Re: [PATCH 1/2] selftests/openat2: fix clang build failures: -static-libasan, LOCAL_HDRS

[John Hubbard <jhubbard@nvidia.com>]

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.

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,
-- 
John Hubbard
NVIDIA

>>
>> The way it works is that lib.mk adds $(LOCAL_HDRS) to the dependencies list,
>> but then filters precisely that same set *out* of the list that it provides
>> to the compile invocation.
>>
>> The other way to implement this requirement of "some things need to be
>> Make dependencies, and some need to be both dependencies and compilation
>> inputs", is to add everything to the dependency list, but then use a
>> separate list of files to pass to the compiler. For an example of that,
>> see $(EXTRA_FILES) in patch 1/7 [1] of my selftests/x86 cleanup.
>>
>> [1] https://lore.kernel.org/all/20240503030214.86681-2-jhubbard@nvidia.com/
>>
>> thanks,
>> John Hubbard
>>
>>>
>>> [1]
>>> https://lore.kernel.org/linux-kselftest/20240417160740.2019530-1-ryan.roberts@arm.com/
>>>
>>> Thanks,
>>> Ryan
>>>
>>>
>>>> ---
>>>>    tools/testing/selftests/openat2/Makefile | 14 ++++++++++++--
>>>>    1 file changed, 12 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/tools/testing/selftests/openat2/Makefile
>>>> b/tools/testing/selftests/openat2/Makefile
>>>> index 254d676a2689..185dc76ebb5f 100644
>>>> --- a/tools/testing/selftests/openat2/Makefile
>>>> +++ b/tools/testing/selftests/openat2/Makefile
>>>> @@ -1,8 +1,18 @@
>>>>    # SPDX-License-Identifier: GPL-2.0-or-later
>>>>    -CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined
>>>> -static-libasan
>>>> +CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined
>>>>    TEST_GEN_PROGS := openat2_test resolve_test rename_attack_test
>>>>    +# gcc requires -static-libasan in order to ensure that Address Sanitizer's
>>>> +# library is the first one loaded. However, clang already statically links the
>>>> +# Address Sanitizer if -fsanitize is specified. Therefore, simply omit
>>>> +# -static-libasan for clang builds.
>>>> +ifeq ($(LLVM),)
>>>> +    CFLAGS += -static-libasan
>>>> +endif
>>>> +
>>>> +LOCAL_HDRS += helpers.h
>>>> +
>>>>    include ../lib.mk
>>>>    -$(TEST_GEN_PROGS): helpers.c helpers.h
>>>> +$(TEST_GEN_PROGS): helpers.c
>>>>
>>>> base-commit: ddb4c3f25b7b95df3d6932db0b379d768a6ebdf7
>>>> prerequisite-patch-id: b901ece2a5b78503e2fb5480f20e304d36a0ea27
>>>
>>
>> thanks,
>