drm/i915: Avoid undefined behaviour of "u32 >> 32"

When computing a hash for looking up relocation target handles in an
execbuf, we start with a large size for the hashtable and proceed to
halve it until the allocation succeeds. The final attempt is with an
order of 0 (i.e. a single element). This means that we then pass bits=0
to hash_32() which then computes "hash >> (32 - 0)" to lookup the single
element. Right shifting a value by the width of the operand is
undefined, so limit the smallest hash table we use to order 1.

v2: Keep the retry allocation flag for the final pass

Fixes: 4ff4b44c ("drm/i915: Store a direct lookup from object handle to vma")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170629150425.27508-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -288,20 +288,26 @@ static int eb_create(struct i915_execbuffer *eb)
 		 * direct lookup.
 		 */
 		do {
+			unsigned int flags;
+
+			/* While we can still reduce the allocation size, don't
+			 * raise a warning and allow the allocation to fail.
+			 * On the last pass though, we want to try as hard
+			 * as possible to perform the allocation and warn
+			 * if it fails.
+			 */
+			flags = GFP_TEMPORARY;
+			if (size > 1)
+				flags |= __GFP_NORETRY | __GFP_NOWARN;
+
 			eb->buckets = kzalloc(sizeof(struct hlist_head) << size,
-					      GFP_TEMPORARY |
-					      __GFP_NORETRY |
-					      __GFP_NOWARN);
+					      flags);
 			if (eb->buckets)
 				break;
 		} while (--size);
 
-		if (unlikely(!eb->buckets)) {
-			eb->buckets = kzalloc(sizeof(struct hlist_head),
-					      GFP_TEMPORARY);
-			if (unlikely(!eb->buckets))
-				return -ENOMEM;
-		}
+		if (unlikely(!size))
+			return -ENOMEM;
 
 		eb->lut_size = size;
 	} else {
@@ -452,7 +458,7 @@ eb_add_vma(struct i915_execbuffer *eb,
 			return err;
 	}
 
-	if (eb->lut_size >= 0) {
+	if (eb->lut_size > 0) {
 		vma->exec_handle = entry->handle;
 		hlist_add_head(&vma->exec_node,
 			       &eb->buckets[hash_32(entry->handle,
@@ -895,7 +901,7 @@ static void eb_release_vmas(const struct i915_execbuffer *eb)
 static void eb_reset_vmas(const struct i915_execbuffer *eb)
 {
 	eb_release_vmas(eb);
-	if (eb->lut_size >= 0)
+	if (eb->lut_size > 0)
 		memset(eb->buckets, 0,
 		       sizeof(struct hlist_head) << eb->lut_size);
 }
@@ -904,7 +910,7 @@ static void eb_destroy(const struct i915_execbuffer *eb)
 {
 	GEM_BUG_ON(eb->reloc_cache.rq);
 
-	if (eb->lut_size >= 0)
+	if (eb->lut_size > 0)
 		kfree(eb->buckets);
 }
 
@@ -2180,8 +2186,11 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 		}
 	}
 
-	if (eb_create(&eb))
-		return -ENOMEM;
+	err = eb_create(&eb);
+	if (err)
+		goto err_out_fence;
+
+	GEM_BUG_ON(!eb.lut_size);
 
 	err = eb_select_context(&eb);
 	if (unlikely(err))
@@ -2341,6 +2350,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	i915_gem_context_put(eb.ctx);
 err_destroy:
 	eb_destroy(&eb);
+err_out_fence:
 	if (out_fence_fd != -1)
 		put_unused_fd(out_fence_fd);
 err_in_fence: