proc: test /proc/thread-self symlink

Same story: I have WIP patch to make it faster, so better have a test
as well.

Link: http://lkml.kernel.org/r/20180627195209.GC18113@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/proc/.gitignore

@@ -10,3 +10,4 @@
 /proc-uptime-002
 /read
 /self
+/thread-self

tools/testing/selftests/proc/Makefile

@@ -14,5 +14,6 @@ TEST_GEN_PROGS += proc-uptime-001
 TEST_GEN_PROGS += proc-uptime-002
 TEST_GEN_PROGS += read
 TEST_GEN_PROGS += self
+TEST_GEN_PROGS += thread-self
 
 include ../lib.mk

tools/testing/selftests/proc/proc.h

@@ -14,6 +14,11 @@ static inline pid_t sys_getpid(void)
 	return syscall(SYS_getpid);
 }
 
+static inline pid_t sys_gettid(void)
+{
+	return syscall(SYS_gettid);
+}
+
 static inline bool streq(const char *s1, const char *s2)
 {
 	return strcmp(s1, s2) == 0;

tools/testing/selftests/proc/thread-self.c

@@ -0,0 +1,64 @@
+/*
+ * Copyright © 2018 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+// Test that /proc/thread-self gives correct TGID/PID.
+#undef NDEBUG
+#include <assert.h>
+#include <sched.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+
+#include "proc.h"
+
+int f(void *arg)
+{
+	char buf1[64], buf2[64];
+	pid_t pid, tid;
+	ssize_t rv;
+
+	pid = sys_getpid();
+	tid = sys_gettid();
+	snprintf(buf1, sizeof(buf1), "%u/task/%u", pid, tid);
+
+	rv = readlink("/proc/thread-self", buf2, sizeof(buf2));
+	assert(rv == strlen(buf1));
+	buf2[rv] = '\0';
+	assert(streq(buf1, buf2));
+
+	if (arg)
+		exit(0);
+	return 0;
+}
+
+int main(void)
+{
+	const int PAGE_SIZE = sysconf(_SC_PAGESIZE);
+	pid_t pid;
+	void *stack;
+
+	/* main thread */
+	f((void *)0);
+
+	stack = mmap(NULL, 2 * PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+	assert(stack != MAP_FAILED);
+	/* side thread */
+	pid = clone(f, stack + PAGE_SIZE, CLONE_THREAD|CLONE_SIGHAND|CLONE_VM, (void *)1);
+	assert(pid > 0);
+	pause();
+
+	return 0;
+}