Message-Id: <110BE776-AA74-4DDC-87F1-453B164D25D2@gpost.dk>
Date: Tue, 7 Jan 2025 12:44:28 +0100
From: Gil Pedersen <gpdev@...st.dk>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org,
 linux-serial@...r.kernel.org,
 Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH 1/1] tty: respond to TIOCGWINSZ when hung

> On 23 Dec 2024, at 18.56, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> 
> On Thu, Nov 21, 2024 at 12:12:54PM +0100, Gil Pedersen wrote:
>> Userspace libc implementations of the isatty() POSIX system interface
>> are currently unable to reliably determine if a fd is really a tty when
>> it is hung.
>> 
>> Specifically glibc libc returns the success status of a TCGETS ioctl.
>> This will return an incorrect result when the TTY is hung, since an EIO
>> is unconditionally returned. Ie. an isatty() will return 0, wrongly
>> indicating that something that definitely is a TTY, is not a TTY.
>> 
>> Userspace implementations could potentially remap EIO errors to a
>> success to work around this. This will likely work in 99.99% of cases,
>> but there is no guarantee that a TCGETS ioctl on a non-TTY fd will not
>> also return EIO, making the isatty() call return a false positive!
>> 
>> This commit enables a specific non-driver, non-ldisc, ioctl to continue
>> working after the TTY is hung. The TIOCGWINSZ ioctl was chosen since it
>> is readonly, and only access tty_struct.winsize (and its mutex), and is
>> already used for the isatty() implementation in musl. The glibc
>> implementation will need to be updated to use the TIOCGWINSZ ioctl,
>> either as a direct replacement, or more conservatively, as a fallback
>> test when the TCGETS ioctl fails with EIO.
> 
> This is a fun "hack", yes, but now you are encoding an odd "side affect"
> into the system that everyone is going to rely on, well, eventually rely
> on.  What code needs to be changed in userspace to determine this?

The patch can definitely be considered a hack, but viewed with another
lens: a bugfix.

There is no specific reason that the call should return an EIO on hung
terminals, so making it always return the current value could be
considered more correct. POSIX tcgetwinsize(), which this ioctl maps
to, does not consider hung terminals, and expects it to return suitable
values whenever possible.

Userspace implementations will have to reconsider their handling of an
EIO error, as the isatty() call could still return an EIO if calling
into a non-TTY device. Unconditionally mapping it to a success, like
isatty_safe() in systemd, would be an error. Supporting both versions
would require a runtime check to determine which variant is used, where
the legacy version would accept the risk of a "wrong" EIO, while the
new version would treat it as a proper error.

> Why not just have a new ioctl that tells you if the tty really is hung
> or not?  Why does isatty() need to know this, does POSIX require it?
> And if it does, what does it say the ioctl command should be?

isatty() should not need to know if the TTY is hung, and besides cannot
safely call any ioctl to check this before it knows that it is indeed a
TTY. POSIX does not seem to include the concept of hung terminals.

A case could be made for introducing a new ioctl though, but it would
need a more generic approach, like the BSD FIODTYPE ioctl that exposes
a d_type property on chardev & block driver interfaces. If implemented
before calling into the VFS layer, it could make the isatty() call 100%
safe (on kernels that support the ioctl). Additionally, this would mean
that it can never return EIO, which makes userspace adaptions simpler,
since it can know that any returned EIO means that it is running on an
unpatched/legacy kernel and/or libc.

/Gil

Link: https://pubs.opengroup.org/onlinepubs/9799919799/
Link: https://github.com/systemd/systemd/blob/83c0b95f63417a36e67305fe9ad16a89ed53ef52/src/basic/terminal-util.c#L63-L79

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