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, 39 insertions, 0 deletions

diff --git a/tools/testing/selftests/proc/proc.h b/tools/testing/selftests/proc/proc.h
new file mode 100644
index 000000000000..4e178166fd84
--- /dev/null
+++ b/tools/testing/selftests/proc/proc.h
@@ -0,0 +1,39 @@
+#pragma once
+#undef NDEBUG
+#include <assert.h>
+#include <dirent.h>
+#include <errno.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+static inline bool streq(const char *s1, const char *s2)
+{
+        return strcmp(s1, s2) == 0;
+}
+
+static unsigned long long xstrtoull(const char *p, char **end)
+{
+        if (*p == '0') {
+                *end = (char *)p + 1;
+                return 0;
+        } else if ('1' <= *p && *p <= '9') {
+                unsigned long long val;
+
+                errno = 0;
+                val = strtoull(p, end, 10);
+                assert(errno == 0);
+                return val;
+        } else
+                assert(0);
+}
+
+static struct dirent *xreaddir(DIR *d)
+{
+        struct dirent *de;
+
+        errno = 0;
+        de = readdir(d);
+        assert(de || errno == 0);
+        return de;
+}