On Fri, Jul 12, 2024 at 08:52:15AM -0400, nerdopolis wrote: > Hi > > Background:-------------------------------------------------------------------- > This issue becomes evident on VT-less kernels. As when there is no /dev/tty0 > device, the console defaults to being /dev/ttyS0 instead. Although this can > also be replicated if booting a standard kernel with 'console=ttyS0' and ensure > nothing is plugged into /dev/ttyS0. > > This issue prevents systemd from logging to the console. > systemd when logging to /dev/console, long story short it calls isatty() on > /dev/console, and when /dev/console is actually /dev/ttyS0, and nothing is > connected to /dev/ttyS0, isatty() fails on /dev/console due to an input/output > error, causing systemd to not log the console output, because it rejects > /dev/console as not being a terminal. > > > This is noticed on a VT-less system with Plymouth. Plymouth calls the TIOCCONS > ioctl on a pty device it requests, to redirect console output, and in newer > versions, it displays the console logs on its own without the assistance of a > VT. > > This part of it works, Plymouth is able to 'see' what gets written to > /dev/console, log output from processes that write to /dev/console directly > (for example 'echo hi > /dev/console") do appear in plymouth's > /var/log/boot.log, it is just that systemd is not writing to /dev/console > because isatty() fails to report /dev/console as a tty device. > > The alternate fix in for systemds https://github.com/systemd/systemd/pull/33690[1] > is believed to be that when TIOCCONS is called on a PTY, or another terminal > device, that trying to call TCGETS on /dev/console should no longer result > in an error. > > > Replicating the issue:--------------------------------------------------------- > > This program replicates it: > ------------------------------------------------------------------------------- > #include <stdio.h> > #include <fcntl.h> > #include <unistd.h> > #include <errno.h> > #include <string.h> > > int main(void) > { > int fd; > > if (getuid() != 0) { > printf("Must be root\n"); > return 1; > } > > fd = open ("/dev/console", O_RDONLY); > if (!isatty(fd)) { > printf("err on /dev/console: %s\n", strerror(errno)); > } > return 0; > } > ------------------------------------------------------------------------------- > > When the kernel console is /dev/ttyS0 and /dev/ttySO has no device connected, > it prints "err on /dev/console: Input/output error" > > When I strace it, the relevant line is: > ioctl(3</dev/console<char 5:1>>, TCGETS, 0x7f...) = -1 EIO (Input/output error) Do you have a proposed kernel change for this that solves this for your tests here? thanks, greg k-h
