[PATCH v2] tracing: silence GCC 9 array bounds warning

[Miguel Ojeda]

Starting with GCC 9, -Warray-bounds detects cases when memset is called
starting on a member of a struct but the size to be cleared ends up
writing over further members.

Such a call happens in the trace code to clear, at once, all members
after and including `seq` on struct trace_iterator:

    In function 'memset',
        inlined from 'ftrace_dump' at kernel/trace/trace.c:8914:3:
    ./include/linux/string.h:344:9: warning: '__builtin_memset' offset
    [8505, 8560] from the object at 'iter' is out of the bounds of
    referenced subobject 'seq' with type 'struct trace_seq' at offset
    4368 [-Warray-bounds]
      344 |  return __builtin_memset(p, c, size);
          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

In order to avoid GCC complaining about it, we compute the address
ourselves by adding the offsetof distance instead of referring
directly to the member.

Since there are two places doing this clear (trace.c and trace_kdb.c),
take the chance to move the workaround into a single place in
the internal header.

Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
---
Steven, let me know if you still prefer (char *) or anything else
(or just change it in your side if you want :-)

 kernel/trace/trace.c     |  6 +-----
 kernel/trace/trace.h     | 14 ++++++++++++++
 kernel/trace/trace_kdb.c |  6 +-----
 3 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2c92b3d9ea30..1c80521fd436 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8910,12 +8910,8 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 
 		cnt++;
 
-		/* reset all but tr, trace, and overruns */
-		memset(&iter.seq, 0,
-		       sizeof(struct trace_iterator) -
-		       offsetof(struct trace_iterator, seq));
+		trace_iterator_reset(&iter);
 		iter.iter_flags |= TRACE_FILE_LAT_FMT;
-		iter.pos = -1;
 
 		if (trace_find_next_entry_inc(&iter) != NULL) {
 			int ret;
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 1974ce818ddb..f9a12003f137 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1967,4 +1967,18 @@ static inline void tracer_hardirqs_off(unsigned long a0, unsigned long a1) { }
 
 extern struct trace_iterator *tracepoint_print_iter;
 
+/* reset all but tr, trace, and overruns */
+static __always_inline void trace_iterator_reset(struct trace_iterator *iter)
+{
+	/*
+	 * We do not simplify the start address to &iter->seq in order to let
+	 * GCC 9 know that we really want to overwrite more members than
+	 * just iter->seq (-Warray-bounds).
+	 */
+	const size_t offset = offsetof(struct trace_iterator, seq);
+	memset((char *)(iter) + offset, 0, sizeof(struct trace_iterator) - offset);
+
+	iter->pos = -1;
+}
+
 #endif /* _LINUX_KERNEL_TRACE_H */
diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 6c1ae6b752d1..cca65044c14c 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -37,12 +37,8 @@ static void ftrace_dump_buf(int skip_entries, long cpu_file)
 	if (skip_entries)
 		kdb_printf("(skipping %d entries)\n", skip_entries);
 
-	/* reset all but tr, trace, and overruns */
-	memset(&iter.seq, 0,
-		   sizeof(struct trace_iterator) -
-		   offsetof(struct trace_iterator, seq));
+	trace_iterator_reset(&iter);
 	iter.iter_flags |= TRACE_FILE_LAT_FMT;
-	iter.pos = -1;
 
 	if (cpu_file == RING_BUFFER_ALL_CPUS) {
 		for_each_tracing_cpu(cpu) {
-- 
2.17.1