Re: Add login_tty

[Felix Janda <felix.janda@posteo.de>]

Rich Felker wrote:
> One more issue:
> 
> On Sat, Nov 01, 2014 at 11:27:29PM +0100, Felix Janda wrote:
> >  int forkpty(int *m, char *name, const struct termios *tio, const struct winsize *ws)
> >  {
> > @@ -13,12 +12,7 @@ int forkpty(int *m, char *name, const struct termios *tio, const struct winsize
> >  	pid = fork();
> >  	if (!pid) {
> >  		close(*m);
> > -		setsid();
> > -		ioctl(s, TIOCSCTTY, (char *)0);
> > -		dup2(s, 0);
> > -		dup2(s, 1);
> > -		dup2(s, 2);
> > -		if (s>2) close(s);
> > +		login_tty(s);
> 
> Here there's no checking for failure of login_tty. Note that checking
> for failure would not be useful, because there's no valid response,
> but:
> 
> > diff --git a/src/misc/login_tty.c b/src/misc/login_tty.c
> > new file mode 100644
> > index 0000000..f0be0a0
> > --- /dev/null
> > +++ b/src/misc/login_tty.c
> > @@ -0,0 +1,14 @@
> > +#include <utmp.h>
> > +#include <sys/ioctl.h>
> > +#include <unistd.h>
> > +
> > +int login_tty(int fd)
> > +{
> > +	setsid();
> > +	if (ioctl(fd, TIOCSCTTY, (char *)0)) return -1;
> > +	dup2(fd, 0);
> > +	dup2(fd, 1);
> > +	dup2(fd, 2);
> > +	if (fd>2) close(fd);
> > +	return 0;
> > +}
> 
> Unlike the code moved out of forkpty, this code errors out early if
> the ioctl fails, thereby skipping the dup2's and close. In the case of
> forkpty, this leaves the child process in a very messed-up state with
> regard to its file descriptors; it will clobber the parent's
> stdin/out/err without knowing anything is wrong.
> 
> In the case of forkpty, the error just needs to be ignored, I think,
> like it is now. I'm not sure what login_tty should do, though.
> Reporting the error is good, but leaving the process and its file
> descriptors in an unpredictable state is not good. Is there any
> documentation for how they're left on error?

I have taken from the man page that if the ioctl fails login_tty will
also fail. So how about

int login_tty(int fd)
{
	int ret;
	setsid();
	ret = ioctl(fd, TIOCSCTTY, (char *)0);
	dup2(fd, 0);
	dup2(fd, 1);
	dup2(fd, 2);
	if (fd>2) close(fd);
	return ret;
}

Felix