linux-kernel - Re: [PATCH] samples: pidfd: Fix compile error seen if __NR_pidfd_send

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Message-ID: <3C2871DE-8F84-4B5E-81CA-04B073E4B27D@brauner.io>
Date:   Thu, 30 May 2019 13:55:06 +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 1:50:31 PM GMT+02:00, Guenter Roeck <linux@...ck-us.net> wrote:
>On 5/30/19 4:43 AM, Christian Brauner wrote:
>> 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.
>> 
>
>syscall numbers can differ from architecture to architecture, and the
>provided solution is used in other test code. Please feel free to
>submit
>a different patch, though - I am only interested in a fix, which
>doesn't
>have to be mine.
>
>Note that this fails in mips builds.
>
>Thanks,
>Guenter

The syscall number is the same on all arches for this syscall.
It's been added after the syscall numbering
work by Arnd.

Christian