commit 9109b7679e13aa34a54834cfb4949cac4b96e576
author Kir Kolyshkin <kolyshkin@gmail.com> Sun Sep 01 17:21:42 2019 -0700
committer Tobias Klauser <tobias.klauser@gmail.com> Mon Sep 02 13:37:55 2019 +0000
tree 65e4f56bd141f04b9dc13c61a20bc6ef01ed1de5
parent 1e83adbbebd0f5dc971915fd7e5db032c3d2b731

unix: add IoctlRetInt for Linux

Add IoctlRetInt() for ioctls that use syscall's return value
as an output parameter. Add a unit test for Linux, and the
NS_GET_ defines that it requires (see ioctl_ns(2)).

Motivation:

Currently, x/sys/unix provides a few functions to deal with ioctls.
In particular, IoctlGetInt(), which "performs an ioctl operation
which gets an integer value". It does that by passing a pointer
to an integer to a syscall and returning that integer. The value
returned from syscall is treated as success/failure flag
(0 means success, -1 means failure, and in such case errno
is used to figure out the underlying error).

It appears that there are a few ioctls in Linux and at least one
in Solaris, which do not use the above way to return an int, instead
they use the syscall's return value (in case it's not negative).

As Linux ioctl(2) man page says,

> RETURN VALUE
> Usually, on success zero is returned. A few ioctl() requests
> use the return value as an output parameter and return
> a nonnegative value on success. On error, -1 is returned,
> and errno is set appropriately.

Currently I am aware of at least 6 Linux ioctls that do that
(return the value directly):

* LOOP_CTL_* ioctls on /dev/loop-control (all 3 of them). Source: loop(4)
* NS_* ioctls (3 out of 4) on /proc/PID/ns/*. Source: ioctl_ns(2)

And one in Solaris:

* I_FIND ioctl, source: streamio(7i).

There might be some more ioctls like the ones above, but since
ioctls are scarcely documented, it is hard to say how many more.

Obviously, using IoctlGetInt() for such ioctls would be a big
mistake, as 0 will always be returned. For example, there was
a bug in Docker's pkg/loopback (moby/moby#39801).

[v6: make it linux-specific for now]

Fixes golang/go#33966

Change-Id: Ie64f72fd84101b955ba14519a357e06a66d685d0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/192780
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>