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Subject: Re: [PATCH] uapi: Make generic uapi headers compile standalone.
To:     Randy Dunlap <rdunlap@infradead.org>, linux-kernel@vger.kernel.org
Cc:     akpm@linux-foundation.org, kernel-team@android.com,
        linux-kbuild@vger.kernel.org
References: <20180606231602.231326-1-jchowdhary@google.com>
 <b6b6dd56-c948-54e8-df95-3d170a3c1e7d@infradead.org>
From:   Jayant Chowdhary <jchowdhary@google.com>
Message-ID: <fe1d69e1-1a0f-a3ea-e45f-994698d53dbd@google.com>
Date:   Fri, 8 Jun 2018 11:36:21 -0700
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Hi Randy,

On 06/07/2018 05:07 PM, Randy Dunlap wrote:
> On 06/06/2018 04:16 PM, Jayant Chowdhary wrote:
>> In order for static analysis tools to analyze each of the uapi headers,
>> we need to enable them to compile stand-alone. Some uapi headers were
>> missing dependencies which would not make them compile stand-alone in
>> user-land. This patch adds those dependencies.
> 
> Hi,
> 
> Thanks for getting started on this.  I see that the kbuild robot and I were
> still not able to do successful builds even after this patch is applied.
> We were building different targets though.  robot was doing kernel builds
> and I was doing a large "all-uapi" build.
> 
> I started on my all-uapi work about 1 week ago but haven't posted anything yet,
> but it's posted (attached) below.  It's not yet up to kernel quality yet (for a
> Makefile), and I have made very little progress toward a successful userspace build.
> 
> If anyone is interested, just put these 3 files in <kernel_tree>/tools/build
> and then run:
> 
> make ARCH=$some_arch O=build_dir headers_check
> so that the headers will be cleaned up and installed in build_dir/usr/.
> 
> Then run 'make -f all-uapi.mk'
> and the userspace program with all header files found in build_dir/usr/include
> will be built -- i.e., attempted (not successfully).
> 
> (note: chmod +x hdr-fix-lines.pl)
> 
> 

Thank you for this. This is surely a more formal way of finding out and
verifying problems w.r.t the uapi headers, being compiled from user-land. I do
have one concern with this approach though: the make target 'all-uapi.o' depends
on all-uapi.h, which includes all the uapi headers installed in <build-dir>. So
this does make sure that when all the uapi headers are included, a user-land
program will be able to compile fine, without adding any additional
dependencies. However, this could be masking some cases of dependency exclusion.
An example can be seen as follows:

Header dependency chain:
A.h->B.h, however A.h doesn't actually include B.h
C.h->B.h, C.h does include B.h

all-uapi.h
#include<C.h>
#include<A.h>

all-uapi.h will compile fine, since the inclusion of C.h before A.h satisfies
A.h's dependency on B.h. However, if a user-land program includes just A.h, it
will not compile without adding B.h. So the target being built successfully
masked an issue with the uapi header, no?

I guess it might be better to just have a Makefile which produces a target for
each header in the set produced by 'make headers_install' and compile those,
with a -I flag as <build-dir>/usr/include ?

Also, do you think it might be better to do this work in phases ? I'm guessing
we'll be seeing many headers to fix (especially arch specific ones). Fixing
these separately: first the generic ones, followed by the ones for the arches,
might be easier ?

Thanks,
Jayant