Re: [PATCH] Fix getlogin() to check only stdin to get a valid tty

[Jeff Johnston <jjohnstn@redhat.com>]

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I think that when in doubt, going with glibc is the right answer.

I'll give Corinna time to pipe in if she wishes, otherwise, I'll check in
the glibc-like patch.

-- Jeff J.

On Thu, Jul 13, 2023 at 12:58 PM Jordi Sanfeliu via Newlib <
newlib@sourceware.org> wrote:

> Hello,
>
> So we have two possible patches to solve this situation:
>
> My first original patch that only checks if stdin is a tty and seems to
> match with glibc comments provided by Torbjörn. The only problem is that
> it won't work in the scenario when someone pipes some data on stdin to
> the application:
>
> $ echo | ./test
> $
>
>
> diff --git a/newlib/libc/unix/getlogin.c b/newlib/libc/unix/getlogin.c
> index da4f47a95..e646bcb08 100644
> --- a/newlib/libc/unix/getlogin.c
> +++ b/newlib/libc/unix/getlogin.c
> @@ -16,9 +16,7 @@ getlogin ()
>     extern char *ttyname ();
>     char *tty;
>
> -  if (((tty = ttyname (0)) == 0)
> -      || ((tty = ttyname (1)) == 0)
> -      || ((tty = ttyname (2)) == 0))
> +  if ((tty = ttyname (0)) == 0)
>       return 0;
>
>     if ((utmp_fd = open (UTMP_FILE, O_RDONLY)) == -1)
>
>
>
> And a patch based on the comments of Torbjörn that checks all three fds
> before returning NULL:
>
> diff --git a/newlib/libc/unix/getlogin.c b/newlib/libc/unix/getlogin.c
> index da4f47a95..5a3f172e7 100644
> --- a/newlib/libc/unix/getlogin.c
> +++ b/newlib/libc/unix/getlogin.c
> @@ -16,10 +16,10 @@ getlogin ()
>     extern char *ttyname ();
>     char *tty;
>
> -  if (((tty = ttyname (0)) == 0)
> -      || ((tty = ttyname (1)) == 0)
> -      || ((tty = ttyname (2)) == 0))
> -    return 0;
> +  if ((tty = ttyname (0)) == 0)
> +      if ((tty = ttyname (1)) == 0)
> +          if ((tty = ttyname (2)) == 0)
> +              return 0;
>
>     if ((utmp_fd = open (UTMP_FILE, O_RDONLY)) == -1)
>       return 0;
>
>
> Any thoughts?
> Thanks.
>
>
> On 7/13/23 18:25, Torbjorn SVENSSON wrote:
>
> > I took a sneak peak at how glibc does this and there is this comment:
> >
> >    /* Get name of tty connected to fd 0.  Return NULL if not a tty or
> >       if fd 0 isn't open.  Note that a lot of documentation says that
> >       getlogin() is based on the controlling terminal---what they
> >       really mean is "the terminal connected to standard input".  The
> >       getlogin() implementation of DEC Unix, SunOS, Solaris, HP-UX all
> >       return NULL if fd 0 has been closed, so this is the compatible
> >       thing to do.  Note that ttyname(open("/dev/tty")) on those
> >       systems returns /dev/tty, so that is not a possible solution for
> >       getlogin().  */
> >
> > Based on this comment, I guess it would be sane to drop the check on
> > stdout and stderr, but it would have the consequence that you are not
> > able to pipe some data on stdin to the application that calls getlogin
> > as it would fail in that scenario.
> >
> > I'm not a maintainer of newlib so I don't really have anything to say
> > about what path you decide to go.
>
> --
> Jordi Sanfeliu
> FIBRANET Network Services Provider
> https://www.fibranet.cat
>
>