[DTrace-devel] [PATCH v4] trace: print alloca pointers as actual pointer values

[Kris Van Hees]

Because alloca pointers are stored internally as offsets into the
scratchmem area, they were printed as small integers.  They are
now printed as actual pointer values into kernel space.

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_cg.c                          | 46 ++++++++++++++++------
 test/unittest/actions/trace/tst.alloca.d   | 24 +++++++++++
 test/unittest/actions/trace/tst.alloca.r   |  1 +
 test/unittest/actions/trace/tst.alloca.r.p | 11 ++++++
 4 files changed, 71 insertions(+), 11 deletions(-)
 create mode 100644 test/unittest/actions/trace/tst.alloca.d
 create mode 100644 test/unittest/actions/trace/tst.alloca.r
 create mode 100755 test/unittest/actions/trace/tst.alloca.r.p

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 7d542521..1f120e1d 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -1687,18 +1687,42 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 		align = vtype.dtdt_align;
 
 		/*
-		 * A DEREF of a REF node does not get resolved in dt_cg_node()
-		 * because the ref node already holds the pointer.  But for
-		 * alloca pointers, that will be the offset into scratchmem so
-		 * we still need to turn it into a real pointer here.
+		 * Alloca pointers are stored as an offset into scratchmem, so
+		 * they need to be converted into real pointers before we go on.
+		 * An alloca pointer value either has the ALLOCA flag set, or
+		 * the value node is a DEREF of an alloca pointer child.
+		 * If the alloca pointer is a REF or ref-by-value is requested,
+		 * we need to do bounds checking before turning the alloca
+		 * pointer into a real pointer.
+		 * If not, we should scalarize it so that the BPF verifier does
+		 * not complain.
 		 */
-		if (dnp->dn_kind == DT_NODE_OP1 &&
-		    dnp->dn_op == DT_TOK_DEREF && (dnp->dn_flags & DT_NF_REF) &&
-		    (dnp->dn_child->dn_flags & DT_NF_ALLOCA)) {
-			dt_cg_alloca_access_check(dlp, drp, dnp->dn_reg,
-						  DT_ISIMM, size);
-			dt_cg_alloca_ptr(dlp, drp, dnp->dn_reg, dnp->dn_reg);
-			not_null = 1;
+		if ((dnp->dn_flags & DT_NF_ALLOCA) ||
+		    (dnp->dn_kind == DT_NODE_OP1 &&
+		     dnp->dn_op == DT_TOK_DEREF &&
+		     (dnp->dn_flags & DT_NF_REF) &&
+		     (dnp->dn_child->dn_flags & DT_NF_ALLOCA))) {
+			if ((dnp->dn_flags & DT_NF_REF) || (arg & DT_NF_REF)) {
+				dt_cg_alloca_access_check(dlp, drp, dnp->dn_reg,
+							  DT_ISIMM, size);
+
+				dt_cg_alloca_ptr(dlp, drp, dnp->dn_reg, dnp->dn_reg);
+				not_null = 1;
+			} else {
+				int	reg;
+
+				dt_regset_xalloc(drp, BPF_REG_0);
+				emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_FP, DT_STK_DCTX));
+				if ((reg = dt_regset_alloc(drp)) == -1)
+					longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
+				emit(dlp, BPF_LOAD(BPF_DW, reg, BPF_REG_0, DCTX_SCRATCHMEM));
+				emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_0, DCTX_MST));
+				emit(dlp, BPF_STORE(BPF_DW, BPF_REG_0, DMST_SCALARIZER, reg));
+				emit(dlp, BPF_LOAD(BPF_DW, reg, BPF_REG_0, DMST_SCALARIZER));
+				dt_regset_free(drp, BPF_REG_0);
+				emit(dlp,  BPF_ALU64_REG(BPF_ADD, dnp->dn_reg, reg));
+				dt_regset_free(drp, reg);
+			}
 		}
 	}
 
diff --git a/test/unittest/actions/trace/tst.alloca.d b/test/unittest/actions/trace/tst.alloca.d
new file mode 100644
index 00000000..d2ff5152
--- /dev/null
+++ b/test/unittest/actions/trace/tst.alloca.d
@@ -0,0 +1,24 @@
+#pragma D option quiet
+
+BEGIN
+{
+	arr = (int *)alloca(5 * sizeof(int));
+	idx = 4;
+	arr[0] = 1;
+	arr[1] = 22;
+	arr[2] = 333;
+	arr[3] = 4444;
+	arr[4] = 55555;
+	trace(arr);
+	trace(" ");
+	trace(*arr);
+	trace(" ");
+	trace(arr + 2);
+	trace(" ");
+	trace(*(arr + 2));
+	trace(" ");
+	trace(arr + idx);
+	trace(" ");
+	trace(*(arr + idx));
+	exit(0);
+}
diff --git a/test/unittest/actions/trace/tst.alloca.r b/test/unittest/actions/trace/tst.alloca.r
new file mode 100644
index 00000000..e9bbf2f5
--- /dev/null
+++ b/test/unittest/actions/trace/tst.alloca.r
@@ -0,0 +1 @@
+OK 1 OK 333 OK 55555
diff --git a/test/unittest/actions/trace/tst.alloca.r.p b/test/unittest/actions/trace/tst.alloca.r.p
new file mode 100755
index 00000000..8515861a
--- /dev/null
+++ b/test/unittest/actions/trace/tst.alloca.r.p
@@ -0,0 +1,11 @@
+#!/usr/bin/gawk -f
+
+{
+	$1 = $1 > 0x7fffffff ? "OK" : "BAD";
+	$3 = $3 > 0x7fffffff ? "OK" : "BAD";
+	$5 = $5 > 0x7fffffff ? "OK" : "BAD";
+}
+
+{
+	print;
+}
-- 
2.43.5