[Intel-gfx] [PATCH] drm/i915: Use unsigned for overflow checks in execbuf

[Daniel Vetter]

There's actually no real risk since we already check for stricter
constraints earlier (using UINT_MAX / sizeof (struct
drm_i915_gem_exec_object2) as the limit). But in eb_create we use
signed integers, which steals a factor of 2. Luckily struct
drm_i915_gem_exec_object2 for this to not matter.

Still, be consistent and use unsigned integers.

Similar use unsinged integers when checking for overflows in the
relocation entry processing.

I've also added a new subtests to igt/gem_reloc_overflow to also
test for overflowing args->buffer_count values.

v2: Give the variables again tighter scope to make it clear that the
computation is purely local and doesn't leak out to the 2nd block.
Requested by Chris Wilson.

v3: Add a comment why we don't need to recheck for overflows.

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index ccfb8e6..f71eb6c 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -47,16 +47,22 @@ eb_create(struct drm_i915_gem_execbuffer2 *args, struct i915_address_space *vm)
 {
 	struct eb_vmas *eb = NULL;
 
+	/*
+	 * We already check for potential overflows of args->buffer_count before
+	 * calling i915_gem_do_execbuffer. So here we just need to make sure
+	 * that we don't overflow the by using a different type than unsigned
+	 * integers.
+	 */
 	if (args->flags & I915_EXEC_HANDLE_LUT) {
-		int size = args->buffer_count;
+		unsigned size = args->buffer_count;
 		size *= sizeof(struct i915_vma *);
 		size += sizeof(struct eb_vmas);
 		eb = kmalloc(size, GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
 	}
 
 	if (eb == NULL) {
-		int size = args->buffer_count;
-		int count = PAGE_SIZE / sizeof(struct hlist_head) / 2;
+		unsigned size = args->buffer_count;
+		unsigned count = PAGE_SIZE / sizeof(struct hlist_head) / 2;
 		BUILD_BUG_ON_NOT_POWER_OF_2(PAGE_SIZE / sizeof(struct hlist_head));
 		while (count > 2*size)
 			count >>= 1;
@@ -667,7 +673,7 @@ i915_gem_execbuffer_relocate_slow(struct drm_device *dev,
 	bool need_relocs;
 	int *reloc_offset;
 	int i, total, ret;
-	int count = args->buffer_count;
+	unsigned count = args->buffer_count;
 
 	if (WARN_ON(list_empty(&eb->vmas)))
 		return 0;
@@ -818,8 +824,8 @@ validate_exec_list(struct drm_i915_gem_exec_object2 *exec,
 		   int count)
 {
 	int i;
-	int relocs_total = 0;
-	int relocs_max = INT_MAX / sizeof(struct drm_i915_gem_relocation_entry);
+	unsigned relocs_total = 0;
+	unsigned relocs_max = UINT_MAX / sizeof(struct drm_i915_gem_relocation_entry);
 
 	for (i = 0; i < count; i++) {
 		char __user *ptr = to_user_ptr(exec[i].relocs_ptr);
-- 
1.8.4.rc3