selftests/tty: add TIOCSTI test suite

TIOCSTI is a TTY ioctl command that allows inserting characters into
the terminal input queue, making it appear as if the user typed those
characters. This functionality has behavior that varies based on system
configuration and process credentials.

The dev.tty.legacy_tiocsti sysctl introduced in commit 83efeeeb3d04
("tty: Allow TIOCSTI to be disabled") controls TIOCSTI usage. When
disabled, TIOCSTI requires CAP_SYS_ADMIN capability.

The current implementation checks the current process's credentials via
capable(CAP_SYS_ADMIN), but does not validate against the file opener's
credentials stored in file->f_cred. This creates different behavior when
file descriptors are passed between processes via SCM_RIGHTS.

Add a test suite with 16 test variants using fixture variants to verify
TIOCSTI behavior when dev.tty.legacy_tiocsti is enabled/disabled:

- Basic TIOCSTI tests (8 variants): Direct testing with different
  capability and controlling terminal combinations
- FD passing tests (8 variants): Test behavior when file descriptors
  are passed between processes with different capabilities

The FD passing tests document this behavior - some tests show different
results than expected based on file opener credentials, demonstrating
that TIOCSTI uses current process credentials rather than file opener
credentials.

The tests validate proper enforcement of the legacy_tiocsti sysctl. Test
implementation uses openpty(3) with TIOCSCTTY for isolated PTY
environments. See tty_ioctl(4) for details on TIOCSTI behavior and
security requirements.

Signed-off-by: Abhinav Saxena <xandfury@gmail.com>
Link: https://patch.msgid.link/20250903-toicsti-bug-v4-1-4894b6649ef8@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions