[PATCH -tip 2/2] perf probe: Find fentry mcount fuzzed parameterlocation

[Masami Hiramatsu]

At this point, --fentry (mcount founction entry) option for gcc
fuzzes the debuginfo variable locations by skipping the mcount
instruction offset (on x86, this is a 5 byte call instruction).
This makes variable searching failure at the entry of functions
which are mcount'ed.

e.g.)
# perf probe --vars vfs_read
Available variables at vfs_read
        @<vfs_read+0>
                (No matched variables)

This patch adds additional location search at the function
entry point to solve this issue, which tries to find the earliest
address for the variable location. Note that this is only works
with function parameters (formal parameters) because any local
variables should not exist on the function entry address (those
are not initialized yet).

With this patch, perf probe shows correct parameters if possible;
# perf probe --vars vfs_read
Available variables at vfs_read
        @<vfs_read+0>
                char*   buf
                loff_t* pos
                size_t  count
                struct file*    file

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxxxxxxxxxx>
---
 tools/perf/util/probe-finder.c |   39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 5a46be9..c044052 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -273,12 +273,15 @@ static struct probe_trace_arg_ref *alloc_trace_arg_ref(long offs)
 /*
  * Convert a location into trace_arg.
  * If tvar == NULL, this just checks variable can be converted.
+ * If fentry == true and vr_die is a parameter, do huristic search
+ * for the location fuzzed by function entry mcount.
  */
 static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
-				     Dwarf_Op *fb_ops,
+				     Dwarf_Op *fb_ops, Dwarf_Die *sp_die,
 				     struct probe_trace_arg *tvar)
 {
 	Dwarf_Attribute attr;
+	Dwarf_Addr tmp = 0;
 	Dwarf_Op *op;
 	size_t nops;
 	unsigned int regn;
@@ -291,12 +294,29 @@ static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
 		goto static_var;
 
 	/* TODO: handle more than 1 exprs */
-	if (dwarf_attr(vr_die, DW_AT_location, &attr) == NULL ||
-	    dwarf_getlocation_addr(&attr, addr, &op, &nops, 1) <= 0 ||
-	    nops == 0) {
-		/* TODO: Support const_value */
+	if (dwarf_attr(vr_die, DW_AT_location, &attr) == NULL)
+		return -EINVAL;	/* Broken DIE ? */
+	if (dwarf_getlocation_addr(&attr, addr, &op, &nops, 1) <= 0) {
+		ret = dwarf_entrypc(sp_die, &tmp);
+		if (ret || addr != tmp ||
+		    dwarf_tag(vr_die) != DW_TAG_formal_parameter ||
+		    dwarf_highpc(sp_die, &tmp))
+			return -ENOENT;
+		/*
+		 * This is fuzzed by fentry mcount. We try to find the
+		 * parameter location at the earliest address.
+		 */
+		for (addr += 1; addr <= tmp; addr++) {
+			if (dwarf_getlocation_addr(&attr, addr, &op,
+						   &nops, 1) > 0)
+				goto found;
+		}
 		return -ENOENT;
 	}
+found:
+	if (nops == 0)
+		/* TODO: Support const_value */
+		return -ENOENT;
 
 	if (op->atom == DW_OP_addr) {
 static_var:
@@ -600,7 +620,7 @@ static int convert_variable(Dwarf_Die *vr_die, struct probe_finder *pf)
 		 dwarf_diename(vr_die));
 
 	ret = convert_variable_location(vr_die, pf->addr, pf->fb_ops,
-					pf->tvar);
+					&pf->sp_die, pf->tvar);
 	if (ret == -ENOENT)
 		pr_err("Failed to find the location of %s at this address.\n"
 		       " Perhaps, it has been optimized out.\n", pf->pvar->var);
@@ -1148,13 +1168,15 @@ struct local_vars_finder {
 static int copy_variables_cb(Dwarf_Die *die_mem, void *data)
 {
 	struct local_vars_finder *vf = data;
+	struct probe_finder *pf = vf->pf;
 	int tag;
 
 	tag = dwarf_tag(die_mem);
 	if (tag == DW_TAG_formal_parameter ||
 	    tag == DW_TAG_variable) {
 		if (convert_variable_location(die_mem, vf->pf->addr,
-					      vf->pf->fb_ops, NULL) == 0) {
+					      vf->pf->fb_ops, &pf->sp_die,
+					      NULL) == 0) {
 			vf->args[vf->nargs].var = (char *)dwarf_diename(die_mem);
 			if (vf->args[vf->nargs].var == NULL) {
 				vf->ret = -ENOMEM;
@@ -1302,7 +1324,8 @@ static int collect_variables_cb(Dwarf_Die *die_mem, void *data)
 	if (tag == DW_TAG_formal_parameter ||
 	    tag == DW_TAG_variable) {
 		ret = convert_variable_location(die_mem, af->pf.addr,
-						af->pf.fb_ops, NULL);
+						af->pf.fb_ops, &af->pf.sp_die,
+						NULL);
 		if (ret == 0) {
 			ret = die_get_varname(die_mem, buf, MAX_VAR_LEN);
 			pr_debug2("Add new var: %s\n", buf);


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