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

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Date:   Tue, 10 Mar 2020 17:11:47 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.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,
        "skh >> Shuah Khan" <skhan@...uxfoundation.org>
Subject: Re: [PATCH v2 2/4] selftests: Fix seccomp to support relocatable
 build (O=objdir)

On 3/5/20 10:42 AM, Kees Cook wrote:
> 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).
> 

Yeah. TEST_CUSTOM_PROGS isn't really needed for this custom case.
I can refine it and get rid of the 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 was way off with my analysis. :(

>> 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")
> 

Odd that you would have empty objdir in the cross-compile case.

In the cross-compile case, you would have cross-built kernel first in
the object directory. Your objdir won't be empty.

This is no different from kselftest build dependency on kernel build
even when srcdir=objdir

So for cross-build case, the following  is the workflow to build kernel
first and then the tests:

make O=/../objdir ARCH=arm64 HOSTCC=gcc CROSS_COMPILE=aarch64-linux-gnu- 
defconfig

make O=/../objdir ARCH=arm64 HOSTCC=gcc CROSS_COMPILE=aarch64-linux-gnu- all

make kselftest-install O=/../objdir ARCH=arm64 HOSTCC=gcc 
CROSS_COMPILE=aarch64-linux-gnu- TARGETS=seccomp

You can isolate a single test when you are do native build:

make kselftest-install O=/../objdir TARGETS=seccomp

The above won't fail even if objdir doesn't exist and/or empty.

thanks,
-- Shuah