Re: drm/i915/ttm Initialize the ttm device and memory managers

[Matthew Auld]

On Mon, 12 Jul 2021 at 16:17, Colin Ian King <colin.king@xxxxxxxxxxxxx> wrote:
>
> Hi,
>
> Static analysis with Coverity on linux-next has found a potential issue
> in drivers/gpu/drm/i915/selftests/intel_memory_region.c in function
> igt_mock_fill - the problematic commit is as follows:
>
> commit d148738923fdb5077089e48ec15555e6008100d0
> Author: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Date:   Wed Jun 2 10:38:08 2021 +0200
>
>     drm/i915/ttm Initialize the ttm device and memory managers
>
> The analysis is as follows:
>
>
>  49 static int igt_mock_fill(void *arg)
>  50 {
>  51        struct intel_memory_region *mem = arg;
>  52        resource_size_t total = resource_size(&mem->region);
>  53        resource_size_t page_size;
>  54        resource_size_t rem;
>  55        unsigned long max_pages;
>  56        unsigned long page_num;
>  57        LIST_HEAD(objects);
>  58        int err = 0;
>  59
>  60        page_size = mem->chunk_size;
>  61        rem = total;
>  62 retry:
>
> value_overwrite: Overwriting previous write to max_pages with value from
> div64_u64(rem, page_size).
>
>  63        max_pages = div64_u64(rem, page_size);
>  64
>  65        for_each_prime_number_from(page_num, 1, max_pages) {
>  66                resource_size_t size = page_num * page_size;
>  67                struct drm_i915_gem_object *obj;
>  68
>  69                obj = i915_gem_object_create_region(mem, size, 0);
>  70                if (IS_ERR(obj)) {
>  71                        err = PTR_ERR(obj);
>  72                        break;
>  73                }
>  74
>  75                err = i915_gem_object_pin_pages_unlocked(obj);
>  76                if (err) {
>  77                        i915_gem_object_put(obj);
>  78                        break;
>  79                }
>  80
>  81                list_add(&obj->st_link, &objects);
>  82                rem -= size;
>  83        }
>  84
>  85        if (err == -ENOMEM)
>  86                err = 0;
>  87        if (err == -ENXIO) {
>  88                if (page_num * page_size <= rem) {
>  89                        if (mem->is_range_manager && max_pages > 1) {
>
> Unused value (UNUSED_VALUE)
> assigned_value: Assigning value from max_pages >> 1 to max_pages here,
> but that stored value is overwritten before it can be used.

Yeah, that doesn't look good.

AFAIK this should be fixed with d53ec322dc7d ("drm/i915/ttm: switch
over to ttm_buddy_man"), which is in drm-tip, but I guess has not made
its way over to linux-next yet.

Thanks for the report.

>
>  90                                max_pages >>= 1;
>  91                                goto retry;
>  92                        }
>  93
>
> The right shift to max_pages on line 90 is being overwritten on line 64
> on the retry.
>
> Colin