proc: test /proc/self symlink

There are plans to change how /proc/self result is calculated,
for that a test is necessary.

Use direct system call because of this whole getpid caching story.

Link: http://lkml.kernel.org/r/20180627195103.GB18113@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

@@ -9,3 +9,4 @@
 /proc-uptime-001
 /proc-uptime-002
 /read
+/self

tools/testing/selftests/proc/Makefile

@@ -1,4 +1,5 @@
 CFLAGS += -Wall -O2 -Wno-unused-function
+CFLAGS += -D_GNU_SOURCE
 
 TEST_GEN_PROGS :=
 TEST_GEN_PROGS += fd-001-lookup
@@ -12,5 +13,6 @@ TEST_GEN_PROGS += proc-self-wchan
 TEST_GEN_PROGS += proc-uptime-001
 TEST_GEN_PROGS += proc-uptime-002
 TEST_GEN_PROGS += read
+TEST_GEN_PROGS += self
 
 include ../lib.mk

tools/testing/selftests/proc/proc.h

@@ -6,6 +6,13 @@
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+
+static inline pid_t sys_getpid(void)
+{
+	return syscall(SYS_getpid);
+}
 
 static inline bool streq(const char *s1, const char *s2)
 {

tools/testing/selftests/proc/self.c

@@ -0,0 +1,39 @@
+/*
+ * 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/self gives correct TGID.
+#undef NDEBUG
+#include <assert.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "proc.h"
+
+int main(void)
+{
+	char buf1[64], buf2[64];
+	pid_t pid;
+	ssize_t rv;
+
+	pid = sys_getpid();
+	snprintf(buf1, sizeof(buf1), "%u", pid);
+
+	rv = readlink("/proc/self", buf2, sizeof(buf2));
+	assert(rv == strlen(buf1));
+	buf2[rv] = '\0';
+	assert(streq(buf1, buf2));
+
+	return 0;
+}