btrfs-progs: simplify ioctl name copy and null termination

In the places where we copy a string into the name
member of btrfs_ioctl_vol_args or btrfs_ioctl_vol_args_v2,
we use strncopy (to not overflow the name array) and then
set the last position to the null character.

Howver, in both cases the arrays are defined with:

        char name[MAX+1];

hence the last array position is name[MAX].

In most cases, we now insert the null at name[MAX-1]
which deprives us of one useful character.

Even the above isn't consistent through the code, so
make some helper code to make it simple, i.e.
strncpy_null(dest, src) which automatically does the
right thing based on the size of dest.

Thanks to Zach Brown for the macro suggestion.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>

btrfsctl.c

@@ -242,10 +242,9 @@ int main(int ac, char **av)
 		fd = btrfsctl_open_file_or_dir(fname);
 	 }
 
-	if (name) {
-                strncpy(args.name, name, BTRFS_PATH_NAME_MAX + 1);
-                args.name[BTRFS_PATH_NAME_MAX] = 0;
-	} else
+	if (name)
+		strncpy_null(args.name, name);
+	else
 		args.name[0] = '\0';
 
 	if (command == BTRFS_IOC_SNAP_CREATE) {

cmds-device.c

@@ -116,8 +116,7 @@ static int cmd_add_dev(int argc, char **argv)
 		}
 		close(devfd);
 
-		strncpy(ioctl_args.name, argv[i], BTRFS_PATH_NAME_MAX);
-		ioctl_args.name[BTRFS_PATH_NAME_MAX-1] = 0;
+		strncpy_null(ioctl_args.name, argv[i]);
 		res = ioctl(fdmnt, BTRFS_IOC_ADD_DEV, &ioctl_args);
 		e = errno;
 		if(res<0){
@@ -161,8 +160,7 @@ static int cmd_rm_dev(int argc, char **argv)
 		struct	btrfs_ioctl_vol_args arg;
 		int	res;
 
-		strncpy(arg.name, argv[i], BTRFS_PATH_NAME_MAX);
-		arg.name[BTRFS_PATH_NAME_MAX-1] = 0;
+		strncpy_null(arg.name, argv[i]);
 		res = ioctl(fdmnt, BTRFS_IOC_RM_DEV, &arg);
 		e = errno;
 		if(res<0){
@@ -227,8 +225,7 @@ static int cmd_scan_dev(int argc, char **argv)
 
 		printf("Scanning for Btrfs filesystems in '%s'\n", argv[i]);
 
-		strncpy(args.name, argv[i], BTRFS_PATH_NAME_MAX);
-		args.name[BTRFS_PATH_NAME_MAX-1] = 0;
+		strncpy_null(args.name, argv[i]);
 		/*
 		 * FIXME: which are the error code returned by this ioctl ?
 		 * it seems that is impossible to understand if there no is

cmds-filesystem.c

@@ -478,8 +478,7 @@ static int cmd_resize(int argc, char **argv)
 	}
 
 	printf("Resize '%s' of '%s'\n", path, amount);
-	strncpy(args.name, amount, BTRFS_PATH_NAME_MAX);
-	args.name[BTRFS_PATH_NAME_MAX-1] = 0;
+	strncpy_null(args.name, amount);
 	res = ioctl(fd, BTRFS_IOC_RESIZE, &args);
 	e = errno;
 	close(fd);

cmds-subvolume.c

@@ -32,6 +32,7 @@
 
 #include "ctree.h"
 #include "commands.h"
+#include "utils.h"
 #include "btrfs-list.h"
 #include "utils.h"
 
@@ -138,8 +139,7 @@ static int cmd_subvol_create(int argc, char **argv)
 		struct btrfs_ioctl_vol_args_v2	args;
 
 		memset(&args, 0, sizeof(args));
-		strncpy(args.name, newname, BTRFS_SUBVOL_NAME_MAX);
-		args.name[BTRFS_SUBVOL_NAME_MAX-1] = 0;
+		strncpy_null(args.name, newname);
 		args.flags |= BTRFS_SUBVOL_QGROUP_INHERIT;
 		args.size = qgroup_inherit_size(inherit);
 		args.qgroup_inherit = inherit;
@@ -149,8 +149,7 @@ static int cmd_subvol_create(int argc, char **argv)
 		struct btrfs_ioctl_vol_args	args;
 
 		memset(&args, 0, sizeof(args));
-		strncpy(args.name, newname, BTRFS_PATH_NAME_MAX);
-		args.name[BTRFS_PATH_NAME_MAX-1] = 0;
+		strncpy_null(args.name, newname);
 
 		res = ioctl(fddst, BTRFS_IOC_SUBVOL_CREATE, &args);
 	}
@@ -250,8 +249,7 @@ again:
 	}
 
 	printf("Delete subvolume '%s/%s'\n", dname, vname);
-	strncpy(args.name, vname, BTRFS_PATH_NAME_MAX);
-	args.name[BTRFS_PATH_NAME_MAX-1] = 0;
+	strncpy_null(args.name, vname);
 	res = ioctl(fd, BTRFS_IOC_SNAP_DESTROY, &args);
 	e = errno;
 
@@ -597,8 +595,7 @@ static int cmd_snapshot(int argc, char **argv)
 		args.size = qgroup_inherit_size(inherit);
 		args.qgroup_inherit = inherit;
 	}
-	strncpy(args.name, newname, BTRFS_SUBVOL_NAME_MAX);
-	args.name[BTRFS_SUBVOL_NAME_MAX-1] = 0;
+	strncpy_null(args.name, newname);
 	res = ioctl(fddst, BTRFS_IOC_SNAP_CREATE_V2, &args);
 	e = errno;
 

utils.c

@@ -1125,6 +1125,26 @@ char *pretty_sizes(u64 size)
 	return pretty;
 }
 
+/*
+ * __strncpy__null - strncpy with null termination
+ * @dest:	the target array
+ * @src:	the source string
+ * @n:		maximum bytes to copy (size of *dest)
+ *
+ * Like strncpy, but ensures destination is null-terminated.
+ *
+ * Copies the string pointed to by src, including the terminating null
+ * byte ('\0'), to the buffer pointed to by dest, up to a maximum
+ * of n bytes.  Then ensure that dest is null-terminated.
+ */
+char *__strncpy__null(char *dest, const char *src, size_t n)
+{
+	strncpy(dest, src, n);
+	if (n > 0)
+		dest[n - 1] = '\0';
+	return dest;
+}
+
 /*
  * Checks to make sure that the label matches our requirements.
  * Returns:

utils.h

@@ -53,4 +53,9 @@ int get_device_info(int fd, u64 devid,
 		    struct btrfs_ioctl_dev_info_args *di_args);
 int get_fs_info(int fd, char *path, struct btrfs_ioctl_fs_info_args *fi_args,
 		struct btrfs_ioctl_dev_info_args **di_ret);
+
+char *__strncpy__null(char *dest, const char *src, size_t n);
+/* Helper to always get proper size of the destination string */
+#define strncpy_null(dest, src) __strncpy__null(dest, src, sizeof(dest))
+
 #endif