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Date:   Thu, 30 May 2019 13:43:31 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Guenter Roeck <linux@...ck-us.net>, Jann Horn <jannh@...gle.com>
CC:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] samples: pidfd: Fix compile error seen if __NR_pidfd_send_signal is undefined

On May 30, 2019 1:40:47 PM GMT+02:00, Guenter Roeck <linux@...ck-us.net> wrote:
>To make pidfd-metadata compile on all arches, irrespective of whether
>or not syscall numbers are assigned, define the syscall number to -1
>if it isn't to cause the kernel to return -ENOSYS.
>
>Fixes: 43c6afee48d4 ("samples: show race-free pidfd metadata access")
>Cc: Christian Brauner <christian@...uner.io>
>Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>---
> samples/pidfd/pidfd-metadata.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
>diff --git a/samples/pidfd/pidfd-metadata.c
>b/samples/pidfd/pidfd-metadata.c
>index 640f5f757c57..1e125ddde268 100644
>--- a/samples/pidfd/pidfd-metadata.c
>+++ b/samples/pidfd/pidfd-metadata.c
>@@ -21,6 +21,10 @@
> #define CLONE_PIDFD 0x00001000
> #endif
> 
>+#ifndef __NR_pidfd_send_signal
>+#define __NR_pidfd_send_signal	-1
>+#endif
>+
> static int do_child(void *args)
> {
> 	printf("%d\n", getpid());

Couldn't you just use the actual syscall number? 
That should still fail if the kernel is to old 
and still work on kernels that support it
but for whatever reason the unistd.h h
header doesn't have it defined.

Thanks!
Christian

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