[PATCH] perf probe: Add support for DW_OP_call_frame_cfa vars

[Daniel Shaulov]

Add support for probes on variables with DW_OP_call_frame_cfa
as the dwarf operation in the debug info.

Some compilers (specifically Golang compiler) output
DW_OP_call_frame_cfa instead of DW_OP_fbreg for variables
on the stack. If DW_OP_call_frame_cfa is the only expression
than it is the same as DW_OP_fbreg with an offset of zero.
In the case of the Golang compiler, DW_OP_call_frame_cfa may
be followed by DW_OP_consts, with a number and than DW_OP_plus.
This trio is the same as DW_OP_fbreg with the number from
DW_OP_consts as the offset.

With this change, probing on functions in Golang with variables works.

Signed-off-by: Daniel Shaulov <daniel.shaulov@xxxxxxxxxxxx>
---
 tools/perf/util/probe-finder.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index e4cff49384f4..866b17aea263 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -240,11 +240,23 @@ static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
 	}
 
 	/* If this is based on frame buffer, set the offset */
-	if (op->atom == DW_OP_fbreg) {
+	if (op->atom == DW_OP_fbreg || op->atom == DW_OP_call_frame_cfa) {
 		if (fb_ops == NULL)
 			return -ENOTSUP;
 		ref = true;
-		offs = op->number;
+		if (op->atom == DW_OP_fbreg) {
+			offs = op->number;
+		} else if (nops == 3) {
+			/*
+			 * In the case of DW_OP_call_frame_cfa, we either have
+			 * an offset of 0 or we have two more expressions that
+			 * add a const
+			 */
+			if ((op + 1)->atom != DW_OP_consts ||
+			    (op + 2)->atom != DW_OP_plus)
+				return -ENOTSUP;
+			offs = (op + 1)->number;
+		}
 		op = &fb_ops[0];
 	}
 
-- 
2.22.0