linux-kernel - Re: [PATCH 0/1] Fix to allow more correct isatty()

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Message-ID: <16989967.geO5KgaWL5@nerdopolis2>
Date: Fri, 22 Nov 2024 18:54:17 -0500
From: nerdopolis <bluescreen_avenger@...izon.net>
To: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
 Gil Pedersen <gpdev@...st.dk>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH 0/1] Fix to allow more correct isatty()

On Thursday, November 21, 2024 6:12:53 AM EST Gil Pedersen wrote:
> This patch comes from an issue discovered in systemd, where it can fail to
> restore a text TTY session after a GUI session is stopped when compiled
> with musl libc.
> 
> The specific systemd integration issue is currently resolved in the musl
> master branch by closer aligning the implementation with glibc, but the
> underlying isatty() implementation is still flawed since it can return 0
> (false) for something that is definitely a TTY. Essentially both musl
> and glibc give up correctly handling this case on Linux due to
> inadequate/buggy kernel APIs.
> 
> Thus I am proposing this patch as a solution to fix the kernel APIs. An
> alternative fix could be to add a new IOCTL to specifically handle this,
> but that seems overkill.
> 
This is pretty cool as it fixes the /dev/console-is-a-disconnected-/dev/ttyS0
issue I found this year
https://github.com/systemd/systemd/pull/33690
and

https://lore.kernel.org/all/2669238.7s5MMGUR32@nerdopolis2/
namely on vt-less kernels that now have /dev/console as /dev/ttyS0 instead of
/dev/tty0 or a virtual device.


So yeah, when you have hardware and you've been using vt-enabled kernels
on it for years (where your /dev/ttyS0 doesn't even have an rs232 port, let
alone connected to anything), and then when you upgrade to a vt-less kernel,
sometime in the future, because of systemd's isatty() check failing, and
refusing to write to /dev/console, Plymouth's new log display facility in turn
gets far fewer logs from systemd, because of the ioctl failure in isatty()


MY idea for this solution was that kernels turning off CONFIG_VT_CONSOLE could
then turn on a CONFIG_NULL_TTY_CONSOLE that would get selected at the same time
the VT console would have, so that if distros choose to do so, the virtual
/dev/ttynull device to always work for User Space. (that way kernels that never
had CONFIG_VT_CONSOLE turned on won't have a behavior change as well)


(the one benefit to my idea is that Plymouth goes into text mode logging when
the console device is /dev/ttyS0, so Desktop distros would need to add a new
"plymouth.graphical" to their grub commands to get the splash to work again,
however when the console device is /dev/ttynull it automatically assumes that
the user wants a splash screen.) This is more specific to Plymouth though


However testing this patch and it also works, and this won't need a new
configuration change.
> Link: https://github.com/systemd/systemd/pull/34039
> Link: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/70711
> Link: https://www.openwall.com/lists/musl/2024/08/20/2
> Link: https://git.musl-libc.org/cgit/musl/commit/src/unistd/isatty.c?id=c94a0c16f08894ce3be6dafb0fe80baa77a6ff2a
> Link: https://sourceware.org/bugzilla/show_bug.cgi?id=32103
> 
> Gil Pedersen (1):
>   tty: respond to TIOCGWINSZ when hung
> 
>  drivers/tty/tty_io.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
> 
>