From 2c7778ecca7e896f692c7bd0b59fca1bf574a963 Mon Sep 17 00:00:00 2001
From: Jing Chen <chjing@google.com>
Date: Fri, 25 Aug 2023 15:14:44 -0700
Subject: [PATCH] Support MS_STRICTATIME flag for the syscall mount.

PiperOrigin-RevId: 560207305
---
 pkg/sentry/syscalls/linux/sys_mount.go |  5 ++---
 test/syscalls/linux/mount.cc           | 29 ++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/pkg/sentry/syscalls/linux/sys_mount.go b/pkg/sentry/syscalls/linux/sys_mount.go
index c3212c071..f5576641f 100644
--- a/pkg/sentry/syscalls/linux/sys_mount.go
+++ b/pkg/sentry/syscalls/linux/sys_mount.go
@@ -46,8 +46,7 @@ func Mount(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr,
 
 	// Silently allow MS_NOSUID, since we don't implement set-id bits anyway.
 	const unsupported = linux.MS_REMOUNT | linux.MS_SLAVE |
-		linux.MS_UNBINDABLE | linux.MS_MOVE | linux.MS_REC | linux.MS_NODIRATIME |
-		linux.MS_STRICTATIME
+		linux.MS_UNBINDABLE | linux.MS_MOVE | linux.MS_REC | linux.MS_NODIRATIME
 
 	// Linux just allows passing any flags to mount(2) - it won't fail when
 	// unknown or unsupported flags are passed. Since we don't implement
@@ -109,7 +108,7 @@ func Mount(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr,
 		}
 	}
 	var opts vfs.MountOptions
-	if flags&linux.MS_NOATIME == linux.MS_NOATIME {
+	if flags&(linux.MS_NOATIME|linux.MS_STRICTATIME) == linux.MS_NOATIME {
 		opts.Flags.NoATime = true
 	}
 	if flags&linux.MS_NOEXEC == linux.MS_NOEXEC {
diff --git a/test/syscalls/linux/mount.cc b/test/syscalls/linux/mount.cc
index d989ef675..c95a5f521 100644
--- a/test/syscalls/linux/mount.cc
+++ b/test/syscalls/linux/mount.cc
@@ -45,6 +45,7 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
+#include "absl/time/clock.h"
 #include "absl/time/time.h"
 #include "test/util/capability_util.h"
 #include "test/util/file_descriptor.h"
@@ -389,6 +390,34 @@ TEST(MountTest, MountNoAtime) {
   EXPECT_EQ(before, after);
 }
 
+TEST(MountTest, MountWithStrictAtime) {
+  SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
+
+  auto const dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
+  auto const mount = ASSERT_NO_ERRNO_AND_VALUE(Mount(
+      "", dir.path(), "tmpfs", MS_NOATIME | MS_STRICTATIME, "mode=0777", 0));
+
+  std::string const contents = "No no no, don't follow the instructions!";
+  auto const file = ASSERT_NO_ERRNO_AND_VALUE(
+      TempPath::CreateFileWith(dir.path(), contents, 0777));
+
+  absl::Time const before = ASSERT_NO_ERRNO_AND_VALUE(ATime(file.path()));
+
+  absl::SleepFor(absl::Milliseconds(100));
+
+  // MS_STRICTATIME should override MS_NOATIME and update the file's atime.
+  auto const fd = ASSERT_NO_ERRNO_AND_VALUE(Open(file.path(), O_RDWR));
+  char buf[100];
+  int read_n;
+  ASSERT_THAT(read_n = read(fd.get(), buf, sizeof(buf)), SyscallSucceeds());
+  EXPECT_EQ(std::string(buf, read_n), contents);
+
+  absl::Time const after = ASSERT_NO_ERRNO_AND_VALUE(ATime(file.path()));
+
+  // The after atime is expected to be larger than the before atime.
+  EXPECT_LT(before, after);
+}
+
 TEST(MountTest, MountNoExec) {
   SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));