tty: skip ioctl(TIOCSLCKTRMIOS) if possible

[mihalicyn]

If ioctl(TIOCSLCKTRMIOS) fails with EPERM it means that a CRIU process lacks of CAP_SYS_ADMIN capability. But we can use ioctl(TIOCGLCKTRMIOS) to read current ->termios_locked value from the kernel and if it's the same as we already have we can skip failing ioctl(TIOCSLCKTRMIOS) safely.

Adrian has recently posted [1] a very good patch to allow ioctl(TIOCSLCKTRMIOS) for processes that have CAP_CHECKPOINT_RESTORE (right now it requires CAP_SYS_ADMIN).

[1] https://lore.kernel.org/all/20231206134340.7093-1-areber@redhat.com/

Suggested-by: Andrei Vagin

[mihalicyn]

I have tested this with the example from our wiki (https://criu.org/Simple_loop#A_shell_job):

echo 0 > /proc/sys/kernel/yama/ptrace_scope
setcap cap_checkpoint_restore+eip ./criu/criu

./criu/criu dump -vvvv -o dump.log -t 184826 --shell-job --unprivileged
./criu/criu restore -vvvv -o restore.log --shell-job --unprivileged

And it works perfectly fine for this case.

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (0da1ab2) 70.86% compared to head (14d9b45) 70.85%.

❗ Current head 14d9b45 differs from pull request most recent head 6448805. Consider uploading reports for the commit 6448805 to get more accurate results

Files	Patch %	Lines
criu/tty.c	10.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           criu-dev    #2311      +/-   ##
============================================
- Coverage     70.86%   70.85%   -0.02%     
============================================
  Files           133      133              
  Lines         32725    32733       +8     
============================================
+ Hits          23191    23192       +1     
- Misses         9534     9541       +7     

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[avagin]

lgtm. Thanks.

[rst0git]

LGTM