linux-kernel - [PATCH] perf probe: Add support for DW_OP_call_frame

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Message-Id: <20200401161954.44640-1-daniel.shaulov@granulate.io>
Date:   Wed,  1 Apr 2020 19:19:52 +0300
From:   Daniel Shaulov <daniel.shaulov@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Daniel Shaulov <daniel.shaulov@...nulate.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] perf probe: Add support for DW_OP_call_frame_cfa vars

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@...nulate.io>
---
 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