[Intel-gfx] [PATCH] drm/i915: Avoid undefined behaviour of "u32 >> 32"

[Chris Wilson]

When computing a hash for looking up relcoation target handles in an
execbuf, we start with a large size for the hashtable and proceed to
reduce it until the allocation suceeds. 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 by a value the width of the operand is
undefined, so limit the smallest hash table we use to order 1.

Fixes: 4ff4b44cbb70 ("drm/i915: Store a direct lookup from object handle to vma")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 718bb75ad387..54791bcb8ccb 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -296,12 +296,8 @@ static int eb_create(struct i915_execbuffer *eb)
 				break;
 		} while (--size);
 
-		if (unlikely(!eb->buckets)) {
-			eb->buckets = kzalloc(sizeof(struct hlist_head),
-					      GFP_TEMPORARY);
-			if (unlikely(!eb->buckets))
-				return -ENOMEM;
-		}
+		if (unlikely(!eb->buckets))
+			return -ENOMEM;
 
 		eb->lut_size = size;
 	} else {
@@ -453,7 +449,7 @@ 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,
@@ -897,7 +893,7 @@ 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);
 }
@@ -906,7 +902,7 @@ 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);
 }
 
@@ -2185,8 +2181,11 @@ 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))
@@ -2346,6 +2345,7 @@ 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:
-- 
2.13.1