[RFC PATCH] perf tools powerpc: Fix callchain ip filtering

[Sandipan Das]

For powerpc64, if a probe is added for a function without specifying
a line number, the corresponding trap instruction is placed at offset
0 (for big endian) or 8 (for little endian) from the start address of
the function. This address is in the function prologue and the trap
instruction preceeds the instructions to set up the stack frame.

Therefore, at this point during execution, the return address for the
function is yet to be written to its caller's stack frame. So, the LR
value at index 2 of the callchain ips provided by the kernel is still
valid and must not be skipped.

This can be observed on a powerpc64le system running Fedora 27 as
shown below.

 # perf probe -x /usr/lib64/libc-2.26.so -a inet_pton
 # perf record -e probe_libc:inet_pton/max-stack=3/ ping -6 -c 1 ::1
 # perf script

Without this patch, the output is:

 ping 27909 [007] 532219.943481: probe_libc:inet_pton: (7fff99b0af28)
                   15af28 __GI___inet_pton (/usr/lib64/libc-2.26.so)
                   1105b4 getaddrinfo (/usr/lib64/libc-2.26.so)

With this patch applied, the output is:

 ping 27909 [007] 532219.943481: probe_libc:inet_pton: (7fff99b0af28)
                   15af28 __GI___inet_pton (/usr/lib64/libc-2.26.so)
                   10fa54 gaih_inet.constprop.7 (/usr/lib64/libc-2.26.so)
                   1105b4 getaddrinfo (/usr/lib64/libc-2.26.so)

Fixes: a60335ba3298 ("perf tools powerpc: Adjust callchain based on DWARF debug info")
Signed-off-by: Sandipan Das <sandipan@xxxxxxxxxxxxxxxxxx>
---
 tools/perf/arch/powerpc/util/skip-callchain-idx.c | 58 ++++++++++++++++-------
 1 file changed, 41 insertions(+), 17 deletions(-)

diff --git a/tools/perf/arch/powerpc/util/skip-callchain-idx.c b/tools/perf/arch/powerpc/util/skip-callchain-idx.c
index 0c370f81e002..1533d85191fd 100644
--- a/tools/perf/arch/powerpc/util/skip-callchain-idx.c
+++ b/tools/perf/arch/powerpc/util/skip-callchain-idx.c
@@ -212,6 +212,37 @@ static int check_return_addr(struct dso *dso, u64 map_start, Dwarf_Addr pc)
 	return rc;
 }
 
+/*
+ * Return:
+ *	0 if return address for the program counter @pc is on stack
+ *	1 if return address is in LR and no new stack frame was allocated
+ *	2 if return address is in LR and a new frame was allocated (but not
+ *		yet used)
+ *	-1 in case of errors
+ */
+static int get_return_addr(struct thread *thread, u64 ip)
+{
+	struct addr_location al;
+	struct dso *dso = NULL;
+	int rc = -1;
+
+	thread__find_addr_location(thread, PERF_RECORD_MISC_USER,
+			MAP__FUNCTION, ip, &al);
+
+	if (!al.map || !al.map->dso) {
+		pr_debug("%" PRIx64 " dso is NULL\n", ip);
+		return rc;
+	}
+
+	dso = al.map->dso;
+	rc = check_return_addr(dso, al.map->start, ip);
+
+	pr_debug("[DSO %s, sym %s, ip 0x%" PRIx64 "] rc %d\n",
+				dso->long_name, al.sym->name, ip, rc);
+
+	return rc;
+}
+
 /*
  * The callchain saved by the kernel always includes the link register (LR).
  *
@@ -237,32 +268,25 @@ static int check_return_addr(struct dso *dso, u64 map_start, Dwarf_Addr pc)
  */
 int arch_skip_callchain_idx(struct thread *thread, struct ip_callchain *chain)
 {
-	struct addr_location al;
-	struct dso *dso = NULL;
 	int rc;
-	u64 ip;
 	u64 skip_slot = -1;
 
 	if (chain->nr < 3)
 		return skip_slot;
 
-	ip = chain->ips[2];
+	rc = get_return_addr(thread, chain->ips[1]);
 
-	thread__find_addr_location(thread, PERF_RECORD_MISC_USER,
-			MAP__FUNCTION, ip, &al);
-
-	if (al.map)
-		dso = al.map->dso;
-
-	if (!dso) {
-		pr_debug("%" PRIx64 " dso is NULL\n", ip);
+	if (rc == 1)
+		/* Return address is still in LR and has not been updated
+		 * in caller's stack frame. This is because the probe was
+		 * placed at an offset from the start of the function that
+		 * comes before the prologue code to set up the stack frame.
+		 * So, an attempt to skip an entry based on chain->ips[2],
+		 * i.e. the LR value, should not be made.
+		 */
 		return skip_slot;
-	}
 
-	rc = check_return_addr(dso, al.map->start, ip);
-
-	pr_debug("[DSO %s, sym %s, ip 0x%" PRIx64 "] rc %d\n",
-				dso->long_name, al.sym->name, ip, rc);
+	rc = get_return_addr(thread, chain->ips[2]);
 
 	if (rc == 0) {
 		/*
-- 
2.14.3