From cc441778d1b77ee0b206751180046fb385f9c9fc Mon Sep 17 00:00:00 2001
From: Tomas Glozar <tglozar@gmail.com>
Date: Mon, 3 Oct 2022 10:38:50 +0200
Subject: [PATCH] Use absolute module file path when possible

Follow symlink in /proc/.../root and prepend it to module filename instead
of prepending /proc/.../root. This allows the file to be found when doing
symbol resolution even when the process has already exited by that time.

The symlink value is stored in ProcStat::root_ and is checked for changes
in ProcStat::is_stale, its change triggering reloading modules to get the
correct path.

If the symlink reading fails, ProcStat::root_ is set to /proc/.../root.
---
 src/cc/bcc_syms.cc | 37 +++++++++++++++++++++++++++++--------
 src/cc/syms.h      |  4 ++++
 2 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/src/cc/bcc_syms.cc b/src/cc/bcc_syms.cc
index fdacfe77d9e0..5968ad8a1b45 100644
--- a/src/cc/bcc_syms.cc
+++ b/src/cc/bcc_syms.cc
@@ -14,25 +14,27 @@
  * limitations under the License.
  */
 
+#include "bcc_syms.h"
+
 #include <cxxabi.h>
-#include <cstring>
 #include <fcntl.h>
+#include <limits.h>
 #include <linux/elf.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <unistd.h>
+
 #include <cstdio>
+#include <cstring>
 
 #include "bcc_elf.h"
 #include "bcc_perf_map.h"
 #include "bcc_proc.h"
-#include "bcc_syms.h"
 #include "common.h"
-#include "vendor/tinyformat.hpp"
-
 #include "syms.h"
+#include "vendor/tinyformat.hpp"
 
 bool ProcStat::getinode_(ino_t &inode) {
   struct stat s;
@@ -44,13 +46,31 @@ bool ProcStat::getinode_(ino_t &inode) {
   }
 }
 
+bool ProcStat::refresh_root() {
+  char ns_root[PATH_MAX];
+  std::string original_root = root_;
+  int readlink_read = readlink(root_symlink_.c_str(), ns_root, PATH_MAX);
+  if (readlink_read >= 0 && readlink_read < PATH_MAX) {
+    // readlink succeded, save link target to root_
+    ns_root[readlink_read] = 0;
+    root_ = std::string(ns_root);
+    return original_root != root_;
+  }
+  // readlink failed, keep old value of root_
+  return false;
+}
+
 bool ProcStat::is_stale() {
   ino_t cur_inode;
-  return getinode_(cur_inode) && (cur_inode != inode_);
+  return getinode_(cur_inode) && (cur_inode != inode_) && refresh_root();
 }
 
-ProcStat::ProcStat(int pid) : procfs_(tfm::format("/proc/%d/exe", pid)) {
+ProcStat::ProcStat(int pid)
+    : procfs_(tfm::format("/proc/%d/exe", pid)),
+      root_symlink_(tfm::format("/proc/%d/root", pid)),
+      root_(root_symlink_) {
   getinode_(inode_);
+  refresh_root();
 }
 
 void KSyms::_add_symbol(const char *symname, const char *modname, uint64_t addr, void *p) {
@@ -134,8 +154,9 @@ void ProcSyms::refresh() {
 
 int ProcSyms::_add_module(mod_info *mod, int enter_ns, void *payload) {
   ProcSyms *ps = static_cast<ProcSyms *>(payload);
-  std::string ns_relative_path = tfm::format("/proc/%d/root%s", ps->pid_, mod->name);
-  const char *modpath = enter_ns && ps->pid_ != -1 ? ns_relative_path.c_str() : mod->name;
+  std::string ns_modpath = ps->procstat_.get_root() + mod->name;
+  const char *modpath =
+      enter_ns && ps->pid_ != -1 ? ns_modpath.c_str() : mod->name;
   auto it = std::find_if(
       ps->modules_.begin(), ps->modules_.end(),
       [=](const ProcSyms::Module &m) { return m.name_ == mod->name; });
diff --git a/src/cc/syms.h b/src/cc/syms.h
index 86e84e221cdf..f61537dd8482 100644
--- a/src/cc/syms.h
+++ b/src/cc/syms.h
@@ -29,11 +29,15 @@
 
 class ProcStat {
   std::string procfs_;
+  std::string root_symlink_;
+  std::string root_;
   ino_t inode_;
   bool getinode_(ino_t &inode);
 
  public:
   ProcStat(int pid);
+  bool refresh_root();
+  const std::string &get_root() { return root_; }
   bool is_stale();
   void reset() { getinode_(inode_); }
 };