[DTrace-devel] [PATCH v4 3/5] cg: add argument mapping in the trampoline

[Nick Alcock]

Before now, argument mapping was restricted to the args[] array, and was
implemented in dt_cg_array_op in the same place where we decide whether
to automatically copyin() userspace strings.

But existing DTrace testcases suggest that arg mapping is also applied to
argN for USDT, even though this is inconsistent with SDT and arguably less
flexible than having argN be the unmapped arguments in all cases.  Add a new
function dt_cg_tramp_map_args(), which can be called from trampolines to
apply mappings (destructively, but it keeps a copy of the old args in the
save area so you can unapply them if you need to).  No existing providers do
any mapping, so this is not yet called, but it's about to be.

Signed-off-by: Nick Alcock <nick.alcock at oracle.com>
---
 libdtrace/dt_cg.c | 36 +++++++++++++++++++++++++++++++++++-
 libdtrace/dt_cg.h |  1 +
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 39c27ab0ec0b..b90757bcb5cb 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -831,6 +831,35 @@ dt_cg_tramp_restore_args(dt_pcb_t *pcb)
 	}
 }
 
+/*
+ * Move arguments according to the mappings in the array of arguments given.
+ * The original arguments are overwritten if necessary (but saved).  In effect,
+ * this changes the native arguments: the caller should adjust the mappings
+ * array accordingly, and shuffle the native arg types to match.
+ *
+ * The caller must ensure that %r7 contains the value set by the
+ * dt_cg_tramp_prologue*() functions.
+ */
+void
+dt_cg_tramp_map_args(dt_pcb_t *pcb, dt_argdesc_t *args, size_t nargs)
+{
+	dt_irlist_t	*dlp = &pcb->pcb_ir;
+	int		i;
+
+	/*
+	 * Work from saved args so we don't need to worry about overwriting regs
+	 * we're about to map into different positions.
+	 */
+	dt_cg_tramp_save_args(pcb);
+
+	for (i = 0; i < nargs; i++) {
+		if (args[i].mapping != i) {
+			emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_7, DMST_ORIG_ARG(args[i].mapping)));
+			emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(i), BPF_REG_0));
+		}
+	}
+}
+
 typedef struct {
 	dt_irlist_t	*dlp;
 	dt_activity_t	act;
@@ -5060,7 +5089,12 @@ dt_cg_array_op(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
 	 * array index according to the static argument mapping (if any),
 	 * unless the argument reference is provided by a dynamic translator.
 	 * If we're using a dynamic translator for args[], then just set dn_reg
-	 * to an invalid reg and return: DIF_OP_XLARG will fetch the arg later.
+	 *
+	 * If dt_cg_tramp_map_args is in use, you should apply the mapping at
+	 * the probe_info stage, since the effective "native" arg positions will
+	 * be changed.
+	 *
+	 * If this is a userland variable, note that we need to copy it in.
 	 */
 	if (idp->di_id == DIF_VAR_ARGS) {
 		if ((idp->di_kind == DT_IDENT_XLPTR ||
diff --git a/libdtrace/dt_cg.h b/libdtrace/dt_cg.h
index 0a7c7ba6a8c5..fb26c125adbc 100644
--- a/libdtrace/dt_cg.h
+++ b/libdtrace/dt_cg.h
@@ -34,6 +34,7 @@ extern void dt_cg_tramp_get_var(dt_pcb_t *pcb, const char *name, int isstore,
 extern void dt_cg_tramp_del_var(dt_pcb_t *pcb, const char *name);
 extern void dt_cg_tramp_save_args(dt_pcb_t *pcb);
 extern void dt_cg_tramp_restore_args(dt_pcb_t *pcb);
+extern void dt_cg_tramp_map_args(dt_pcb_t *pcb, dt_argdesc_t *args, size_t nargs);
 extern void dt_cg_tramp_call_clauses(dt_pcb_t *pcb, const dt_probe_t *prp,
 				     dt_activity_t act);
 extern void dt_cg_tramp_return(dt_pcb_t *pcb);
-- 
2.46.0.278.g36e3a12567