[PATCH v2 2/2] perf uprobe: Skip prologue if program compiled without optimization

[Ravi Bangoria]

Function prologue prepares stack and registers before executing function
logic. When target program is compiled without optimization, function
parameter information is only valid after prologue. When we probe entrypc
of the function, and try to record function parameter, it contains
garbage value.

For example,
  $ vim test.c
    #include <stdio.h>

    void foo(int i)
    {
       printf("i: %d\n", i);
    }

    int main()
    {
      foo(42);
      return 0;
    }

  $ gcc -g test.c -o test
  $ objdump -dl test | less
    foo():
    /home/ravi/test.c:4
      400536:       55                      push   %rbp
      400537:       48 89 e5                mov    %rsp,%rbp
      40053a:       48 83 ec 10             sub    -bashx10,%rsp
      40053e:       89 7d fc                mov    %edi,-0x4(%rbp)
    /home/ravi/test.c:5
      400541:       8b 45 fc                mov    -0x4(%rbp),%eax
    ...
    ...
    main():
    /home/ravi/test.c:9
      400558:       55                      push   %rbp
      400559:       48 89 e5                mov    %rsp,%rbp
    /home/ravi/test.c:10
      40055c:       bf 2a 00 00 00          mov    -bashx2a,%edi
      400561:       e8 d0 ff ff ff          callq  400536 <foo>
    /home/ravi/test.c:11

  $ ./perf probe -x ./test 'foo i'
  $ cat /sys/kernel/debug/tracing/uprobe_events
     p:probe_test/foo /home/ravi/test:0x0000000000000536 i=-12(%sp):s32

  $ ./perf record -e probe_test:foo ./test
  $ ./perf script
     test  5778 [001]  4918.562027: probe_test:foo: (400536) i=0

Here variable 'i' is passed via stack which is pushed on stack at
0x40053e. But we are probing at 0x400536.

To resolve this issues, we need to probe on next instruction after
prologue. gdb and systemtap also does same thing. I've implemented
this patch based on approach systemtap has used.

After applying patch:

  $ ./perf probe -x ./test 'foo i'
  $ cat /sys/kernel/debug/tracing/uprobe_events
    p:probe_test/foo /home/ravi/test:0x0000000000000541 i=-4(%bp):s32

  $ ./perf record -e probe_test:foo ./test
  $ ./perf script
    test  6300 [001]  5877.879327: probe_test:foo: (400541) i=42

No need to skip prologue for optimized case since debug info is correct
for each instructions for -O2 -g. For more details please visit:
        https://bugzilla.redhat.com/show_bug.cgi?id=612253#c6

Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
---
Changes in v2:
  - Skipping prologue only when any ARG is either C variable, $params
    or $vars.
  - Probe on line(:1) may not be always possible. Recommend only address
    to force probe on function entry.

 tools/perf/util/probe-finder.c | 164 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 164 insertions(+)

diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index f2d9ff0..b2bc77c 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -892,6 +892,169 @@ static int find_probe_point_lazy(Dwarf_Die *sp_die, struct probe_finder *pf)
 	return die_walk_lines(sp_die, probe_point_lazy_walker, pf);
 }
 
+static bool var_has_loclist(Dwarf_Die *die)
+{
+	Dwarf_Attribute loc;
+	int tag = dwarf_tag(die);
+
+	if (tag != DW_TAG_formal_parameter &&
+	    tag != DW_TAG_variable)
+		return false;
+
+	return (dwarf_attr_integrate(die, DW_AT_location, &loc) &&
+		dwarf_whatform(&loc) == DW_FORM_sec_offset);
+}
+
+/*
+ * For any object in given CU whose DW_AT_location is a location list,
+ * target program is compiled with optimization.
+ */
+static bool optimized_target(Dwarf_Die *die)
+{
+	Dwarf_Die tmp_die;
+
+	if (var_has_loclist(die))
+		return true;
+
+	if (!dwarf_child(die, &tmp_die) && optimized_target(&tmp_die))
+		return true;
+
+	if (!dwarf_siblingof(die, &tmp_die) && optimized_target(&tmp_die))
+		return true;
+
+	return false;
+}
+
+static bool get_entrypc_idx(Dwarf_Lines *lines, unsigned long nr_lines,
+			    Dwarf_Addr pf_addr, unsigned long *entrypc_idx)
+{
+	unsigned long i;
+	Dwarf_Addr addr;
+
+	for (i = 0; i < nr_lines; i++) {
+		if (dwarf_lineaddr(dwarf_onesrcline(lines, i), &addr))
+			return false;
+
+		if (addr == pf_addr) {
+			*entrypc_idx = i;
+			return true;
+		}
+	}
+	return false;
+}
+
+static bool get_postprologue_addr(unsigned long entrypc_idx,
+				  Dwarf_Lines *lines,
+				  unsigned long nr_lines,
+				  Dwarf_Addr highpc,
+				  Dwarf_Addr *postprologue_addr)
+{
+	unsigned long i;
+	int entrypc_lno, lno;
+	Dwarf_Line *line;
+	Dwarf_Addr addr;
+	bool p_end;
+
+	/* entrypc_lno is actual source line number */
+	line = dwarf_onesrcline(lines, entrypc_idx);
+	if (dwarf_lineno(line, &entrypc_lno))
+		return false;
+
+	for (i = entrypc_idx; i < nr_lines; i++) {
+		line = dwarf_onesrcline(lines, i);
+
+		if (dwarf_lineaddr(line, &addr) ||
+		    dwarf_lineno(line, &lno)    ||
+		    dwarf_lineprologueend(line, &p_end))
+			return false;
+
+		/* highpc is exclusive. [entrypc,highpc) */
+		if (addr >= highpc)
+			break;
+
+		/* clang supports prologue-end marker */
+		if (p_end)
+			break;
+
+		/* Actual next line in source */
+		if (lno != entrypc_lno)
+			break;
+
+		/*
+		 * Single source line can have multiple line records.
+		 * For Example,
+		 *     void foo() { printf("hello\n"); }
+		 * contains two line records. One points to declaration and
+		 * other points to printf() line. Variable 'lno' won't get
+		 * incremented in this case but 'i' will.
+		 */
+		if (i != entrypc_idx)
+			break;
+	}
+
+	dwarf_lineaddr(line, postprologue_addr);
+	if (*postprologue_addr >= highpc)
+		dwarf_lineaddr(dwarf_onesrcline(lines, i - 1),
+			       postprologue_addr);
+
+	return true;
+}
+
+static void __skip_prologue(Dwarf_Die *sp_die, struct probe_finder *pf)
+{
+	unsigned long nr_lines = 0, entrypc_idx = 0;
+	Dwarf_Lines *lines = NULL;
+	Dwarf_Addr postprologue_addr;
+	Dwarf_Addr highpc;
+
+	if (dwarf_highpc(sp_die, &highpc))
+		return;
+
+	if (dwarf_getsrclines(&pf->cu_die, &lines, &nr_lines))
+		return;
+
+	if (!get_entrypc_idx(lines, nr_lines, pf->addr, &entrypc_idx))
+		return;
+
+	if (!get_postprologue_addr(entrypc_idx, lines, nr_lines,
+				   highpc, &postprologue_addr))
+		return;
+
+	pf->addr = postprologue_addr;
+}
+
+static void skip_prologue(Dwarf_Die *sp_die, struct probe_finder *pf)
+{
+	struct perf_probe_point *pp = &pf->pev->point;
+
+	/* Not uprobe? */
+	if (!pf->pev->uprobes)
+		return;
+
+	/* Compiled with optimization? */
+	if (optimized_target(&pf->cu_die))
+		return;
+
+	/* Don't know entrypc? */
+	if (!pf->addr)
+		return;
+
+	/* Only FUNC and FUNC@SRC are eligible. */
+	if (!pp->function || pp->line || pp->retprobe || pp->lazy_line ||
+	    pp->offset || pp->abs_address)
+		return;
+
+	/* Not interested in func parameter? */
+	if (!perf_probe_with_var(pf->pev))
+		return;
+
+	pr_info("Target program is compiled without optimization. Skipping prologue.\n"
+		"Probe on address 0x%lx to force probing at the function entry.\n\n",
+		pf->addr);
+
+	__skip_prologue(sp_die, pf);
+}
+
 static int probe_point_inline_cb(Dwarf_Die *in_die, void *data)
 {
 	struct probe_finder *pf = data;
@@ -954,6 +1117,7 @@ static int probe_point_search_cb(Dwarf_Die *sp_die, void *data)
 		if (pp->lazy_line)
 			param->retval = find_probe_point_lazy(sp_die, pf);
 		else {
+			skip_prologue(sp_die, pf);
 			pf->addr += pp->offset;
 			/* TODO: Check the address in this function */
 			param->retval = call_probe_finder(sp_die, pf);
-- 
2.5.5