Eliminate runtime function pointer mods in autotools checks

PaX/GrSecurity patched kernels implement a dialect of C that relies on a
GCC plugin for enforcement. A basic idea in this dialect is that
function pointers in structures should not change during runtime.
This causes code that modifies function pointers at runtime to fail to
compile in many instances. The autotools checks rely on whether or
not small test cases compile against a given kernel. Some
autotools checks assume some default case if other cases fail. When one
of these autotools checks tests a PaX/GrSecurity patched kernel by
modifying a function pointer at runtime, the default case will be used.

Early detection of such situations is possible by relying on compiler
warnings, which are compiler errors when --enable-debug is used.
Unfortunately, very few people build ZFS with --enable-debug. The more
common situation is that these issues manifest themselves as runtime
failures in the form of NULL pointer exceptions.

Previous patches that addressed such issues with PaX/GrSecurity
compatibility largely relied on rewriting autotools checks to avoid
runtime function pointer modification or the addition of PaX/GrSecurity
specific checks. This patch takes the previous work to its logical
conclusion by eliminating the use of runtime function pointer
modification. This permits the removal of PaX-specific autotools checks
in favor of ones that work across all supported kernels.

This should resolve issues that were reported to occur with
PaX/GrSecurity-patched Linux 3.7.5 kernels on Gentoo Linux.

https://bugs.gentoo.org/show_bug.cgi?id=457176

We should be able to prevent future regressions in PaX/GrSecurity
compatibility by ensuring that all changes to ZFSOnLinux avoid runtime
function pointer modification. At the same time, this does not solve the
issue of silent failures triggering default cases in the autotools
check, which is what permitted these regressions to become runtime
failures in the first place. This will need to be addressed in a future
patch.

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

1 files changed, 16 insertions, 29 deletions

diff --git a/config/kernel-fallocate.m4 b/config/kernel-fallocate.m4
index 6ac5ae65c..550906472 100644
--- a/config/kernel-fallocate.m4
+++ b/config/kernel-fallocate.m4
@@ -5,12 +5,16 @@ AC_DEFUN([ZFS_AC_KERNEL_FILE_FALLOCATE], [
 	AC_MSG_CHECKING([whether fops->fallocate() exists])
 	ZFS_LINUX_TRY_COMPILE([
 		#include <linux/fs.h>
-	],[
-		long (*fallocate) (struct file *, int, loff_t, loff_t) = NULL;
-		struct file_operations fops __attribute__ ((unused)) = {
-			.fallocate = fallocate,
+
+		long test_fallocate(struct file *file, int mode,
+		    loff_t offset, loff_t len) { return 0; }
+
+		static const struct file_operations
+		    fops __attribute__ ((unused)) = {
+			.fallocate = test_fallocate,
 		};
 	],[
+	],[
 		AC_MSG_RESULT(yes)
 		AC_DEFINE(HAVE_FILE_FALLOCATE, 1, [fops->fallocate() exists])
 	],[
@@ -25,34 +29,18 @@ AC_DEFUN([ZFS_AC_KERNEL_INODE_FALLOCATE], [
 	AC_MSG_CHECKING([whether iops->fallocate() exists])
 	ZFS_LINUX_TRY_COMPILE([
 		#include <linux/fs.h>
-	],[
-		long (*fallocate) (struct inode *, int, loff_t, loff_t) = NULL;
-		struct inode_operations fops __attribute__ ((unused)) = {
-			.fallocate = fallocate,
-		};
-	],[
-		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_INODE_FALLOCATE, 1, [fops->fallocate() exists])
-	],[
-		AC_MSG_RESULT(no)
-	])
-])
 
-dnl #
-dnl # PaX Linux 2.6.38 - 3.x API
-dnl #
-AC_DEFUN([ZFS_AC_PAX_KERNEL_FILE_FALLOCATE], [
-	AC_MSG_CHECKING([whether fops->fallocate() exists])
-	ZFS_LINUX_TRY_COMPILE([
-		#include <linux/fs.h>
-	],[
-		long (*fallocate) (struct file *, int, loff_t, loff_t) = NULL;
-		struct file_operations_no_const fops __attribute__ ((unused)) = {
-			.fallocate = fallocate,
+		long test_fallocate(struct inode *inode, int mode,
+		    loff_t offset, loff_t len) { return 0; }
+
+		static const struct inode_operations
+		    fops __attribute__ ((unused)) = {
+			.fallocate = test_fallocate,
 		};
 	],[
+	],[
 		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_FILE_FALLOCATE, 1, [fops->fallocate() exists])
+		AC_DEFINE(HAVE_INODE_FALLOCATE, 1, [fops->fallocate() exists])
 	],[
 		AC_MSG_RESULT(no)
 	])
@@ -65,5 +53,4 @@ dnl #
 AC_DEFUN([ZFS_AC_KERNEL_FALLOCATE], [
 	ZFS_AC_KERNEL_FILE_FALLOCATE
 	ZFS_AC_KERNEL_INODE_FALLOCATE
-	ZFS_AC_PAX_KERNEL_FILE_FALLOCATE
 ])