linux-kernel - Re: x86 - clang / objtool status

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Date:   Tue, 23 Jul 2019 21:43:24 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: x86 - clang / objtool status

On Thu, Jul 18, 2019 at 10:40:09PM +0200, Thomas Gleixner wrote:

>   drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x86: redundant UACCESS disable

Looking at this one, I think I agree with objtool.

PeterZ, Linus, I know y'all discussed this code a few months ago.

__copy_from_user() already does a CLAC in its error path.  So isn't the
user_access_end() redundant for the __copy_from_user() error path?

Untested fix:

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 5fae0e50aad0..41dab9ea33cd 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -1628,6 +1628,7 @@ static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
 
 static int eb_copy_relocations(const struct i915_execbuffer *eb)
 {
+	struct drm_i915_gem_relocation_entry *relocs;
 	const unsigned int count = eb->buffer_count;
 	unsigned int i;
 	int err;
@@ -1635,7 +1636,6 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 	for (i = 0; i < count; i++) {
 		const unsigned int nreloc = eb->exec[i].relocation_count;
 		struct drm_i915_gem_relocation_entry __user *urelocs;
-		struct drm_i915_gem_relocation_entry *relocs;
 		unsigned long size;
 		unsigned long copied;
 
@@ -1663,14 +1663,8 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 
 			if (__copy_from_user((char *)relocs + copied,
 					     (char __user *)urelocs + copied,
-					     len)) {
-end_user:
-				user_access_end();
-end:
-				kvfree(relocs);
-				err = -EFAULT;
-				goto err;
-			}
+					     len))
+				goto end;
 
 			copied += len;
 		} while (copied < size);
@@ -1699,10 +1693,14 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 
 	return 0;
 
+end_user:
+	user_access_end();
+end:
+	kvfree(relocs);
+	err = -EFAULT;
 err:
 	while (i--) {
-		struct drm_i915_gem_relocation_entry *relocs =
-			u64_to_ptr(typeof(*relocs), eb->exec[i].relocs_ptr);
+		relocs = u64_to_ptr(typeof(*relocs), eb->exec[i].relocs_ptr);
 		if (eb->exec[i].relocation_count)
 			kvfree(relocs);
 	}