util: allow to run criu under strace

[avagin]

fork_and_ptrace_attach has to fork a child with CLONE_UNTRACED, so that strace doesn't trace it.

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (c20fb83) 70.55% compared to head (c3890aa) 70.53%.
Report is 1 commits behind head on criu-dev.

❗ Current head c3890aa differs from pull request most recent head af919e8. Consider uploading reports for the commit af919e8 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##           criu-dev    #2273      +/-   ##
============================================
- Coverage     70.55%   70.53%   -0.03%     
============================================
  Files           132      135       +3     
  Lines         33511    33526      +15     
============================================
+ Hits          23643    23646       +3     
- Misses         9868     9880      +12     

Files	Coverage Δ
criu/util.c	63.10% <14.28%> (-0.19%)	⬇️

... and 16 files with indirect coverage changes

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[rst0git]

LGTM, but I was not able to replicate the problem in #2269 (comment).

[avagin]

@rst0git it is triggered each time when kerndat_has_ptrace_get_rseq_conf is called. If you can't reproduce it, you need to find out why kerndat_has_ptrace_get_rseq_conf isn't executed in your case.

criu/criu/kerndat.c

Line 978 in 5bf7652

pid = fork_and_ptrace_attach(NULL);

[rst0git]

@avagin Thanks! I was able to reproduce the problem and confirm that the patch works.