perf tools: Try chroot'ed filename when opening dso/symbol

Currently it doesn't handle tasks in chroot properly.  As filenames in
MMAP records base on their root directory, it's different than what perf
tool can see from outside.

Add filename_with_chroot() helper to deal with those cases.  The
function returns a new filename only if it's in a different root
directory.  Since it needs to access /proc for the process, it only
works until the task exits.

With this change, I can see symbols in my program like below.

  # perf record -o- chroot myroot myprog 3 | perf report -i-
  ...
  #
  # Overhead  Command  Shared Object      Symbol
  # ........  .......  .................  .............................
  #
      99.83%  myprog   myprog             [.] loop
       0.04%  chroot   [kernel.kallsyms]  [k] fxregs_fixup
       0.04%  chroot   [kernel.kallsyms]  [k] rsm_load_seg_32
  ...

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20220202070828.143303-3-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 31 insertions, 0 deletions

diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
index fb4f6616b5fa..f8571a66d063 100644
--- a/tools/perf/util/util.c
+++ b/tools/perf/util/util.c
@@ -431,3 +431,34 @@ void perf_debuginfod_setup(struct perf_debuginfod *di)
 
 	pr_debug("DEBUGINFOD_URLS=%s\n", getenv("DEBUGINFOD_URLS"));
 }
+
+/*
+ * Return a new filename prepended with task's root directory if it's in
+ * a chroot.  Callers should free the returned string.
+ */
+char *filename_with_chroot(int pid, const char *filename)
+{
+	char buf[PATH_MAX];
+	char proc_root[32];
+	char *new_name = NULL;
+	int ret;
+
+	scnprintf(proc_root, sizeof(proc_root), "/proc/%d/root", pid);
+	ret = readlink(proc_root, buf, sizeof(buf) - 1);
+	if (ret <= 0)
+		return NULL;
+
+	/* readlink(2) does not append a null byte to buf */
+	buf[ret] = '\0';
+
+	if (!strcmp(buf, "/"))
+		return NULL;
+
+	if (strstr(buf, "(deleted)"))
+		return NULL;
+
+	if (asprintf(&new_name, "%s/%s", buf, filename) < 0)
+		return NULL;
+
+	return new_name;
+}