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Subject: Re: [PATCH] vt: vt_ioctl: Add new ioctl to read vt switch lock state
To:     Johannes Hostert <jojohostert@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Eric Biggers <ebiggers@google.com>,
        Eric Dumazet <edumazet@google.com>,
        linux-kernel@vger.kernel.org
References: <20200425231436.535671-1-jojohostert@gmail.com>
From:   Jiri Slaby <jslaby@suse.com>
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Message-ID: <ea60aefe-f294-7e58-b53c-5f1e72b7af9e@suse.com>
Date:   Sun, 26 Apr 2020 09:08:33 +0200
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On 26. 04. 20, 1:14, Johannes Hostert wrote:
> The existing ioctls VT_LOCKSWITCH and VT_UNLOCKSWITCH can be used to
> allow/disallow switching the virtual terminal. However, no mechanism
> exists that allows software to read this lock state.
> 
> Userspace programs that try to switch to another virtual terminal
> like chvt have no mechanism to figure out whether they will be able
> to actually switch the terminal. When eg. chvt is run while terminal
> switching is disabled, it simply sleeps forever waiting for the target
> terminal to become active.
> 
> This commit introduces a new ioctl VT_GETLOCKSWITCH that allows
> reading the current state of the switch lock flag. Userspace
> software can then use that flag and handle not being able to switch
> virtual terminals.
> 
> Example program using this:
> 
> #include <linux/vt.h>
> #include <sys/ioctl.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <errno.h>
> #include <error.h>
> #include <unistd.h>
> const char* console_device = "/dev/tty0";
> int main(int argc, char* argv[]) {
> 	int fd;
> 	while ((fd = open(console_device, O_RDWR)) == -1 && errno == EINTR);
> 	if (fd < 0)
> 		error(1, errno, "Opening %s", console_device);
>         int ret;
> 	while ((ret = ioctl(fd, VT_GETLOCKSWITCH, 1)) == -1 && errno == EINTR);
> 	if (ret == -1)
> 		error(1, errno, "%s: VT_GETLOCKSWITCH", console_device);

And now if you do VT_LOCKSWITCH even when VT_GETLOCKSWITCH returns 0,
vt_dont_switch can be 1 already.

So NACK.

Please don't add another racy ioctl. Introduce VT_LOCKSWITCH_NONBLOCK or
something like that instead (like respecting O_NONBLOCK).

thanks,
-- 
js
suse labs