Re: [PATCH v3 2/4] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes

On Mon, Dec 16, 2019 at 06:49:37PM -0800, Sargun Dhillon wrote:

On Mon, Dec 16, 2019 at 5:50 PM Christian Brauner
[off-list ref] wrote:

quoted

[Cc Arnd since he fiddled with ioctl()s quite a bit recently.]


That should be pidfd.h and the resulting new file be placed under the
pidfd entry in maintainers:
+F:     include/uapi/linux/pidfd.h

quoted

 enum pid_type
 {

diff --git a/include/uapi/linux/pid.h b/include/uapi/linux/pid.h
new file mode 100644
index 000000000000..4ec02ed8b39a
--- /dev/null
+++ b/include/uapi/linux/pid.h

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_PID_H
+#define _UAPI_LINUX_PID_H
+
+#include <linux/types.h>
+#include <linux/ioctl.h>
+
+/* options to pass in to pidfd_getfd_args flags */
+#define PIDFD_GETFD_CLOEXEC (1 << 0) /* open the fd with cloexec */

Please, make them cloexec by default unless there's a very good reason
not to.

For now then, should I have flags, and just say "reserved for future usage",
or would you prefer that I drop flags entirely?

Hm, you can leave the flags argument imho but maybe someone else has
stronger opinions about this.

quoted

quoted

+
+struct pidfd_getfd_args {
+     __u32 size;             /* sizeof(pidfd_getfd_args) */
+     __u32 fd;       /* the tracee's file descriptor to get */
+     __u32 flags;
+};

I think you want to either want to pad this

+struct pidfd_getfd_args {
+       __u32 size;             /* sizeof(pidfd_getfd_args) */
+       __u32 fd;       /* the tracee's file descriptor to get */
+       __u32 flags;
        __u32 reserved;
+};

or use __aligned_u64 everywhere which I'd personally prefer instead of
this manual padding everywhere.

Wouldn't __attribute__((packed)) achieve a similar thing of making sure
the struct is a constant size across all compilers?

I'll go with __aligned_u64 instead of packed, if you don't want to use packed.

We had a discussion about this in relation to the openat2()
patchset just recently. Florian and a few others raised good points why
we might not want to use packed:
https://lore.kernel.org/lkml/87o8w9bcaf.fsf@mid.deneb.enyo.de/ (local)
https://lore.kernel.org/lkml/a328b91d-fd8f-4f27-b3c2-91a9c45f18c0@rasmusvillemoes.dk/ (local)

Christian