Constify structures containing function pointers

The PaX team modified the kernel's modpost to report writeable function
pointers as section mismatches because they are potential exploit
targets. We could ignore the warnings, but their presence can obscure
actual issues. Proper const correctness can also catch programming
mistakes.

Building the kernel modules against a PaX/GrSecurity patched Linux 3.4.2
kernel reports 133 section mismatches prior to this patch. This patch
eliminates 130 of them. The quantity of writeable function pointers
eliminated by constifying each structure is as follows:

vdev_opts_t             52
zil_replay_func_t       24
zio_compress_info_t     24
zio_checksum_info_t     9
space_map_ops_t         7
arc_byteswap_func_t     5

The remaining 3 writeable function pointers cannot be addressed by this
patch. 2 of them are in zpl_fs_type. The kernel's sget function requires
that this be non-const. The final writeable function pointer is created
by SPL_SHRINKER_DECLARE. The kernel's set_shrinker() and
remove_shrinker() functions also require that this be non-const.

Signed-off-by: Richard Yao <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #1300

1 files changed, 3 insertions, 3 deletions

diff --git a/include/sys/dmu.h b/include/sys/dmu.h
index 2a3a76486..eb6a465c4 100644
--- a/include/sys/dmu.h
+++ b/include/sys/dmu.h
@@ -641,7 +641,7 @@ typedef struct dmu_object_info {
 	uint64_t doi_fill_count;		/* number of non-empty blocks */
 } dmu_object_info_t;
 
-typedef void arc_byteswap_func_t(void *buf, size_t size);
+typedef void (*const arc_byteswap_func_t)(void *buf, size_t size);
 
 typedef struct dmu_object_type_info {
 	dmu_object_byteswap_t	ot_byteswap;
@@ -649,8 +649,8 @@ typedef struct dmu_object_type_info {
 	char			*ot_name;
 } dmu_object_type_info_t;
 
-typedef struct dmu_object_byteswap_info {
-	arc_byteswap_func_t	*ob_func;
+typedef const struct dmu_object_byteswap_info {
+	arc_byteswap_func_t	 ob_func;
 	char			*ob_name;
 } dmu_object_byteswap_info_t;