linuxkpi: Provide a non-NULL value for THIS_MODULE

THIS_MODULE is used to differentiate modules on Linux. We currently
completely stub out any Linux struct module usage, but THIS_MODULE
is still used to populate the "owner" fields of various drivers.
Even though we don't actually dereference these "owner" fields they
are still used by drivers to check if devices/dmabufs/etc come
from different modules. For example, during DRM GEM import some
drivers check if the dmabuf's owner matches the dev's owner. If
they match because they are both NULL drivers may incorrectly think
two resources come from the same module.

This adds a general purpose __this_linker_file which will point to
the linker file of the module that uses it. We can then use that
pointer to have a valid value for THIS_MODULE.

Reviewed by:	bz, jhb
Differential Revision:	https://reviews.freebsd.org/D44306

Author: amshafer

sys/compat/linuxkpi/common/include/linux/module.h

@@ -32,6 +32,8 @@
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/module.h>
+#include <sys/queue.h>
+#include <sys/linker.h>
 
 #include <linux/list.h>
 #include <linux/compiler.h>
@@ -51,7 +53,26 @@
 #define	MODULE_SUPPORTED_DEVICE(name)
 #define	MODULE_IMPORT_NS(_name)
 
+/*
+ * THIS_MODULE is used to differentiate modules on Linux. We currently
+ * completely stub out any Linux struct module usage, but THIS_MODULE is still
+ * used to populate the "owner" fields of various drivers.  Even though we
+ * don't actually dereference these "owner" fields they are still used by
+ * drivers to check if devices/dmabufs/etc come from different modules. For
+ * example, during DRM GEM import some drivers check if the dmabuf's owner
+ * matches the dev's owner. If they match because they are both NULL drivers
+ * may incorrectly think two resources come from the same module.
+ *
+ * To handle this we specify an undefined symbol __this_linker_file, which
+ * will get special treatment from the linker when resolving. This will
+ * populate the usages of __this_linker_file with the linker_file_t of the
+ * module.
+ */
+#ifdef KLD_MODULE
+#define	THIS_MODULE	((struct module *)&__this_linker_file)
+#else
 #define	THIS_MODULE	((struct module *)0)
+#endif
 
 #define	__MODULE_STRING(x) __stringify(x)
 

sys/kern/kern_linker.c

@@ -906,6 +906,20 @@ linker_file_lookup_symbol_internal(linker_file_t file, const char *name,
 	KLD_DPF(SYM, ("linker_file_lookup_symbol: file=%p, name=%s, deps=%d\n",
 	    file, name, deps));
 
+	/*
+	 * Treat the __this_linker_file as a special symbol. This is a
+	 * global that linuxkpi uses to populate the THIS_MODULE
+	 * value.  In this case we can simply return the linker_file_t.
+	 *
+	 * Modules compiled statically into the kernel are assigned NULL.
+	 */
+	if (strcmp(name, "__this_linker_file") == 0) {
+		address = (file == linker_kernel_file) ? NULL : (caddr_t)file;
+		KLD_DPF(SYM, ("linker_file_lookup_symbol: resolving special "
+		    "symbol __this_linker_file to %p\n", address));
+		return (address);
+	}
+
 	if (LINKER_LOOKUP_SYMBOL(file, name, &sym) == 0) {
 		LINKER_SYMBOL_VALUES(file, sym, &symval);
 		if (symval.value == 0)

sys/sys/linker.h

@@ -130,6 +130,12 @@ typedef int linker_predicate_t(linker_file_t, void *);
  */
 extern linker_file_t	linker_kernel_file;
 
+/*
+ * Special symbol which will be replaced by a reference to the linker_file_t
+ * of the module it is used in.
+ */
+extern linker_file_t __this_linker_file;
+
 /*
  * Obtain a reference to a module, loading it if required.
  */