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Message-ID: <1f8711d2-8505-4685-9e0f-180bdc709381@arm.com>
Date: Tue, 7 May 2024 17:56:39 +0100
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Subject: Re: [PATCH 1/2] selftests/openat2: fix clang build failures:
 -static-libasan, LOCAL_HDRS
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To: John Hubbard <jhubbard@nvidia.com>, Shuah Khan <shuah@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
 =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>,
 Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>,
 Christian Brauner <brauner@kernel.org>,
 Muhammad Usama Anjum <usama.anjum@collabora.com>,
 Alexey Gladkov <legion@kernel.org>, Valentin Obst <kernel@valentinobst.de>,
 linux-kselftest@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
 llvm@lists.linux.dev
References: <20240504044336.14411-1-jhubbard@nvidia.com>
 <8fdefaa9-675e-4b37-9456-896b9989d18f@arm.com>
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 <518dd1e3-e31a-41c3-b488-9b75a64b6c8a@arm.com>
 <c1ff82e6-4d6a-4680-925f-5f0226ba32c4@nvidia.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <c1ff82e6-4d6a-4680-925f-5f0226ba32c4@nvidia.com>
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On 07/05/2024 17:47, John Hubbard wrote:
> On 5/7/24 9:34 AM, Ryan Roberts wrote:
>> On 07/05/2024 17:19, John Hubbard wrote:
>>> On 5/7/24 12:45 AM, Ryan Roberts wrote:
>>>> On 04/05/2024 05:43, John Hubbard wrote:
>>> ...
>>>> Hi John,
>>>>
>>>> I sent out a similar fix a couple of weeks ago, see [1]. I don't think it got
>>>> picked up though. It takes a slightly different approach, explicitly adding
>>>> -static-libsan (note no 'a') for clang, instead of relying on its default.
>>>>
>>>> And it just drops helpers.h from the makefile altogether, on the assumption
>>>> that
>>>> it was a mistake; its just a header and shouldn't be compiled directly. I'm not
>>>> exactly sure what the benefit of adding it to LOCAL_HDRS is?
>>>
>>> Ah no, you must not drop headers.h. That's a mistake itself, because
>>> LOCAL_HDRS adds a Make dependency; that's its purpose. If you touch
>>> helpers.h it should cause a rebuild, which won't happen if you remove it
>>> from LOCAL_HDRS.
>>
>> Ahh. I was under the impression that the compiler was configured to output the
>> list of dependencies for make to track (something like -M, from memory ?). Since
>> helpers.h is included from helpers.c I assumed it would be tracked like this - I
>> guess its not that simple?
> 
> This can be done, but it is not automatic with GNU Make. You have to explicitly
> run gcc -M, capture the output in a dependencies list, and track it. Which the
> Kbuild system does, but kselftest does not.

Understood - thanks for the lesson!

> 
> After just now sweeping through kselftest to fix up the clang build, I see a
> lot of mistaken or partial use of the kselftest build's Make variables, because
> people naturally reason based on what they know about Kbuild, and it doesn't
> always translate. And LOCAL_HDRS might need some more documentation too.
> I'll keep thinking about how to clarify this, I have a couple early ideas.
> 
>>
>> Anyway, on the basis that LOCAL_HDRS is the right way to do this, let's go with
>> your version and drop mine:
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>>
> 
> Thanks for the review!
> 
> thanks,