From b2f15782a71fd7d89bed7b3e1f799358779ee828 Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Fri, 16 Oct 2009 21:49:33 +0200
Subject: [PATCH] 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>
---
 fdisk/fdisk.c         | 25 ++++++++++++++++++++++++-
 fdisk/fdisk.h         |  4 +++-
 fdisk/fdiskaixlabel.c |  2 +-
 fdisk/fdiskmaclabel.c |  2 +-
 fdisk/fdisksgilabel.c |  2 +-
 fdisk/fdisksunlabel.c |  2 +-
 6 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/fdisk/fdisk.c b/fdisk/fdisk.c
index aaa01a84..d0b4c5f8 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
@@ -949,6 +949,27 @@ 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)
@@ -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;
diff --git a/fdisk/fdisk.h b/fdisk/fdisk.h
index 6ef68b6e..0286ce0a 100644
--- a/fdisk/fdisk.h
+++ b/fdisk/fdisk.h
@@ -76,7 +76,9 @@ extern int valid_part_table_flag(unsigned char *b);
 extern unsigned int read_int(unsigned int low, unsigned int dflt,
 			     unsigned int high, unsigned int base, char *mesg);
 
-extern unsigned char MBRbuffer[MAX_SECTOR_SIZE];
+extern unsigned char *MBRbuffer;
+extern void zeroize_mbr_buffer(void);
+
 extern unsigned int heads, cylinders, sector_size;
 extern unsigned long long sectors;
 extern char *partition_type(unsigned char type);
diff --git a/fdisk/fdiskaixlabel.c b/fdisk/fdiskaixlabel.c
index 4f6167f9..e7ee95d1 100644
--- a/fdisk/fdiskaixlabel.c
+++ b/fdisk/fdiskaixlabel.c
@@ -44,7 +44,7 @@ aix_nolabel( void )
     aixlabel->magic = 0;
     aix_label = 0;
     partitions = 4;
-    memset( MBRbuffer, 0, sizeof(MBRbuffer) );	/* avoid fdisk cores */
+    zeroize_mbr_buffer();
     return;
 }
 
diff --git a/fdisk/fdiskmaclabel.c b/fdisk/fdiskmaclabel.c
index 06d4a6ed..4e3db20e 100644
--- a/fdisk/fdiskmaclabel.c
+++ b/fdisk/fdiskmaclabel.c
@@ -44,7 +44,7 @@ mac_nolabel( void )
     maclabel->magic = 0;
     mac_label = 0;
     partitions = 4;
-    memset( MBRbuffer, 0, sizeof(MBRbuffer) );	/* avoid fdisk cores */
+    zeroize_mbr_buffer();
     return;
 }
 
diff --git a/fdisk/fdisksgilabel.c b/fdisk/fdisksgilabel.c
index fb2091da..3707a4cd 100644
--- a/fdisk/fdisksgilabel.c
+++ b/fdisk/fdisksgilabel.c
@@ -737,7 +737,7 @@ create_sgilabel(void)
 		}
 	}
 
-	memset(MBRbuffer, 0, sizeof(MBRbuffer));
+	zeroize_mbr_buffer();
 	sgilabel->magic = SSWAP32(SGI_LABEL_MAGIC);
 	sgilabel->boot_part = SSWAP16(0);
 	sgilabel->swap_part = SSWAP16(1);
diff --git a/fdisk/fdisksunlabel.c b/fdisk/fdisksunlabel.c
index be7b9f6a..2f9fad90 100644
--- a/fdisk/fdisksunlabel.c
+++ b/fdisk/fdisksunlabel.c
@@ -203,7 +203,7 @@ void create_sunlabel(void)
 #else
 	other_endian = 0;
 #endif
-	memset(MBRbuffer, 0, sizeof(MBRbuffer));
+	zeroize_mbr_buffer();
 	sunlabel->magic = SSWAP16(SUN_LABEL_MAGIC);
 	sunlabel->sanity = SSWAP32(SUN_LABEL_SANE);
 	sunlabel->version = SSWAP32(SUN_LABEL_VERSION);
-- 
2.39.5