Do not use call_usermodehelper() when testing the shrinker callback.

If the usermode helper enters an infinite loop, we will hang. I caught this
when debugging an infinite loop in `drop_slab()` on Linux 3.12.y.  Instead of
doing an upcall into userland, lets write to the file directly from the kernel.
This does not solve the infinite loop in `shrink_slab()`, but it makes
debugging such issues easier, makes the code cleaner and makes the test suite
slightly faster.

Signed-off-by: Richard Yao <ryao@gentoo.org>

module/splat/splat-linux.c

@@ -23,6 +23,7 @@
  *  Solaris Porting LAyer Tests (SPLAT) Kernel Compatibility Tests.
 \*****************************************************************************/
 
+#include <linux/file.h>
 #include <sys/kmem.h>
 #include "splat-internal.h"
 
@@ -94,32 +95,27 @@ __splat_linux_shrinker_fn(struct shrinker *shrink, struct shrink_control *sc)
 
 SPL_SHRINKER_CALLBACK_WRAPPER(splat_linux_shrinker_fn);
 
-#define DROP_SLAB_CMD \
-	"exec 0</dev/null " \
-	"     1>/proc/sys/vm/drop_caches " \
-	"     2>/dev/null; " \
-	"echo 2"
-
 static int
-splat_linux_drop_slab(struct file *file)
+splat_echo(char *path, char *data, ssize_t len)
 {
-	char *argv[] = { "/bin/sh",
-	                 "-c",
-	                 DROP_SLAB_CMD,
-	                 NULL };
-	char *envp[] = { "HOME=/",
-	                 "TERM=linux",
-	                 "PATH=/sbin:/usr/sbin:/bin:/usr/bin",
-	                 NULL };
+	struct file *fp;
+	mm_segment_t saved_fs;
+	loff_t offset = 0;
 	int rc;
 
-	rc = call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
-	if (rc)
-		splat_vprint(file, SPLAT_LINUX_TEST1_NAME,
-	            "Failed user helper '%s %s %s', rc = %d\n",
-		    argv[0], argv[1], argv[2], rc);
+	fp = filp_open(path, O_WRONLY, 0644);
+	if ((rc = PTR_RET(fp)))
+		return (rc);
 
-	return rc;
+	saved_fs = get_fs();
+	set_fs(get_ds());
+	rc = vfs_write(fp, data, len, &offset);
+	set_fs(saved_fs);
+
+	if (rc == 0)
+		rc = filp_close(fp, 0);
+
+	return (rc);
 }
 
 /*
@@ -152,7 +148,7 @@ splat_linux_test1(struct file *file, void *arg)
 	splat_linux_shrinker_file = file;
 
 	spl_register_shrinker(&splat_linux_shrinker);
-	rc = splat_linux_drop_slab(file);
+	rc = splat_echo("/proc/sys/vm/drop_caches", "2", 1);
 	if (rc)
 		goto out;