Re: x86 - clang / objtool status

[Josh Poimboeuf]

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);
        }