implement a union to pad out file_handle

Cases where name_to_handle_at is used allocated the full struct to be
MAX_HANDLE_SZ, and assigned this size to handle_bytes. This is wrong
since handle_bytes should describe the length of the flexible array
member and not the whole struct.

Define a union type which includes sufficient padding to allow
assignment of MAX_HANDLE_SZ to be correct.

4 files changed, 14 insertions, 15 deletions

diff --git a/src/libudev/libudev-monitor.c b/src/libudev/libudev-monitor.c
index 3f7436b094..0a2ab82a89 100644
--- a/src/libudev/libudev-monitor.c
+++ b/src/libudev/libudev-monitor.c
@@ -108,15 +108,13 @@ static struct udev_monitor *udev_monitor_new(struct udev *udev)
 
 /* we consider udev running when /dev is on devtmpfs */
 static bool udev_has_devtmpfs(struct udev *udev) {
-        struct file_handle *h;
+        union file_handle_union h = { .handle.handle_bytes = MAX_HANDLE_SZ, };
         int mount_id;
         _cleanup_fclose_ FILE *f = NULL;
         char line[LINE_MAX], *e;
         int r;
 
-        h = alloca(MAX_HANDLE_SZ);
-        h->handle_bytes = MAX_HANDLE_SZ;
-        r = name_to_handle_at(AT_FDCWD, "/dev", h, &mount_id, 0);
+        r = name_to_handle_at(AT_FDCWD, "/dev", &h.handle, &mount_id, 0);
         if (r < 0)
                 return false;
 
diff --git a/src/readahead/readahead-common.c b/src/readahead/readahead-common.c
index 5ffa88b78a..49679fc834 100644
--- a/src/readahead/readahead-common.c
+++ b/src/readahead/readahead-common.c
@@ -75,7 +75,7 @@ int fs_on_ssd(const char *p) {
         if (major(st.st_dev) == 0) {
                 _cleanup_fclose_ FILE *f = NULL;
                 int mount_id;
-                struct file_handle *h;
+                union file_handle_union h = { .handle.handle_bytes = MAX_HANDLE_SZ, };
 
                 /* Might be btrfs, which exposes "ssd" as mount flag if it is on ssd.
                  *
@@ -83,9 +83,7 @@ int fs_on_ssd(const char *p) {
                  * and then lookup the mount ID in mountinfo to find
                  * the mount options. */
 
-                h = alloca(MAX_HANDLE_SZ);
-                h->handle_bytes = MAX_HANDLE_SZ;
-                r = name_to_handle_at(AT_FDCWD, p, h, &mount_id, AT_SYMLINK_FOLLOW);
+                r = name_to_handle_at(AT_FDCWD, p, &h.handle, &mount_id, AT_SYMLINK_FOLLOW);
                 if (r < 0)
                         return false;
 
diff --git a/src/shared/util.h b/src/shared/util.h
index 900f1cf54d..891848a1d8 100644
--- a/src/shared/util.h
+++ b/src/shared/util.h
@@ -22,6 +22,7 @@
 ***/
 
 #include <alloca.h>
+#include <fcntl.h>
 #include <inttypes.h>
 #include <time.h>
 #include <sys/time.h>
@@ -914,3 +915,8 @@ uint64_t physical_memory(void);
 char* mount_test_option(const char *haystack, const char *needle);
 
 void hexdump(FILE *f, const void *p, size_t s);
+
+union file_handle_union {
+  struct file_handle handle;
+  char padding[sizeof(struct file_handle) + MAX_HANDLE_SZ];
+};
diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index 33e7cbc05d..04b472da97 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -217,19 +217,16 @@ static bool unix_socket_alive(const char *fn) {
 }
 
 static int dir_is_mount_point(DIR *d, const char *subdir) {
-        struct file_handle *h;
+        union file_handle_union h = { .handle.handle_bytes = MAX_HANDLE_SZ };
         int mount_id_parent, mount_id;
         int r_p, r;
 
-        h = alloca(MAX_HANDLE_SZ);
-
-        h->handle_bytes = MAX_HANDLE_SZ;
-        r_p = name_to_handle_at(dirfd(d), ".", h, &mount_id_parent, 0);
+        r_p = name_to_handle_at(dirfd(d), ".", &h.handle, &mount_id_parent, 0);
         if (r_p < 0)
                 r_p = -errno;
 
-        h->handle_bytes = MAX_HANDLE_SZ;
-        r = name_to_handle_at(dirfd(d), subdir, h, &mount_id, 0);
+        h.handle.handle_bytes = MAX_HANDLE_SZ;
+        r = name_to_handle_at(dirfd(d), subdir, &h.handle, &mount_id, 0);
         if (r < 0)
                 r = -errno;