From e39ed91daaf94689b9aeb20df6733d8d24d2a45d Mon Sep 17 00:00:00 2001
From: Kevin Krakauer <krakauer@google.com>
Date: Thu, 18 Jul 2024 15:09:11 -0700
Subject: [PATCH] sentry: support NULL mount source

NULL mount sources can be valid, e.g. when mounting "proc".

PiperOrigin-RevId: 653769241
---
 pkg/sentry/syscalls/linux/sys_mount.go |  9 ++++++---
 test/syscalls/linux/mount.cc           | 22 ++++++++++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/pkg/sentry/syscalls/linux/sys_mount.go b/pkg/sentry/syscalls/linux/sys_mount.go
index faf09c710..b9776bc42 100644
--- a/pkg/sentry/syscalls/linux/sys_mount.go
+++ b/pkg/sentry/syscalls/linux/sys_mount.go
@@ -116,9 +116,12 @@ func Mount(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr,
 	}
 
 	// Only copy in source, fstype, and data if we are doing a normal mount.
-	source, err := t.CopyInString(sourceAddr, hostarch.PageSize)
-	if err != nil {
-		return 0, nil, err
+	var source string
+	if sourceAddr != 0 {
+		source, err = t.CopyInString(sourceAddr, hostarch.PageSize)
+		if err != nil {
+			return 0, nil, err
+		}
 	}
 	fsType, err := t.CopyInString(typeAddr, hostarch.PageSize)
 	if err != nil {
diff --git a/test/syscalls/linux/mount.cc b/test/syscalls/linux/mount.cc
index 172e774a9..edbabbd79 100644
--- a/test/syscalls/linux/mount.cc
+++ b/test/syscalls/linux/mount.cc
@@ -2328,6 +2328,28 @@ TEST(MountTest, DetachedMountBindFails) {
               SyscallFailsWithErrno(EINVAL));
 }
 
+TEST(MountTest, MountProc) {
+  SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
+
+  // Mount procfs with a NULL source to a temporary directory.
+  const TempPath dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
+  ASSERT_THAT(mount(NULL, dir.path().c_str(), "proc", 0, NULL),
+              SyscallSucceeds());
+  auto cleanup = Cleanup([&dir] {
+    EXPECT_THAT(umount2(dir.path().c_str(), 0), SyscallSucceeds());
+  });
+
+  // Verify that /proc/self/mountinfo describes the device as "none".
+  const std::vector<ProcMountInfoEntry> mountinfo =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcSelfMountInfoEntries());
+  for (auto const& e : mountinfo) {
+    if (e.mount_point == dir.path()) {
+      EXPECT_EQ(e.fstype, "proc");
+      EXPECT_EQ(e.mount_source, "none");
+    }
+  }
+}
+
 }  // namespace
 
 }  // namespace testing