fdisk: fix strict-aliasing bugs

gcc 4.4 produces tons of

  "dereferencing type-punned pointer will break strict-aliasing rules"

warnings for fdisk code where is

  char buffer[BUFSIZ];

  ((struct disklabel *) MBRBuffer)->foo

There are two ways how fix the problem:

  1/ union {
          char buffer[BUFSIZ], struct disklabel label
     } MBRBuffer;

  2/ use allocated buffer, this way seems less invasive.

This patch implements 2/.

Old version:
  $ make -C fdisk | grep -c warning
  236

New version:
  $ make -C fdisk | grep -c warning
  0

About aliasing:
 - http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40665
 - http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html
 - C99

Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 24 insertions, 1 deletions

diff --git a/fdisk/fdisk.c b/fdisk/fdisk.c
index aaa01a84a..d0b4c5f89 100644
--- a/fdisk/fdisk.c
+++ b/fdisk/fdisk.c
@@ -174,7 +174,7 @@ static int type_open = O_RDWR;
  * Raw disk label. For DOS-type partition tables the MBR,
  * with descriptions of the primary partitions.
  */
-unsigned char MBRbuffer[MAX_SECTOR_SIZE];
+unsigned char *MBRbuffer;
 
 /*
  * per partition table entry data
@@ -950,6 +950,27 @@ get_geometry(int fd, struct geom *g) {
 }
 
 /*
+ * Please, always use allocated buffer if you want to cast the buffer to
+ * any struct -- cast non-allocated buffer to any struct is against
+ * strict-aliasing rules.  --kzak 16-Oct-2009
+ */
+static void init_mbr_buffer(void)
+{
+	if (MBRbuffer)
+		return;
+
+	MBRbuffer = calloc(1, MAX_SECTOR_SIZE);
+	if (!MBRbuffer)
+		fatal(out_of_memory);
+}
+
+void zeroize_mbr_buffer(void)
+{
+	if (MBRbuffer)
+		memset(MBRbuffer, 0, MAX_SECTOR_SIZE);
+}
+
+/*
  * Read MBR.  Returns:
  *   -1: no 0xaa55 flag present (possibly entire disk BSD)
  *    0: found or created label
@@ -2658,6 +2679,8 @@ main(int argc, char **argv) {
 			 " be used with one specified device\n"));
 #endif
 
+	init_mbr_buffer();
+
 	if (optl) {
 		nowarn = 1;
 		type_open = O_RDONLY;