Re: [PATCH v3 bpf-next 01/14] samples: bpf: makefile: fix HDR_PROBE "echo"

From: Andreas Schwab <schwab@linux-m68k.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Yonghong Song <yhs@fb.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <jakub.kicinski@netronome.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	john fastabend <john.fastabend@gmail.com>,
	open list <linux-kernel@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	clang-built-linux@googlegroups.com,
	sergei.shtylyov@cogentembedded.com
Subject: Re: [PATCH v3 bpf-next 01/14] samples: bpf: makefile: fix HDR_PROBE "echo"
Date: Mon, 16 Sep 2019 23:35:44 +0200	[thread overview]
Message-ID: <8736gvexfz.fsf@igel.home> (raw)
In-Reply-To: <CAEf4BzZVTjCybmDgM0VBzv_L-LHtF8LcDyyKSWJm0ZA4jtJKcw@mail.gmail.com> (Andrii Nakryiko's message of "Mon, 16 Sep 2019 13:13:23 -0700")

On Sep 16 2019, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Mon, Sep 16, 2019 at 3:59 AM Ivan Khoronzhuk
> <ivan.khoronzhuk@linaro.org> wrote:
>>
>> echo should be replaced with echo -e to handle '\n' correctly, but
>> instead, replace it with printf as some systems can't handle echo -e.
>>
>> Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
>> ---
>>  samples/bpf/Makefile | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>> index 1d9be26b4edd..f50ca852c2a8 100644
>> --- a/samples/bpf/Makefile
>> +++ b/samples/bpf/Makefile
>> @@ -201,7 +201,7 @@ endif
>>
>>  # Don't evaluate probes and warnings if we need to run make recursively
>>  ifneq ($(src),)
>> -HDR_PROBE := $(shell echo "\#include <linux/types.h>\n struct list_head { int a; }; int main() { return 0; }" | \
>> +HDR_PROBE := $(shell printf "\#include <linux/types.h>\n struct list_head { int a; }; int main() { return 0; }" | \
>
> printf change is fine, but I'm confused about \# at the beginning of
> the string.

From the NEWS of make 4.3:

* WARNING: Backward-incompatibility!
  Number signs (#) appearing inside a macro reference or function invocation
  no longer introduce comments and should not be escaped with backslashes:
  thus a call such as:
    foo := $(shell echo '#')
  is legal.  Previously the number sign needed to be escaped, for example:
    foo := $(shell echo '\#')
  Now this latter will resolve to "\#".  If you want to write makefiles
  portable to both versions, assign the number sign to a variable:
    H := \#
    foo := $(shell echo '$H')
  This was claimed to be fixed in 3.81, but wasn't, for some reason.
  To detect this change search for 'nocomment' in the .FEATURES variable.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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