22:14, John Hubbard wrote:
>> On 6/17/24 11:56 PM, David Hildenbrand wrote:
>>> On 18.06.24 04:24, John Hubbard wrote:
>> ...
...
>> I can update the commit description with some of the above, if it helps.
> 
> I think it will. The main concern I had was that we could be ending up including headers with *wrong* data. As long as (a) it compiles where it's supposed to compile (b) it runs where it's supposed to run, we're good :)
> 

OK, I've drafted an updated commit description (below), and in order
to reduce email churn perhaps it's best for me to hold onto it for a
day or two, while we see how v3 fares in linux-next. (Thanks, Andrew,
for patching that up with my Makefile fix.)

Here's the draft:
selftests/mm: mseal, self_elf: fix missing __NR_mseal

The selftests/mm build isn't exactly "broken", according to the current
documentation, which still claims that one must run "make headers",
before building the kselftests. However, according to the new plan to
get rid of that requirement [1], they are future-broken: attempting to
build selftests/mm *without* first running "make headers" will fail due
to not finding __NR_mseal.

Therefore, include asm-generic/unistd.h, which has all of the system
call numbers that are needed, abstracted across the various CPU arches.

Some explanation in support of this "asm-generic" approach:

For most user space programs, the header file inclusion behaves as
per this microblaze example, which comes from David Hildenbrand
(thanks!) :

     arch/microblaze/include/asm/unistd.h
         -> #include <uapi/asm/unistd.h>

     arch/microblaze/include/uapi/asm/unistd.h
         -> #include <asm/unistd_32.h>
         -> Generated during "make headers"

     usr/include/asm/unistd_32.h is generated via
     arch/microblaze/kernel/syscalls/Makefile with the syshdr command.

     So we never end up including asm-generic/unistd.h directly on
     microblaze... [2]

However, those programs are installed on a single computer that has a
single set of asm and kernel headers installed.

In contrast, the kselftests are quite special, because they must provide
a set of user space programs that:

     a) Mostly avoid using the installed (distro) system header files.

     b) Build (and run) on all supported CPU architectures

     c) Occasionally use symbols that have so new that they have not
        yet been included in the distro's header files.

Doing (a) creates a new problem: how to get a set of cross-platform
headers that works in all cases.

Fortunately, asm-generic headers solve that one. Which is why we need to
use them here--at least, for particularly difficult headers such as
unistd.h.

The reason this hasn't really come up yet, is that until now, the
kselftests requirement (which I'm trying to eventually remove) was that
"make headers" must first be run. That allowed the selftests to get a
snapshot of sufficiently new header files that looked just like (and
conflict with) the installed system headers.

And as an aside, this is also an improvement over past practices of
simply open-coding in a single (not per-arch) definition of a new
symbol, directly into the selftest code.

[1] commit e076eaca5906 ("selftests: break the dependency upon local
header files")

[2] https://lore.kernel.org/all/0b152bea-ccb6-403e-9c57-08ed5e828135@redhat.com/

Fixes: 4926c7a52de7 ("selftest mm/mseal memory sealing")
Cc: Jeff Xu <jeffxu@...omium.org>
Cc: David Hildenbrand <david@...hat.com>
Signed-off-by: John Hubbard <jhubbard@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA