[PATCH V2] drm: handle error values properly

[Nicholas Mc Guire]

drm_legacy_ctxbitmap_next() returns idr_alloc() which can return
-ENOMEM, -EINVAL or -ENOSPC none of which are -1. since drm_context_t
is an unsigned int an intermediate variable is used to handle the
error cases, and then cast to drm_context_t after ensuring that the
value is >= 0. The explicit cast is to mark the type conversion as
intentional.

Signed-off-by: Nicholas Mc Guire <hofrat@xxxxxxxxx>
Reported-by: kbuild test robot <lkp@xxxxxxxxx>
Fixes: d530b5f1ca0b ("drm: re-enable error handling")
Fixes: 62968144e673 ("drm: convert drm context code to use Linux idr")
---

kbuild test robot reported:
<snip>
tree:   git://anongit.freedesktop.org/drm/drm-misc for-linux-next-fixes
head:   d530b5f1ca0bb66958a2b714bebe40a1248b9c15
commit: d530b5f1ca0bb66958a2b714bebe40a1248b9c15 [2/2] drm: re-enable error
+handling

smatch warnings:
drivers/gpu/drm/drm_context.c:375 drm_legacy_addctx() warn: unsigned
+'ctx->handle' is never less than zero.
<snip>

V2: The proposed fix in d530b5f1ca0b ("drm: re-enable error handling")
    actually was ineffective as the negative return value check was 
    against a unsigned int and thus always false as reported by
    kbuild test robot <lkp@xxxxxxxxx>. The below patch removes that
    warning and fixes the original problem of missed error handling.

drm_context_t is actually just used in a few placed so the type could be
changed but it is also exported via tools/include/uapi/drm/drm.h so
changing the typedef of drm_context_t could break applications and thus
this is not an option.

Patch was compile tested with: x86_64_defconfig

Patch is against 4.18-rc4 (localversion-next is next-20180717)

 drivers/gpu/drm/drm_context.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_context.c b/drivers/gpu/drm/drm_context.c
index 3c4000f..78f32a3 100644
--- a/drivers/gpu/drm/drm_context.c
+++ b/drivers/gpu/drm/drm_context.c
@@ -361,22 +361,26 @@ int drm_legacy_addctx(struct drm_device *dev, void *data,
 {
 	struct drm_ctx_list *ctx_entry;
 	struct drm_ctx *ctx = data;
+	int ret;
 
 	if (!drm_core_check_feature(dev, DRIVER_KMS_LEGACY_CONTEXT) &&
 	    !drm_core_check_feature(dev, DRIVER_LEGACY))
 		return -EINVAL;
 
 	ctx->handle = drm_legacy_ctxbitmap_next(dev);
-	if (ctx->handle == DRM_KERNEL_CONTEXT) {
+	ret = drm_legacy_ctxbitmap_next(dev);
+	if (ret == DRM_KERNEL_CONTEXT) {
 		/* Skip kernel's context and get a new one. */
-		ctx->handle = drm_legacy_ctxbitmap_next(dev);
+		ret = drm_legacy_ctxbitmap_next(dev);
 	}
-	DRM_DEBUG("%d\n", ctx->handle);
-	if (ctx->handle < 0) {
+	DRM_DEBUG("ctxbitmap is error code %d\n", ret);
+	if (ret < 0) {
 		DRM_DEBUG("Not enough free contexts.\n");
 		/* Should this return -EBUSY instead? */
 		return -ENOMEM;
 	}
+	/* valid context is >= 0 */
+	ctx->handle = (drm_context_t)ret;
 
 	ctx_entry = kmalloc(sizeof(*ctx_entry), GFP_KERNEL);
 	if (!ctx_entry) {
-- 
2.1.4