linux-kernel - Re: [PATCH v3 bpf-next 01/14] samples: bpf: makefile: fix HDR

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Date:   Mon, 16 Sep 2019 15:01:52 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Andreas Schwab <schwab@...ux-m68k.org>
Cc:     Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v3 bpf-next 01/14] samples: bpf: makefile: fix HDR_PROBE "echo"

On Mon, Sep 16, 2019 at 2:35 PM Andreas Schwab <schwab@...ux-m68k.org> wrote:
>
> On Sep 16 2019, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> > On Mon, Sep 16, 2019 at 3:59 AM Ivan Khoronzhuk
> > <ivan.khoronzhuk@...aro.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@...aro.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.

Oh, subtle... Thanks for explaining!

>
> --
> Andreas Schwab, schwab@...ux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> "And now for something completely different."