intel_th: msu: Use memset_startat() for clearing hw header

Bugzilla: https://bugzilla.redhat.com/2159468

commit 9ed0a59
Author: Kees Cook <keescook@chromium.org>
Date:   Mon May 24 21:36:48 2021 -0700

    intel_th: msu: Use memset_startat() for clearing hw header

    In preparation for FORTIFY_SOURCE performing compile-time and run-time
    field bounds checking for memset(), avoid intentionally writing across
    neighboring fields.

    Use memset_startat() so memset() doesn't get confused about writing
    beyond the destination member that is intended to be the starting point
    of zeroing through the end of the struct.

    Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Link: https://lore.kernel.org/lkml/87sfyzi97l.fsf@ashishki-desk.ger.corp.intel.com
    Signed-off-by: Kees Cook <keescook@chromium.org>

Signed-off-by: Eric Chanudet <echanude@redhat.com>

Author: eric-ch

drivers/hwtracing/intel_th/msu.c

@@ -658,13 +658,11 @@ static void msc_buffer_clear_hw_header(struct msc *msc)
 
 	list_for_each_entry(win, &msc->win_list, entry) {
 		unsigned int blk;
-		size_t hw_sz = sizeof(struct msc_block_desc) -
-			offsetof(struct msc_block_desc, hw_tag);
 
 		for_each_sg(win->sgt->sgl, sg, win->nr_segs, blk) {
 			struct msc_block_desc *bdesc = sg_virt(sg);
 
-			memset(&bdesc->hw_tag, 0, hw_sz);
+			memset_startat(bdesc, 0, hw_tag);
 		}
 	}
 }