diff --git a/tests/sys/fs/fusefs/bmap.cc b/tests/sys/fs/fusefs/bmap.cc
--- a/tests/sys/fs/fusefs/bmap.cc
+++ b/tests/sys/fs/fusefs/bmap.cc
@@ -210,6 +210,8 @@
 		_)
 	).WillOnce(Invoke(ReturnImmediate([=](auto in, auto& out) {
 		size_t osize = in.body.read.size;
+
+		assert(osize < sizeof(out.body.bytes));
 		out.header.len = sizeof(struct fuse_out_header) + osize;
 		bzero(out.body.bytes, osize);
 	})));
diff --git a/tests/sys/fs/fusefs/fallocate.cc b/tests/sys/fs/fusefs/fallocate.cc
--- a/tests/sys/fs/fusefs/fallocate.cc
+++ b/tests/sys/fs/fusefs/fallocate.cc
@@ -70,6 +70,7 @@
 		}, Eq(true)),
 		_)
 	).WillOnce(Invoke(ReturnImmediate([=](auto in, auto& out) {
+		assert(in.body.read.size <= sizeof(out.body.bytes));
 		out.header.len = sizeof(struct fuse_out_header) +
 			in.body.read.size;
 		memset(out.body.bytes, 'X', in.body.read.size);
@@ -79,6 +80,8 @@
 			const char *buf = (const char*)in.body.bytes +
 				sizeof(struct fuse_write_in);
 
+			assert(length <= sizeof(in.body.bytes) -
+				sizeof(struct fuse_write_in));
 			return (in.header.opcode == FUSE_WRITE &&
 				in.header.nodeid == ino &&
 				in.body.write.offset == off  &&
diff --git a/tests/sys/fs/fusefs/io.cc b/tests/sys/fs/fusefs/io.cc
--- a/tests/sys/fs/fusefs/io.cc
+++ b/tests/sys/fs/fusefs/io.cc
@@ -141,6 +141,8 @@
 		ssize_t isize = in.body.write.size;
 		off_t iofs = in.body.write.offset;
 
+		assert((size_t)isize <= sizeof(in.body.bytes) -
+			sizeof(struct fuse_write_in));
 		ASSERT_EQ(isize, pwrite(m_backing_fd, buf, isize, iofs))
 			<< strerror(errno);
 		SET_OUT_HEADER_LEN(out, write);
@@ -158,6 +160,7 @@
 		void *buf = out.body.bytes;
 		ssize_t osize;
 
+		assert((size_t)isize <= sizeof(out.body.bytes));
 		osize = pread(m_backing_fd, buf, isize, iofs);
 		ASSERT_LE(0, osize) << strerror(errno);
 		out.header.len = sizeof(struct fuse_out_header) + osize;
diff --git a/tests/sys/fs/fusefs/mockfs.hh b/tests/sys/fs/fusefs/mockfs.hh
--- a/tests/sys/fs/fusefs/mockfs.hh
+++ b/tests/sys/fs/fusefs/mockfs.hh
@@ -206,7 +206,7 @@
 	 * The protocol places no limits on the size of bytes.  Choose
 	 * a size big enough for anything we'll test.
 	 */
-	uint8_t			bytes[0x20000];
+	uint8_t			bytes[0x40000];
 	fuse_entry_out		entry;
 	fuse_entry_out_7_8	entry_7_8;
 	fuse_lk_out		getlk;
diff --git a/tests/sys/fs/fusefs/setattr.cc b/tests/sys/fs/fusefs/setattr.cc
--- a/tests/sys/fs/fusefs/setattr.cc
+++ b/tests/sys/fs/fusefs/setattr.cc
@@ -530,6 +530,7 @@
 		auto osize = std::min(
 			static_cast<uint64_t>(cur_size) - in.body.read.offset,
 			static_cast<uint64_t>(in.body.read.size));
+		assert(osize <= sizeof(out.body.bytes));
 		out.header.len = sizeof(struct fuse_out_header) + osize;
 		if (should_have_data)
 			memset(out.body.bytes, 'X', osize);
diff --git a/tests/sys/fs/fusefs/utils.cc b/tests/sys/fs/fusefs/utils.cc
--- a/tests/sys/fs/fusefs/utils.cc
+++ b/tests/sys/fs/fusefs/utils.cc
@@ -400,6 +400,7 @@
 		}, Eq(true)),
 		_)
 	).WillOnce(Invoke(ReturnImmediate([=](auto in __unused, auto& out) {
+		assert(osize <= sizeof(out.body.bytes));
 		out.header.len = sizeof(struct fuse_out_header) + osize;
 		memmove(out.body.bytes, contents, osize);
 	}))).RetiresOnSaturation();
@@ -502,6 +503,8 @@
 			bool pid_ok;
 			uint32_t wf = in.body.write.write_flags;
 
+			assert(isize <= sizeof(in.body.bytes) -
+				sizeof(struct fuse_write_in));
 			if (wf & FUSE_WRITE_CACHE)
 				pid_ok = true;
 			else
@@ -534,6 +537,9 @@
 			const char *buf = (const char*)in.body.bytes +
 				FUSE_COMPAT_WRITE_IN_SIZE;
 			bool pid_ok = (pid_t)in.header.pid == getpid();
+
+			assert(isize <= sizeof(in.body.bytes) -
+				FUSE_COMPAT_WRITE_IN_SIZE);
 			return (in.header.opcode == FUSE_WRITE &&
 				in.header.nodeid == ino &&
 				in.body.write.fh == FH &&
diff --git a/tests/sys/fs/fusefs/write.cc b/tests/sys/fs/fusefs/write.cc
--- a/tests/sys/fs/fusefs/write.cc
+++ b/tests/sys/fs/fusefs/write.cc
@@ -97,6 +97,8 @@
 			const char *buf = (const char*)in.body.bytes +
 				sizeof(struct fuse_write_in);
 
+			assert(size <= sizeof(in.body.bytes) -
+				sizeof(struct fuse_write_in));
 			return (in.header.opcode == FUSE_WRITE &&
 				in.header.nodeid == ino &&
 				in.body.write.offset == offset  &&