livepatch: Remove .klp.arch

After the previous patch, vmlinux-specific KLP relocations are now
applied early during KLP module load.  This means that .klp.arch
sections are no longer needed for *vmlinux-specific* KLP relocations.

One might think they're still needed for *module-specific* KLP
relocations.  If a to-be-patched module is loaded *after* its
corresponding KLP module is loaded, any corresponding KLP relocations
will be delayed until the to-be-patched module is loaded.  If any
special sections (.parainstructions, for example) rely on those
relocations, their initializations (apply_paravirt) need to be done
afterwards.  Thus the apparent need for arch_klp_init_object_loaded()
and its corresponding .klp.arch sections -- it allows some of the
special section initializations to be done at a later time.

But... if you look closer, that dependency between the special sections
and the module-specific KLP relocations doesn't actually exist in
reality.  Looking at the contents of the .altinstructions and
.parainstructions sections, there's not a realistic scenario in which a
KLP module's .altinstructions or .parainstructions section needs to
access a symbol in a to-be-patched module.  It might need to access a
local symbol or even a vmlinux symbol; but not another module's symbol.
When a special section needs to reference a local or vmlinux symbol, a
normal rela can be used instead of a KLP rela.

Since the special section initializations don't actually have any real
dependency on module-specific KLP relocations, .klp.arch and
arch_klp_init_object_loaded() no longer have a reason to exist.  So
remove them.

As Peter said much more succinctly:

  So the reason for .klp.arch was that .klp.rela.* stuff would overwrite
  paravirt instructions. If that happens you're doing it wrong. Those
  RELAs are core kernel, not module, and thus should've happened in
  .rela.* sections at patch-module loading time.

  Reverting this removes the two apply_{paravirt,alternatives}() calls
  from the late patching path, and means we don't have to worry about
  them when removing module_disable_ro().

[ jpoimboe: Rewrote patch description.  Tweaked klp_init_object_loaded()
	    error path. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Author: Peter Zijlstra

Documentation/livepatch/module-elf-format.rst

@@ -14,8 +14,7 @@ This document outlines the Elf format requirements that livepatch modules must f
    4. Livepatch symbols
       4.1 A livepatch module's symbol table
       4.2 Livepatch symbol format
-   5. Architecture-specific sections
-   6. Symbol table and Elf section access
+   5. Symbol table and Elf section access
 
 1. Background and motivation
 ============================
@@ -298,17 +297,7 @@ Examples:
   Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
   "OS" means OS-specific.
 
-5. Architecture-specific sections
-=================================
-Architectures may override arch_klp_init_object_loaded() to perform
-additional arch-specific tasks when a target module loads, such as applying
-arch-specific sections. On x86 for example, we must apply per-object
-.altinstructions and .parainstructions sections when a target module loads.
-These sections must be prefixed with ".klp.arch.$objname." so that they can
-be easily identified when iterating through a patch module's Elf sections
-(See arch/x86/kernel/livepatch.c for a complete example).
-
-6. Symbol table and Elf section access
+5. Symbol table and Elf section access
 ======================================
 A livepatch module's symbol table is accessible through module->symtab.
 

arch/x86/kernel/Makefile

@@ -90,7 +90,6 @@ obj-$(CONFIG_X86_MPPARSE)	+= mpparse.o
 obj-y				+= apic/
 obj-$(CONFIG_X86_REBOOTFIXUPS)	+= reboot_fixups_32.o
 obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
-obj-$(CONFIG_LIVEPATCH)	+= livepatch.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= ftrace_$(BITS).o
 obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += ftrace.o
 obj-$(CONFIG_FTRACE_SYSCALLS)	+= ftrace.o

arch/x86/kernel/livepatch.c

@@ -195,9 +195,6 @@ struct klp_patch {
 
 int klp_enable_patch(struct klp_patch *);
 
-void arch_klp_init_object_loaded(struct klp_patch *patch,
-				 struct klp_object *obj);
-
 /* Called from the module loader during module coming/going states */
 int klp_module_coming(struct module *mod);
 void klp_module_going(struct module *mod);

include/linux/livepatch.h

@@ -729,12 +729,6 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func)
 			   func->old_sympos ? func->old_sympos : 1);
 }
 
-/* Arches may override this to finish any remaining arch-specific tasks */
-void __weak arch_klp_init_object_loaded(struct klp_patch *patch,
-					struct klp_object *obj)
-{
-}
-
 int klp_apply_object_relocs(struct klp_patch *patch, struct klp_object *obj)
 {
 	int i, ret;
@@ -764,28 +758,25 @@ static int klp_init_object_loaded(struct klp_patch *patch,
 	struct klp_func *func;
 	int ret;
 
-	mutex_lock(&text_mutex);
-	module_disable_ro(patch->mod);
-
 	if (klp_is_module(obj)) {
+
+		mutex_lock(&text_mutex);
+		module_disable_ro(patch->mod);
+
 		/*
 		 * Only write module-specific relocations here
 		 * (.klp.rela.{module}.*).  vmlinux-specific relocations were
 		 * written earlier during the initialization of the klp module
 		 * itself.
 		 */
 		ret = klp_apply_object_relocs(patch, obj);
-		if (ret) {
-			module_enable_ro(patch->mod, true);
-			mutex_unlock(&text_mutex);
-			return ret;
-		}
-	}
 
-	arch_klp_init_object_loaded(patch, obj);
+		module_enable_ro(patch->mod, true);
+		mutex_unlock(&text_mutex);
 
-	module_enable_ro(patch->mod, true);
-	mutex_unlock(&text_mutex);
+		if (ret)
+			return ret;
+	}
 
 	klp_for_each_func(obj, func) {
 		ret = klp_find_object_symbol(obj->name, func->old_name,