libbrillo: ScopedMountNamespace: Add CreateFromPath.

It's useful to be able to create ScopedMountNamespace objects from
a path.

BUG=chromium:985492
TEST=Tested with https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1721070

Change-Id: I51cf338f7ce881e33d082cc57afec32fa46a9e36
Reviewed-on: https://chromium-review.googlesource.com/1784126
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Ready: Jorge Lucangeli Obes <jorgelo@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Mirrored-From: https://chromium.googlesource.com/chromiumos/platform2
Cr-Mirrored-Commit: b3cba12dde210c1fc3533c3e87426772d8a5c1e2

2 files changed, 28 insertions, 8 deletions

diff --git a/brillo/scoped_mount_namespace.cc b/brillo/scoped_mount_namespace.cc
index d136161..0f35e82 100644
--- a/brillo/scoped_mount_namespace.cc
+++ b/brillo/scoped_mount_namespace.cc
@@ -9,11 +9,16 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
+#include <string>
 #include <utility>
 
 #include <base/posix/eintr_wrapper.h>
 #include <base/strings/stringprintf.h>
 
+namespace {
+constexpr char kCurrentMountNamespacePath[] = "/proc/self/ns/mnt";
+}  // anonymous namespace
+
 namespace brillo {
 
 ScopedMountNamespace::ScopedMountNamespace(base::ScopedFD mount_namespace_fd)
@@ -25,26 +30,35 @@ ScopedMountNamespace::~ScopedMountNamespace() {
 }
 
 // static
-std::unique_ptr<ScopedMountNamespace>
-ScopedMountNamespace::CreateForPid(pid_t pid) {
-  constexpr char kCurrentMountNamespacePath[] = "/proc/self/ns/mnt";
+std::unique_ptr<ScopedMountNamespace> ScopedMountNamespace::CreateForPid(
+    pid_t pid) {
+  std::string ns_path = base::StringPrintf("/proc/%d/ns/mnt", pid);
+  return CreateFromPath(base::FilePath(ns_path));
+}
+
+// static
+std::unique_ptr<ScopedMountNamespace> ScopedMountNamespace::CreateFromPath(
+    base::FilePath ns_path) {
   base::ScopedFD original_mount_namespace_fd(
       HANDLE_EINTR(open(kCurrentMountNamespacePath, O_RDONLY)));
   if (!original_mount_namespace_fd.is_valid()) {
-    PLOG(ERROR) << "Failed to get the original mount namespace FD";
+    PLOG(ERROR) << "Failed to open original mount namespace FD at "
+                << kCurrentMountNamespacePath;
     return nullptr;
   }
-  base::ScopedFD mount_namespace_fd(HANDLE_EINTR(
-      open(base::StringPrintf("/proc/%d/ns/mnt", pid).c_str(), O_RDONLY)));
+
+  base::ScopedFD mount_namespace_fd(
+      HANDLE_EINTR(open(ns_path.value().c_str(), O_RDONLY)));
   if (!mount_namespace_fd.is_valid()) {
-    PLOG(ERROR) << "Failed to get PID " << pid << "'s mount namespace FD";
+    PLOG(ERROR) << "Failed to open mount namespace FD at " << ns_path.value();
     return nullptr;
   }
 
   if (setns(mount_namespace_fd.get(), CLONE_NEWNS) != 0) {
-    PLOG(ERROR) << "Failed to enter PID " << pid << "'s mount namespace";
+    PLOG(ERROR) << "Failed to enter mount namespace at " << ns_path.value();
     return nullptr;
   }
+
   return std::make_unique<ScopedMountNamespace>(
       std::move(original_mount_namespace_fd));
 }
diff --git a/brillo/scoped_mount_namespace.h b/brillo/scoped_mount_namespace.h
index e8c91bf..f360221 100644
--- a/brillo/scoped_mount_namespace.h
+++ b/brillo/scoped_mount_namespace.h
@@ -8,6 +8,7 @@
 #include <memory>
 
 #include <base/macros.h>
+#include <base/files/file_path.h>
 #include <base/files/scoped_file.h>
 
 #include <brillo/brillo_export.h>
@@ -24,6 +25,11 @@ class BRILLO_EXPORT ScopedMountNamespace {
   // scope.
   static std::unique_ptr<ScopedMountNamespace> CreateForPid(pid_t pid);
 
+  // Enters the mount namespace identified by |path| and returns a unique_ptr
+  // that restores the original mount namespace when it goes out of scope.
+  static std::unique_ptr<ScopedMountNamespace> CreateFromPath(
+      base::FilePath ns_path);
+
   explicit ScopedMountNamespace(base::ScopedFD mount_namespace_fd);
   ~ScopedMountNamespace();