[Intel-gfx] [PATCH] drm/i915/gem: Comment on inability to check args.pad for MMAP_OFFSET

[Chris Wilson]

Since we didn't check and insist that args.pad must be zero for MMAP_GTT
historically, we cannot insert a check now as old userspace may be
feeding in garbage. As such the lack of check is enshrined into the ABI,
so add a comment to remind us we cannot add the check later.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_mman.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index 3a3f30bc8ac7..15faf95d74ca 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
@@ -620,6 +620,15 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
 	enum i915_mmap_type type;
 	int err;
 
+	/*
+	 * Historically we failed to check args.pad and args.offset
+	 * and so we cannot those fields for user input and we cannot
+	 * add -EINVAL for them as the ABI is fixed, i.e. old userspace
+	 * may be feeding in garbage in those fields.
+	 *
+	 * if (args->pad) return -EINVAL; is verbotten!
+	 */
+
 	err = i915_user_extensions(u64_to_user_ptr(args->extensions),
 				   NULL, 0, NULL);
 	if (err)
-- 
2.24.0