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.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/dso.c

@@ -508,8 +508,19 @@ static int __open_dso(struct dso *dso, struct machine *machine)
 					    root_dir, name, PATH_MAX))
 		goto out;
 
-	if (!is_regular_file(name))
-		goto out;
+	if (!is_regular_file(name)) {
+		char *new_name;
+
+		if (errno != ENOENT || dso->nsinfo == NULL)
+			goto out;
+
+		new_name = filename_with_chroot(dso->nsinfo->pid, name);
+		if (!new_name)
+			goto out;
+
+		free(name);
+		name = new_name;
+	}
 
 	if (dso__needs_decompress(dso)) {
 		char newpath[KMOD_DECOMP_LEN];

tools/perf/util/dsos.c

@@ -2,12 +2,15 @@
 #include "debug.h"
 #include "dsos.h"
 #include "dso.h"
+#include "util.h"
 #include "vdso.h"
 #include "namespaces.h"
+#include <errno.h>
 #include <libgen.h>
 #include <stdlib.h>
 #include <string.h>
 #include <symbol.h> // filename__read_build_id
+#include <unistd.h>
 
 static int __dso_id__cmp(struct dso_id *a, struct dso_id *b)
 {
@@ -76,6 +79,16 @@ bool __dsos__read_build_ids(struct list_head *head, bool with_hits)
 		if (filename__read_build_id(pos->long_name, &pos->bid) > 0) {
 			have_build_id	  = true;
 			pos->has_build_id = true;
+		} else if (errno == ENOENT && pos->nsinfo) {
+			char *new_name = filename_with_chroot(pos->nsinfo->pid,
+							      pos->long_name);
+
+			if (new_name && filename__read_build_id(new_name,
+								&pos->bid) > 0) {
+				have_build_id = true;
+				pos->has_build_id = true;
+			}
+			free(new_name);
 		}
 		nsinfo__mountns_exit(&nsc);
 	}

tools/perf/util/symbol.c

@@ -1864,6 +1864,16 @@ int dso__load(struct dso *dso, struct map *map)
 			nsinfo__mountns_exit(&nsc);
 
 		is_reg = is_regular_file(name);
+		if (!is_reg && errno == ENOENT && dso->nsinfo) {
+			char *new_name = filename_with_chroot(dso->nsinfo->pid,
+							      name);
+			if (new_name) {
+				is_reg = is_regular_file(new_name);
+				strlcpy(name, new_name, PATH_MAX);
+				free(new_name);
+			}
+		}
+
 #ifdef HAVE_LIBBFD_SUPPORT
 		if (is_reg)
 			bfdrc = dso__load_bfd_symbols(dso, name);

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;
+}

tools/perf/util/util.h

@@ -77,4 +77,6 @@ struct perf_debuginfod {
 	bool		 set;
 };
 void perf_debuginfod_setup(struct perf_debuginfod *di);
+
+char *filename_with_chroot(int pid, const char *filename);
 #endif /* GIT_COMPAT_UTIL_H */