tools/testing/selftests/proc: test /proc/*/fd a bit (+ PF_KTHREAD is ABI!)

* 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@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 16 deletions

diff --git a/tools/testing/selftests/proc/read.c b/tools/testing/selftests/proc/read.c
index 1e73c2232097..563e752e6eba 100644
--- a/tools/testing/selftests/proc/read.c
+++ b/tools/testing/selftests/proc/read.c
@@ -31,22 +31,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 
-static inline bool streq(const char *s1, const char *s2)
-{
-	return strcmp(s1, s2) == 0;
-}
-
-static struct dirent *xreaddir(DIR *d)
-{
-	struct dirent *de;
-
-	errno = 0;
-	de = readdir(d);
-	if (!de && errno != 0) {
-		exit(1);
-	}
-	return de;
-}
+#include "proc.h"
 
 static void f_reg(DIR *d, const char *filename)
 {