[DTrace-devel] [PATCH] Fix size of string data in the trace output buffer

[Kris Van Hees]

The size of string data items in the trace output buffer was not taking
into account that we use a 2-byte length prefix and it also did not
include the terminating NUL byte.

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_cg.c                         | 27 +++++++++++++++--------
 test/unittest/codegen/tst.str_data_size.d | 21 ++++++++++++++++++
 test/unittest/codegen/tst.str_data_size.r |  4 ++++
 3 files changed, 43 insertions(+), 9 deletions(-)
 create mode 100644 test/unittest/codegen/tst.str_data_size.d
 create mode 100644 test/unittest/codegen/tst.str_data_size.r

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index a8756581..acb7c6fa 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -939,39 +939,48 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 		int		reg = dt_regset_alloc(drp);
 
 		off = dt_rec_add(pcb->pcb_hdl, dt_cg_fill_gap, kind,
-				 size, 1, pfp, arg);
+				 size + DT_STRLEN_BYTES + 1, 1, pfp, arg);
 
 		/*
 		 * Retrieve the length of the string, limit it to the maximum
-		 * string size, and adjust for the terminating NUL byte and the
-		 * length prefix.
+		 * string size, and store it in the buffer at [%r9 + off].
 		 */
 		dt_cg_strlen(dlp, drp, reg, dnp->dn_reg);
 		emit(dlp,  BPF_BRANCH_IMM(BPF_JLT, reg, size, size_ok));
 		emit(dlp,  BPF_MOV_IMM(reg, size));
 		emitl(dlp, size_ok,
-			   BPF_ALU64_IMM(BPF_ADD, reg, 1 + DT_STRLEN_BYTES));
+			   BPF_MOV_REG(BPF_REG_0, reg));
+		emit(dlp,  BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, 8));
+		emit(dlp,  BPF_STORE(BPF_B, BPF_REG_9, off, BPF_REG_0));
+		emit(dlp,  BPF_STORE(BPF_B, BPF_REG_9, off + 1, reg));
 
 		/*
-		 * Copy string data to the output buffer at [%r9 + off].  The
-		 * amount of bytes copied is the lesser of the data size and
-		 * the maximum string size.
+		 * Copy the string to the output buffer at
+		 * [%r9 + off + DT_STRLEN_BYTES] because we need to skip the
+		 * length prefix.
 		 */
 		if (dt_regset_xalloc_args(drp) == -1)
 			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
 
 		emit(dlp,  BPF_MOV_REG(BPF_REG_1, BPF_REG_9));
-		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off));
+		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off + DT_STRLEN_BYTES));
 		emit(dlp,  BPF_MOV_REG(BPF_REG_2, reg));
 		dt_regset_free(drp, reg);
 		emit(dlp,  BPF_MOV_REG(BPF_REG_3, dnp->dn_reg));
 		dt_regset_free(drp, dnp->dn_reg);
+		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, DT_STRLEN_BYTES));
 		if (dnp->dn_tstring)
 			dt_cg_tstring_free(pcb, dnp);
 		dt_regset_xalloc(drp, BPF_REG_0);
 		emit(dlp,  BPF_CALL_HELPER(BPF_FUNC_probe_read));
 		dt_regset_free_args(drp);
-		dt_regset_free(drp, BPF_REG_0);
+
+		/*
+		 * Write the NUL terminating byte in case we are truncating.
+		 */
+		emit(dlp,  BPF_MOV_REG(BPF_REG_0, BPF_REG_9));
+		emit(dlp,  BPF_ALU64_REG(BPF_ADD, BPF_REG_0, reg));
+		emit(dlp,  BPF_STORE_IMM(BPF_B, BPF_REG_0, off + DT_STRLEN_BYTES, 0));
 
 		return 0;
 	}
diff --git a/test/unittest/codegen/tst.str_data_size.d b/test/unittest/codegen/tst.str_data_size.d
new file mode 100644
index 00000000..a928e829
--- /dev/null
+++ b/test/unittest/codegen/tst.str_data_size.d
@@ -0,0 +1,21 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+#pragma D option rawbytes
+#pragma D option strsize=5
+#pragma D option quiet
+
+BEGIN
+{
+	trace(probeprov);
+	exit(0);
+}
+
+ERROR
+{
+	exit(1);
+}
diff --git a/test/unittest/codegen/tst.str_data_size.r b/test/unittest/codegen/tst.str_data_size.r
new file mode 100644
index 00000000..60483d74
--- /dev/null
+++ b/test/unittest/codegen/tst.str_data_size.r
@@ -0,0 +1,4 @@
+
+             0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f  0123456789abcdef
+         0: 00 05 64 74 72 61 63 00                          ..dtrac.
+
-- 
2.33.0