selftests/mm: use PIDFD_SELF in guard pages test

Now we have PIDFD_SELF available for process_madvise(), make use of it in
the guard pages test.

This is both more convenient and asserts that PIDFD_SELF works as expected.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/r/69fbbe088d3424de9983e145228459cb05a8f13d.1738268370.git.lorenzo.stoakes@oracle.com
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: ljskernel

tools/testing/selftests/mm/guard-pages.c

@@ -19,6 +19,8 @@
 #include <sys/uio.h>
 #include <unistd.h>
 
+#include "../pidfd/pidfd.h"
+
 /*
  * Ignore the checkpatch warning, as per the C99 standard, section 7.14.1.1:
  *
@@ -50,11 +52,6 @@ static void handle_fatal(int c)
 	siglongjmp(signal_jmp_buf, c);
 }
 
-static int pidfd_open(pid_t pid, unsigned int flags)
-{
-	return syscall(SYS_pidfd_open, pid, flags);
-}
-
 static ssize_t sys_process_madvise(int pidfd, const struct iovec *iovec,
 				   size_t n, int advice, unsigned int flags)
 {
@@ -370,14 +367,10 @@ TEST_F(guard_pages, multi_vma)
 TEST_F(guard_pages, process_madvise)
 {
 	const unsigned long page_size = self->page_size;
-	pid_t pid = getpid();
-	int pidfd = pidfd_open(pid, 0);
 	char *ptr_region, *ptr1, *ptr2, *ptr3;
 	ssize_t count;
 	struct iovec vec[6];
 
-	ASSERT_NE(pidfd, -1);
-
 	/* Reserve region to map over. */
 	ptr_region = mmap(NULL, 100 * page_size, PROT_NONE,
 			  MAP_ANON | MAP_PRIVATE, -1, 0);
@@ -425,7 +418,7 @@ TEST_F(guard_pages, process_madvise)
 	ASSERT_EQ(munmap(&ptr_region[99 * page_size], page_size), 0);
 
 	/* Now guard in one step. */
-	count = sys_process_madvise(pidfd, vec, 6, MADV_GUARD_INSTALL, 0);
+	count = sys_process_madvise(PIDFD_SELF, vec, 6, MADV_GUARD_INSTALL, 0);
 
 	/* OK we don't have permission to do this, skip. */
 	if (count == -1 && errno == EPERM)
@@ -446,7 +439,7 @@ TEST_F(guard_pages, process_madvise)
 	ASSERT_FALSE(try_read_write_buf(&ptr3[19 * page_size]));
 
 	/* Now do the same with unguard... */
-	count = sys_process_madvise(pidfd, vec, 6, MADV_GUARD_REMOVE, 0);
+	count = sys_process_madvise(PIDFD_SELF, vec, 6, MADV_GUARD_REMOVE, 0);
 
 	/* ...and everything should now succeed. */
 
@@ -463,7 +456,6 @@ TEST_F(guard_pages, process_madvise)
 	ASSERT_EQ(munmap(ptr1, 10 * page_size), 0);
 	ASSERT_EQ(munmap(ptr2, 5 * page_size), 0);
 	ASSERT_EQ(munmap(ptr3, 20 * page_size), 0);
-	close(pidfd);
 }
 
 /* Assert that unmapping ranges does not leave guard markers behind. */