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livepatch: Remove .klp.arch

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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>
This commit is contained in:
Peter Zijlstra
2020-04-29 10:24:45 -05:00
committed by Jiri Kosina
parent 7c8e2bdd5f
commit 1d05334d28
5 changed files with 11 additions and 88 deletions
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27
kernel/livepatch/core.c
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@@ -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;
@@ -764,10 +758,11 @@ 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
@@ -775,18 +770,14 @@ static int klp_init_object_loaded(struct klp_patch *patch,
* itself.
*/
ret = klp_apply_object_relocs(patch, obj);
if (ret) {
module_enable_ro(patch->mod, true);
mutex_unlock(&text_mutex);
module_enable_ro(patch->mod, true);
mutex_unlock(&text_mutex);
if (ret)
return ret;
}
}
arch_klp_init_object_loaded(patch, obj);
module_enable_ro(patch->mod, true);
mutex_unlock(&text_mutex);
klp_for_each_func(obj, func) {
ret = klp_find_object_symbol(obj->name, func->old_name,
func->old_sympos,