Re: [musl] bug: isatty() can return wrong value

[Rich Felker <dalias@libc.org>]

On Tue, Nov 26, 2024 at 11:03:36AM +0100, Gil Pedersen wrote:
> > On 20 Aug 2024, at 13.11, Rich Felker <dalias@libc.org> wrote:
> > 
> > What guarantee do we have that nothing other than a tty in this state
> > will return EIO for the ioctl? The implementation on the kernel side
> > suggests that, if they tried to put any other device in such a state
> > by replacing its ioctl function the same way, it would also give EIO
> > for these ioctls.
> 
> Yeah, EIO is unconditionally signalled for any (expect TIOCSPGRP) ioctl call when a tty is hung.
> While a TIOCGWINSZ ioctl call that returns EIO will very likely be a tty, there is indeed no such guarantee.
> 
> > I'm hopeful there's some good fix here where we don't have to have
> > either of these bad behaviors, but returning true for isatty() of a
> > device that is absolutely not a tty is a much worse behavior than
> > returning false for a device that is/was a tty when it was opened but
> > that's been replaced by the kernel with a defunct device.
> 
> I have proposed a change to the ioctl handling in the kernel:
> 
> https://lore.kernel.org/linux-serial/20241121111506.4717-1-gpdev@gpost.dk/
> 
> If merged, the issue should be fixed going forward.

Great. AIUI the proposed kernel change would fix this issue without
needing any further action on the libc side, right?

Rich