linux-hardening - Re: [PATCH][next] drm/i915/gem: Fix fall-through warning for Clang

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Date:   Mon, 07 Jun 2021 13:57:06 -0700
From:   Joe Perches <joe@...ches.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH][next] drm/i915/gem: Fix fall-through warning for Clang

On Mon, 2021-06-07 at 15:32 -0500, Gustavo A. R. Silva wrote:
> In preparation to enable -Wimplicit-fallthrough for Clang, fix a
> warning by explicitly adding a fallthrough; statement.
[]
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
[]
> @@ -62,6 +62,7 @@ static void try_to_writeback(struct drm_i915_gem_object *obj,
>  	switch (obj->mm.madv) {
>  	case I915_MADV_DONTNEED:
>  		i915_gem_object_truncate(obj);
> +		fallthrough;
>  	case __I915_MADV_PURGED:
>  		return;
>  	}

I think fallthrough to return is not particularly nice to follow.

This is the current function:

static void try_to_writeback(struct drm_i915_gem_object *obj,
			     unsigned int flags)
{
	switch (obj->mm.madv) {
	case I915_MADV_DONTNEED:
		i915_gem_object_truncate(obj);
	case __I915_MADV_PURGED:
		return;
	}

	if (flags & I915_SHRINK_WRITEBACK)
		i915_gem_object_writeback(obj);
}

One of these might be more typical:

static void try_to_writeback(struct drm_i915_gem_object *obj,
			     unsigned int flags)
{
	switch (obj->mm.madv) {
	case I915_MADV_DONTNEED:
		i915_gem_object_truncate(obj);
		break;
	case __I915_MADV_PURGED:
		break;
	default:
		if (flags & I915_SHRINK_WRITEBACK)
			i915_gem_object_writeback(obj);
		break;
	}
}

or maybe:

static void try_to_writeback(struct drm_i915_gem_object *obj,
			     unsigned int flags)
{
	switch (obj->mm.madv) {
	case I915_MADV_DONTNEED:
		i915_gem_object_truncate(obj);
		return;
	case __I915_MADV_PURGED:
		return;
	}

	if (flags & I915_SHRINK_WRITEBACK)
		i915_gem_object_writeback(obj);
}