perf probe: Support comp_dir to find an absolute source path

Gcc generates DW_AT_comp_dir and stores relative source path if building kernel
without O= option. In that case, perf probe --line sometimes doesn't work
without --source option, because it tries to access relative source path.

This adds DW_AT_comp_dir support to perf probe for finding an absolute source
path when no --source option.

LKML-Reference: <4C36EBE7.3060802@hitachi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

3 files changed, 44 insertions, 12 deletions

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 8d08e75b2dd3..4445a1e7052f 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -201,28 +201,38 @@ static int try_to_find_kprobe_trace_events(struct perf_probe_event *pev,
  * a newly allocated path on success.
  * Return 0 if file was found and readable, -errno otherwise.
  */
-static int get_real_path(const char *raw_path, char **new_path)
+static int get_real_path(const char *raw_path, const char *comp_dir,
+                         char **new_path)
 {
-        if (!symbol_conf.source_prefix) {
-                if (access(raw_path, R_OK) == 0) {
-                        *new_path = strdup(raw_path);
-                        return 0;
-                } else
-                        return -errno;
+        const char *prefix = symbol_conf.source_prefix;
+
+        if (!prefix) {
+                if (raw_path[0] != '/' && comp_dir)
+                        /* If not an absolute path, try to use comp_dir */
+                        prefix = comp_dir;
+                else {
+                        if (access(raw_path, R_OK) == 0) {
+                                *new_path = strdup(raw_path);
+                                return 0;
+                        } else
+                                return -errno;
+                }
         }
 
-        *new_path = malloc((strlen(symbol_conf.source_prefix) +
-                            strlen(raw_path) + 2));
+        *new_path = malloc((strlen(prefix) + strlen(raw_path) + 2));
         if (!*new_path)
                 return -ENOMEM;
 
         for (;;) {
-                sprintf(*new_path, "%s/%s", symbol_conf.source_prefix,
-                        raw_path);
+                sprintf(*new_path, "%s/%s", prefix, raw_path);
 
                 if (access(*new_path, R_OK) == 0)
                         return 0;
 
+                if (!symbol_conf.source_prefix)
+                        /* In case of searching comp_dir, don't retry */
+                        return -errno;
+
                 switch (errno) {
                 case ENAMETOOLONG:
                 case ENOENT:
@@ -318,7 +328,7 @@ int show_line_range(struct line_range *lr)
 
         /* Convert source file path */
         tmp = lr->path;
-        ret = get_real_path(tmp, &lr->path);
+        ret = get_real_path(tmp, lr->comp_dir, &lr->path);
         free(tmp);      /* Free old path */
         if (ret < 0) {
                 pr_warning("Failed to find source file. (%d)\n", ret);
diff --git a/tools/perf/util/probe-event.h b/tools/perf/util/probe-event.h
index bc06d3e8bafa..ed362acff4b6 100644
--- a/tools/perf/util/probe-event.h
+++ b/tools/perf/util/probe-event.h
@@ -86,6 +86,7 @@ struct line_range {
         int                     end;            /* End line number */
         int                     offset;         /* Start line offset */
         char                    *path;          /* Real path name */
+        char                    *comp_dir;      /* Compile directory */
         struct list_head        line_list;      /* Visible lines */
 };
 
diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index a934a364c30f..37dcdb651a69 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -144,6 +144,15 @@ static const char *cu_find_realpath(Dwarf_Die *cu_die, const char *fname)
         return src;
 }
 
+/* Get DW_AT_comp_dir (should be NULL with older gcc) */
+static const char *cu_get_comp_dir(Dwarf_Die *cu_die)
+{
+        Dwarf_Attribute attr;
+        if (dwarf_attr(cu_die, DW_AT_comp_dir, &attr) == NULL)
+                return NULL;
+        return dwarf_formstring(&attr);
+}
+
 /* Compare diename and tname */
 static bool die_compare_name(Dwarf_Die *dw_die, const char *tname)
 {
@@ -1374,6 +1383,7 @@ int find_line_range(int fd, struct line_range *lr)
         size_t cuhl;
         Dwarf_Die *diep;
         Dwarf *dbg;
+        const char *comp_dir;
 
         dbg = dwarf_begin(fd, DWARF_C_READ);
         if (!dbg) {
@@ -1409,6 +1419,17 @@ int find_line_range(int fd, struct line_range *lr)
                 }
                 off = noff;
         }
+
+        /* Store comp_dir */
+        if (lf.found) {
+                comp_dir = cu_get_comp_dir(&lf.cu_die);
+                if (comp_dir) {
+                        lr->comp_dir = strdup(comp_dir);
+                        if (!lr->comp_dir)
+                                ret = -ENOMEM;
+                }
+        }
+
         pr_debug("path: %s\n", lr->path);
         dwarf_end(dbg);