• Alexey Dobriyan's avatar
    tools/testing/selftests/proc: test /proc/*/fd a bit (+ PF_KTHREAD is ABI!) · b2f5de03
    Alexey Dobriyan authored
    * Test lookup in /proc/self/fd.
      "map_files" lookup story showed that lookup is not that simple.
    
    * Test that all those symlinks open the same file.
      Check with (st_dev, st_info).
    
    * Test that kernel threads do not have anything in their /proc/*/fd/
      directory.
    
    Now this is where things get interesting.
    
    First, kernel threads aren't pinned by /proc/self or equivalent,
    thus some "atomicity" is required.
    
    Second, ->comm can contain whitespace and ')'.
    No, they are not escaped.
    
    Third, the only reliable way to check if process is kernel thread
    appears to be field #9 in /proc/*/stat.
    
    This field is struct task_struct::flags in decimal!
    Check is done by testing PF_KTHREAD flags like we do in kernel.
    
    	PF_KTREAD value is a part of userspace ABI !!!
    
    Other methods for determining kernel threadness are not reliable:
    * RSS can be 0 if everything is swapped, even while reading
      from /proc/self.
    
    * ->total_vm CAN BE ZERO if process is finishing
    
    	munmap(NULL, whole address space);
    
    * /proc/*/maps and similar files can be empty because unmapping
      everything works. Read returning 0 can't distinguish between
      kernel thread and such suicide process.
    
    Link: http://lkml.kernel.org/r/20180505000414.GA15090@avx2Signed-off-by: default avatarAlexey Dobriyan <adobriyan@gmail.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    b2f5de03
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