2 files changed, 5 insertions, 3 deletions
diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
index 9e4ea852f636..4c360daa4e24 100644
--- a/tools/perf/util/util.c
+++ b/tools/perf/util/util.c
@@ -281,6 +281,7 @@ static ssize_t ion(bool is_read, int fd, void *buf, size_t n)
 	size_t left = n;
 
 	while (left) {
+		/* buf must be treated as const if !is_read. */
 		ssize_t ret = is_read ? read(fd, buf, left) :
 					write(fd, buf, left);
 
@@ -308,9 +309,10 @@ ssize_t readn(int fd, void *buf, size_t n)
 /*
  * Write exactly 'n' bytes or return an error.
  */
-ssize_t writen(int fd, void *buf, size_t n)
+ssize_t writen(int fd, const void *buf, size_t n)
 {
-	return ion(false, fd, buf, n);
+	/* ion does not modify buf. */
+	return ion(false, fd, (void *)buf, n);
 }
 
 size_t hex_width(u64 v)
diff --git a/tools/perf/util/util.h b/tools/perf/util/util.h
index 062dd20437f7..b136c271125f 100644
--- a/tools/perf/util/util.h
+++ b/tools/perf/util/util.h
@@ -38,7 +38,7 @@ int copyfile_ns(const char *from, const char *to, struct nsinfo *nsi);
 int copyfile_offset(int fromfd, loff_t from_ofs, int tofd, loff_t to_ofs, u64 size);
 
 ssize_t readn(int fd, void *buf, size_t n);
-ssize_t writen(int fd, void *buf, size_t n);
+ssize_t writen(int fd, const void *buf, size_t n);
 
 size_t hex_width(u64 v);
 int hex2u64(const char *ptr, u64 *val);