Use a tmpfs file for shared anonymous and /dev/zero mmap on VFS2.

This is more consistent with Linux (see comment on MM.NewSharedAnonMappable()).
We don't do the same thing on VFS1 for reasons documented by the updated
comment.

PiperOrigin-RevId: 332514849

2 files changed, 19 insertions, 0 deletions

diff --git a/test/syscalls/linux/BUILD b/test/syscalls/linux/BUILD
index e5d43cf2e..451feb8f5 100644
--- a/test/syscalls/linux/BUILD
+++ b/test/syscalls/linux/BUILD
@@ -1673,6 +1673,7 @@ cc_binary(
         gtest,
         "//test/util:memory_util",
         "//test/util:posix_error",
+        "//test/util:proc_util",
         "//test/util:temp_path",
         "//test/util:test_util",
         "//test/util:thread_util",
diff --git a/test/syscalls/linux/proc.cc b/test/syscalls/linux/proc.cc
index 1ba7bf4f5..c1488b06b 100644
--- a/test/syscalls/linux/proc.cc
+++ b/test/syscalls/linux/proc.cc
@@ -63,6 +63,7 @@
 #include "test/util/fs_util.h"
 #include "test/util/memory_util.h"
 #include "test/util/posix_error.h"
+#include "test/util/proc_util.h"
 #include "test/util/temp_path.h"
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
@@ -672,6 +673,23 @@ TEST(ProcSelfMaps, Mprotect) {
                                                    3 * kPageSize, PROT_READ)));
 }
 
+TEST(ProcSelfMaps, SharedAnon) {
+  const Mapping m = ASSERT_NO_ERRNO_AND_VALUE(
+      MmapAnon(kPageSize, PROT_READ, MAP_SHARED | MAP_ANONYMOUS));
+
+  const auto proc_self_maps =
+      ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/self/maps"));
+  for (const auto& line : absl::StrSplit(proc_self_maps, '\n')) {
+    const auto entry = ASSERT_NO_ERRNO_AND_VALUE(ParseProcMapsLine(line));
+    if (entry.start <= m.addr() && m.addr() < entry.end) {
+      // cf. proc(5), "/proc/[pid]/map_files/"
+      EXPECT_EQ(entry.filename, "/dev/zero (deleted)");
+      return;
+    }
+  }
+  FAIL() << "no maps entry containing mapping at " << m.ptr();
+}
+
 TEST(ProcSelfFd, OpenFd) {
   int pipe_fds[2];
   ASSERT_THAT(pipe2(pipe_fds, O_CLOEXEC), SyscallSucceeds());