[for-linus][PATCH 2/3] tracing: Use memset_startat() to zero struct trace_iterator

[Steven Rostedt]

From: Kees Cook <keescook@xxxxxxxxxxxx>

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() to avoid confusing memset() about writing beyond
the target struct member.

Link: https://lkml.kernel.org/r/20211118202217.1285588-1-keescook@xxxxxxxxxxxx

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f9139dc1262c..e3c80cfd4eec 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6706,9 +6706,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 		cnt = PAGE_SIZE - 1;
 
 	/* reset all but tr, trace, and overruns */
-	memset(&iter->seq, 0,
-	       sizeof(struct trace_iterator) -
-	       offsetof(struct trace_iterator, seq));
+	memset_startat(iter, 0, seq);
 	cpumask_clear(iter->started);
 	trace_seq_init(&iter->seq);
 	iter->pos = -1;
-- 
2.33.0