netdev - Re: [PATCH v2 2/4] selftests: Fix seccomp to support relocatable build (O=objdir)

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Date:   Thu, 5 Mar 2020 09:42:13 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     shuah@...nel.org, luto@...capital.net, wad@...omium.org,
        daniel@...earbox.net, kafai@...com, yhs@...com, andriin@...com,
        gregkh@...uxfoundation.org, tglx@...utronix.de,
        khilman@...libre.com, mpe@...erman.id.au,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v2 2/4] selftests: Fix seccomp to support relocatable
 build (O=objdir)

On Thu, Mar 05, 2020 at 09:41:34AM -0700, Shuah Khan wrote:
> On 3/4/20 7:20 PM, Kees Cook wrote:
> > Instead of the TEST_CUSTOM_PROGS+all dance, you can just add an explicit
> > dependency, with the final seccomp/Makefile looking like this:
> > 
> > 
> > # SPDX-License-Identifier: GPL-2.0
> > CFLAGS += -Wl,-no-as-needed -Wall
> > LDFLAGS += -lpthread
> > 
> > TEST_GEN_PROGS := seccomp_bpf seccomp_benchmark
> > 
> 
> TEST_CUSTOM_PROGS is for differentiating test programs that
> can't use lib.mk generic rules. It is appropriate to use
> for seccomp_bpf

I don't follow? This suggested Makefile works for me (i.e. it can use
the lib.mk generic rules since CFLAGS and LDFLAGS can be customized
first, and it just adds an additional dependency).

> > include ../lib.mk
> > 
> > # Additional dependencies
> > $(OUTPUT)/seccomp_bpf: ../kselftest_harness.h

BTW, I see a lot of other targets that use kselftest_harness.h appear to
be missing this Makefile dependency, but that's a different problem. :)

> > (Though this fails in the same way as above when run from the top-level
> > directory.)
> > 
> 
> I didn't see this because I have been the same directory I used
> for relocated cross-build kernel. :(
> 
> Thanks for testing this. I know the problem here. all is a dependency
> for install step and $(OUTPUT) is referencing the objdir before it
> gets created. It is a Makefile/lib.mk problem to fix.
> 
> I will do a separate patch for this. This will show up in any test
> that is using $(OUTPUT) to relocate objects mainly the ones that
> require custom build rule like seeccomp.

Okay, cool. It looked to me like it lost track of the top level source
directory (i.e. "make: entering $output" ... "can't find
../other/files")

Anyway, I look forward to the next version and I'll get it tested. :)
Thanks for fixing this! I really like having a top-level "make" command
that can extract a single selftest; that's very handy!

-- 
Kees Cook