| Message ID | 20250314160749.3286153-1-adamsimonelli@gmail.com |
|---|---|
| Headers | show |
| Series | Optionally allow ttynull to be selected as a default console | expand |
On Fri 2025-03-14 12:07:49, adamsimonelli@gmail.com wrote: > From: Adam Simonelli <adamsimonelli@gmail.com> > > The new option is CONFIG_NULL_TTY_DEFAULT_CONSOLE. > > if enabled, and CONFIG_VT is disabled, ttynull will become the default > primary console device. > > ttynull will be the only console device usually with this option enabled. > Some architectures do call add_preferred_console() which may add another > console though. > > Motivation: > > Many distributions ship with CONFIG_VT enabled. On tested desktop hardware > if CONFIG_VT is disabled, the default console device falls back to > /dev/ttyS0 instead of /dev/tty. > > This could cause issues in user space, and hardware problems: > > 1. The user space issues include the case where /dev/ttyS0 is > disconnected, and the TCGETS ioctl, which some user space libraries use > as a probe to determine if a file is a tty, is called on /dev/console and > fails. Programs that call isatty() on /dev/console and get an incorrect > false value may skip expected logging to /dev/console. > > 2. The hardware issues include the case if a user has a science instrument > or other device connected to the /dev/ttyS0 port, and they were to upgrade > to a kernel that is disabling the CONFIG_VT option, kernel logs will then be > sent to the device connected to /dev/ttyS0 unless they edit their kernel > command line manually. > > The new CONFIG_NULL_TTY_DEFAULT_CONSOLE option will give users and > distribution maintainers an option to avoid this. Disabling CONFIG_VT and > enabling CONFIG_NULL_TTY_DEFAULT_CONSOLE will ensure the default kernel > console behavior is not dependent on hardware configuration by default, and > avoid unexpected new behavior on devices connected to the /dev/ttyS0 serial > port. > > Reviewed-by: Petr Mladek <pmladek@suse.com> > Tested-by: Petr Mladek <pmladek@suse.com> > Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com> JFYI, the patch have been comitted into printk/linux.git, branch for-6.15. Best Regards, Petr
On Monday, March 17, 2025 11:42:25 AM EDT Petr Mladek wrote: > On Fri 2025-03-14 12:07:49, adamsimonelli@gmail.com wrote: > > From: Adam Simonelli <adamsimonelli@gmail.com> > > > > The new option is CONFIG_NULL_TTY_DEFAULT_CONSOLE. > > > > if enabled, and CONFIG_VT is disabled, ttynull will become the default > > primary console device. > > > > ttynull will be the only console device usually with this option enabled. > > Some architectures do call add_preferred_console() which may add another > > console though. > > > > Motivation: > > > > Many distributions ship with CONFIG_VT enabled. On tested desktop hardware > > if CONFIG_VT is disabled, the default console device falls back to > > /dev/ttyS0 instead of /dev/tty. > > > > This could cause issues in user space, and hardware problems: > > > > 1. The user space issues include the case where /dev/ttyS0 is > > disconnected, and the TCGETS ioctl, which some user space libraries use > > as a probe to determine if a file is a tty, is called on /dev/console and > > fails. Programs that call isatty() on /dev/console and get an incorrect > > false value may skip expected logging to /dev/console. > > > > 2. The hardware issues include the case if a user has a science instrument > > or other device connected to the /dev/ttyS0 port, and they were to upgrade > > to a kernel that is disabling the CONFIG_VT option, kernel logs will then be > > sent to the device connected to /dev/ttyS0 unless they edit their kernel > > command line manually. > > > > The new CONFIG_NULL_TTY_DEFAULT_CONSOLE option will give users and > > distribution maintainers an option to avoid this. Disabling CONFIG_VT and > > enabling CONFIG_NULL_TTY_DEFAULT_CONSOLE will ensure the default kernel > > console behavior is not dependent on hardware configuration by default, and > > avoid unexpected new behavior on devices connected to the /dev/ttyS0 serial > > port. > > > > Reviewed-by: Petr Mladek <pmladek@suse.com> > > Tested-by: Petr Mladek <pmladek@suse.com> > > Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com> > > JFYI, the patch have been comitted into printk/linux.git, > branch for-6.15. > > Best Regards, > Petr > Oh wow! Thank you so much!
From: Adam Simonelli <adamsimonelli@gmail.com> When switching to a CONFIG_VT=n world, at least on x86 systems, /dev/console becomes /dev/ttyS0. This can cause some undesired effects. /dev/console's behavior is now tied to the physical /dev/ttyS0, which when disconnected can cause isatty() to fail when /dev/ttyS0 is disconnected, and users who upgrade to a theoretical vt-less kernel from their distribution who have a device such as a science instrument connected to their /dev/ttyS0 port will suddenly see it receive kernel log messages. When the new CONFIG_NULL_TTY_DEFAULT_CONSOLE option is turned on, this will allow the ttynull device to be leveraged as the default console. Distributions that had CONFIG_VT turned on before will be able to leverage this option to where /dev/console is still backed by a pseudo device, avoiding these issues, without needing to enable the entire VT subsystem. v2: rebase v3: Clarify commit messages. Guard the all the register_console()s in ttynull to prevent it from being registered twice. Only change the link order if CONFIG_NULL_TTY_CONSOLE is enabled, otherwise use the existing order for ttynull if only CONFIG_NULL_TTY is enabled. Document why the link order changes in the drivers/tty/Makefile file. Replace #ifdefs v4: Remember to actually include the changes to v3 in the cover letter. v5: Correct code formatting in Makefile comment. v6: Change to CONFIG_NULL_TTY_DEFAULT_CONSOLE Set the index to -1, and don't set the flags Use add_preferred_console() instead of register_console() in ttynull's console_initcall when CONFIG_NULL_TTY_DEFAULT_CONSOLE is enabled v7 Add a commit message to the first commit, and the Suggested-by Correct Kconfig tabs/spaces formatting Invert the console_set_on_cmdline check v8 Update Documentation. Remove the "CONFIG_" in the help text when mentioning other configuration options. Update commit message v9 Spell pseudo correctly in the cover letter. Use more clear, and suggested help text for NULL_TTY_DEFAULT_CONSOLE. Do not set ttynull's index to -1 anymore. Do not rely on link order, call add_preferred_console() in console_init(). Merge the Documentation/admin-guide/serial-console.rst change into one commit. v10 Fix commit message to mention the option correctly. Don't mention the older commit option from older revisions. v11 Fix spacing Fix spelling error in commit message, remove no longer true statement from commit message, and add clarifying line. v12 Add tags from Petr Mladek in the commit message Adam Simonelli (1): printk: Add an option to allow ttynull to be a default console device Documentation/admin-guide/serial-console.rst | 4 +++- drivers/tty/Kconfig | 19 ++++++++++++++++++- kernel/printk/printk.c | 5 +++++ 3 files changed, 26 insertions(+), 2 deletions(-)