Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Mar 01, 2019 at 11:34:52AM +0100, Peter Zijlstra wrote:

> Let me do an allmodconfig build to see how much pain is caused by that
> redundant CLAC warning.

arch/x86/lib/usercopy_64.o: warning: objtool: .altinstr_replacement+0x30: redundant UACCESS disable
drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x22: redundant UACCESS disable
drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x64: redundant UACCESS disable
drivers/xen/privcmd.o: warning: objtool: privcmd_ioctl()+0x1c0: call to {dynamic}() with UACCESS enabled

The usercopy one is difficult, that's copy_user_handle_tail(), it is
buggered though, because that lacks notrace and thus has a __fentry__
call in.

Also, afaict all exception jumps into copy_user_handle_tail() will have
AC=1, but the __{get,put}_user_nocheck() things do STAC/CLAC all over
again.

So what do we do? Annotate that we start with AC=1 and then immediately
do the clac, and then let __{get,put}_user_nocheck() do their own thing?
or make it use the unsafe stuff?

---
diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index ee42bb0cbeb3..e1ab9a50937c 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -59,7 +59,7 @@ EXPORT_SYMBOL(clear_user);
  * Since protection fault in copy_from/to_user is not a normal situation,
  * it is not necessary to optimize tail handling.
  */
-__visible unsigned long
+__visible notrace unsigned long
 copy_user_handle_tail(char *to, char *from, unsigned len)
 {
 	for (; len; --len, to++) {
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 485b259127c3..695212c5bd07 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1606,6 +1606,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 					     len)) {
 end_user:
 				user_access_end();
+end:
 				kvfree(relocs);
 				err = -EFAULT;
 				goto err;
@@ -1625,7 +1626,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 		 * relocations were valid.
 		 */
 		if (!user_access_begin(urelocs, size))
-			goto end_user;
+			goto end;
 
 		for (copied = 0; copied < nreloc; copied++)
 			unsafe_put_user(-1,
@@ -2616,7 +2617,7 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
 		 * when we did the "copy_from_user()" above.
 		 */
 		if (!user_access_begin(user_exec_list, count * sizeof(*user_exec_list)))
-			goto end_user;
+			goto end;
 
 		for (i = 0; i < args->buffer_count; i++) {
 			if (!(exec2_list[i].offset & UPDATE))
@@ -2630,6 +2631,7 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
 		}
 end_user:
 		user_access_end();
+end:;
 	}
 
 	args->flags &= ~__I915_EXEC_UNKNOWN_FLAGS;