Re: [PATCH V2] drm: handle error values properly

[Sean Paul]

On Tue, Jul 17, 2018 at 03:28:21PM +0200, Nicholas Mc Guire wrote:
> 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);

Remove this call?

Sean

> -	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
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS