1 files changed, 514 insertions, 0 deletions
diff --git a/test/syscalls/linux/mremap.cc b/test/syscalls/linux/mremap.cc
new file mode 100644
index 000000000..ededab336
--- /dev/null
+++ b/test/syscalls/linux/mremap.cc
@@ -0,0 +1,514 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <errno.h>
+#include <string.h>
+#include <sys/mman.h>
+
+#include <string>
+
+#include "gmock/gmock.h"
+#include "absl/strings/string_view.h"
+#include "test/util/file_descriptor.h"
+#include "test/util/logging.h"
+#include "test/util/memory_util.h"
+#include "test/util/multiprocess_util.h"
+#include "test/util/posix_error.h"
+#include "test/util/temp_path.h"
+#include "test/util/test_util.h"
+
+using ::testing::_;
+
+namespace gvisor {
+namespace testing {
+
+namespace {
+
+// Wrapper for mremap that returns a PosixErrorOr<>, since the return type of
+// void* isn't directly compatible with SyscallSucceeds.
+PosixErrorOr<void*> Mremap(void* old_address, size_t old_size, size_t new_size,
+                           int flags, void* new_address) {
+  void* rv = mremap(old_address, old_size, new_size, flags, new_address);
+  if (rv == MAP_FAILED) {
+    return PosixError(errno, "mremap failed");
+  }
+  return rv;
+}
+
+// Returns true if the page containing addr is mapped.
+bool IsMapped(uintptr_t addr) {
+  int const rv = msync(reinterpret_cast<void*>(addr & ~(kPageSize - 1)),
+                       kPageSize, MS_ASYNC);
+  if (rv == 0) {
+    return true;
+  }
+  TEST_PCHECK_MSG(errno == ENOMEM, "msync failed with unexpected errno");
+  return false;
+}
+
+// Fixture for mremap tests parameterized by mmap flags.
+using MremapParamTest = ::testing::TestWithParam<int>;
+
+TEST_P(MremapParamTest, Noop) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+
+  ASSERT_THAT(Mremap(m.ptr(), kPageSize, kPageSize, 0, nullptr),
+              IsPosixErrorOkAndHolds(m.ptr()));
+  EXPECT_TRUE(IsMapped(m.addr()));
+}
+
+TEST_P(MremapParamTest, InPlace_ShrinkingWholeVMA) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // N.B. we must be in a single-threaded subprocess to ensure a
+    // background thread doesn't concurrently map the second page.
+    void* addr = mremap(m.ptr(), 2 * kPageSize, kPageSize, 0, nullptr);
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == m.ptr());
+    MaybeSave();
+
+    TEST_CHECK(IsMapped(m.addr()));
+    TEST_CHECK(!IsMapped(m.addr() + kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, InPlace_ShrinkingPartialVMA) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(3 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    void* addr = mremap(m.ptr(), 2 * kPageSize, kPageSize, 0, nullptr);
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == m.ptr());
+    MaybeSave();
+
+    TEST_CHECK(IsMapped(m.addr()));
+    TEST_CHECK(!IsMapped(m.addr() + kPageSize));
+    TEST_CHECK(IsMapped(m.addr() + 2 * kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, InPlace_ShrinkingAcrossVMAs) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(3 * kPageSize, PROT_READ, GetParam()));
+  // Changing permissions on the first page forces it to become a separate vma.
+  ASSERT_THAT(mprotect(m.ptr(), kPageSize, PROT_NONE), SyscallSucceeds());
+
+  const auto rest = [&] {
+    // Both old_size and new_size now span two vmas; mremap
+    // shouldn't care.
+    void* addr = mremap(m.ptr(), 3 * kPageSize, 2 * kPageSize, 0, nullptr);
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == m.ptr());
+    MaybeSave();
+
+    TEST_CHECK(IsMapped(m.addr()));
+    TEST_CHECK(IsMapped(m.addr() + kPageSize));
+    TEST_CHECK(!IsMapped(m.addr() + 2 * kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, InPlace_ExpansionSuccess) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap the second page so that the first can be expanded back into it.
+    //
+    // N.B. we must be in a single-threaded subprocess to ensure a
+    // background thread doesn't concurrently map this page.
+    TEST_PCHECK(
+        munmap(reinterpret_cast<void*>(m.addr() + kPageSize), kPageSize) == 0);
+    MaybeSave();
+
+    void* addr = mremap(m.ptr(), kPageSize, 2 * kPageSize, 0, nullptr);
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == m.ptr());
+    MaybeSave();
+
+    TEST_CHECK(IsMapped(m.addr()));
+    TEST_CHECK(IsMapped(m.addr() + kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, InPlace_ExpansionFailure) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(3 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap the second page, leaving a one-page hole. Trying to expand the
+    // first page to three pages should fail since the original third page
+    // is still mapped.
+    TEST_PCHECK(
+        munmap(reinterpret_cast<void*>(m.addr() + kPageSize), kPageSize) == 0);
+    MaybeSave();
+
+    void* addr = mremap(m.ptr(), kPageSize, 3 * kPageSize, 0, nullptr);
+    TEST_CHECK_MSG(addr == MAP_FAILED, "mremap unexpectedly succeeded");
+    TEST_PCHECK_MSG(errno == ENOMEM, "mremap failed with wrong errno");
+    MaybeSave();
+
+    TEST_CHECK(IsMapped(m.addr()));
+    TEST_CHECK(!IsMapped(m.addr() + kPageSize));
+    TEST_CHECK(IsMapped(m.addr() + 2 * kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, MayMove_Expansion) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(3 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap the second page, leaving a one-page hole. Trying to expand the
+    // first page to three pages with MREMAP_MAYMOVE should force the
+    // mapping to be relocated since the original third page is still
+    // mapped.
+    TEST_PCHECK(
+        munmap(reinterpret_cast<void*>(m.addr() + kPageSize), kPageSize) == 0);
+    MaybeSave();
+
+    void* addr2 =
+        mremap(m.ptr(), kPageSize, 3 * kPageSize, MREMAP_MAYMOVE, nullptr);
+    TEST_PCHECK_MSG(addr2 != MAP_FAILED, "mremap failed");
+    MaybeSave();
+
+    const Mapping m2 = Mapping(addr2, 3 * kPageSize);
+    TEST_CHECK(m.addr() != m2.addr());
+
+    TEST_CHECK(!IsMapped(m.addr()));
+    TEST_CHECK(!IsMapped(m.addr() + kPageSize));
+    TEST_CHECK(IsMapped(m.addr() + 2 * kPageSize));
+    TEST_CHECK(IsMapped(m2.addr()));
+    TEST_CHECK(IsMapped(m2.addr() + kPageSize));
+    TEST_CHECK(IsMapped(m2.addr() + 2 * kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, Fixed_SourceAndDestinationCannotOverlap) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+
+  ASSERT_THAT(Mremap(m.ptr(), kPageSize, kPageSize,
+                     MREMAP_MAYMOVE | MREMAP_FIXED, m.ptr()),
+              PosixErrorIs(EINVAL, _));
+  EXPECT_TRUE(IsMapped(m.addr()));
+}
+
+TEST_P(MremapParamTest, Fixed_SameSize) {
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap dst to create a hole.
+    TEST_PCHECK(munmap(dst.ptr(), kPageSize) == 0);
+    MaybeSave();
+
+    void* addr = mremap(src.ptr(), kPageSize, kPageSize,
+                        MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr());
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == dst.ptr());
+    MaybeSave();
+
+    TEST_CHECK(!IsMapped(src.addr()));
+    TEST_CHECK(IsMapped(dst.addr()));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, Fixed_SameSize_Unmapping) {
+  // Like the Fixed_SameSize case, but expect mremap to unmap the destination
+  // automatically.
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    void* addr = mremap(src.ptr(), kPageSize, kPageSize,
+                        MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr());
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == dst.ptr());
+    MaybeSave();
+
+    TEST_CHECK(!IsMapped(src.addr()));
+    TEST_CHECK(IsMapped(dst.addr()));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, Fixed_ShrinkingWholeVMA) {
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap dst so we can check that mremap does not keep the
+    // second page.
+    TEST_PCHECK(munmap(dst.ptr(), 2 * kPageSize) == 0);
+    MaybeSave();
+
+    void* addr = mremap(src.ptr(), 2 * kPageSize, kPageSize,
+                        MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr());
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == dst.ptr());
+    MaybeSave();
+
+    TEST_CHECK(!IsMapped(src.addr()));
+    TEST_CHECK(!IsMapped(src.addr() + kPageSize));
+    TEST_CHECK(IsMapped(dst.addr()));
+    TEST_CHECK(!IsMapped(dst.addr() + kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, Fixed_ShrinkingPartialVMA) {
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(3 * kPageSize, PROT_NONE, GetParam()));
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap dst so we can check that mremap does not keep the
+    // second page.
+    TEST_PCHECK(munmap(dst.ptr(), 2 * kPageSize) == 0);
+    MaybeSave();
+
+    void* addr = mremap(src.ptr(), 2 * kPageSize, kPageSize,
+                        MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr());
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == dst.ptr());
+    MaybeSave();
+
+    TEST_CHECK(!IsMapped(src.addr()));
+    TEST_CHECK(!IsMapped(src.addr() + kPageSize));
+    TEST_CHECK(IsMapped(src.addr() + 2 * kPageSize));
+    TEST_CHECK(IsMapped(dst.addr()));
+    TEST_CHECK(!IsMapped(dst.addr() + kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, Fixed_ShrinkingAcrossVMAs) {
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(3 * kPageSize, PROT_READ, GetParam()));
+  // Changing permissions on the first page forces it to become a separate vma.
+  ASSERT_THAT(mprotect(src.ptr(), kPageSize, PROT_NONE), SyscallSucceeds());
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unlike flags=0, MREMAP_FIXED requires that [old_address,
+    // old_address+new_size) only spans a single vma.
+    void* addr = mremap(src.ptr(), 3 * kPageSize, 2 * kPageSize,
+                        MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr());
+    TEST_CHECK_MSG(addr == MAP_FAILED, "mremap unexpectedly succeeded");
+    TEST_PCHECK_MSG(errno == EFAULT, "mremap failed with wrong errno");
+    MaybeSave();
+
+    TEST_CHECK(IsMapped(src.addr()));
+    TEST_CHECK(IsMapped(src.addr() + kPageSize));
+    // Despite failing, mremap should have unmapped [old_address+new_size,
+    // old_address+old_size) (i.e. the third page).
+    TEST_CHECK(!IsMapped(src.addr() + 2 * kPageSize));
+    // Despite failing, mremap should have unmapped the destination pages.
+    TEST_CHECK(!IsMapped(dst.addr()));
+    TEST_CHECK(!IsMapped(dst.addr() + kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST_P(MremapParamTest, Fixed_Expansion) {
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, GetParam()));
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(2 * kPageSize, PROT_NONE, GetParam()));
+
+  const auto rest = [&] {
+    // Unmap dst so we can check that mremap actually maps all pages
+    // at the destination.
+    TEST_PCHECK(munmap(dst.ptr(), 2 * kPageSize) == 0);
+    MaybeSave();
+
+    void* addr = mremap(src.ptr(), kPageSize, 2 * kPageSize,
+                        MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr());
+    TEST_PCHECK_MSG(addr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(addr == dst.ptr());
+    MaybeSave();
+
+    TEST_CHECK(!IsMapped(src.addr()));
+    TEST_CHECK(IsMapped(dst.addr()));
+    TEST_CHECK(IsMapped(dst.addr() + kPageSize));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+INSTANTIATE_TEST_CASE_P(PrivateShared, MremapParamTest,
+                        ::testing::Values(MAP_PRIVATE, MAP_SHARED));
+
+// mremap with old_size == 0 only works with MAP_SHARED after Linux 4.14
+// (dba58d3b8c50 "mm/mremap: fail map duplication attempts for private
+// mappings").
+
+TEST(MremapTest, InPlace_Copy) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, MAP_SHARED));
+  EXPECT_THAT(Mremap(m.ptr(), 0, kPageSize, 0, nullptr),
+              PosixErrorIs(ENOMEM, _));
+}
+
+TEST(MremapTest, MayMove_Copy) {
+  Mapping const m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, MAP_SHARED));
+
+  // Remainder of this test executes in a subprocess to ensure that if mremap
+  // incorrectly removes m, it is not remapped by another thread.
+  const auto rest = [&] {
+    void* ptr = mremap(m.ptr(), 0, kPageSize, MREMAP_MAYMOVE, nullptr);
+    MaybeSave();
+    TEST_PCHECK_MSG(ptr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(ptr != m.ptr());
+    TEST_CHECK(IsMapped(m.addr()));
+    TEST_CHECK(IsMapped(reinterpret_cast<uintptr_t>(ptr)));
+  };
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+TEST(MremapTest, MustMove_Copy) {
+  Mapping const src =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, MAP_SHARED));
+  Mapping const dst =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, MAP_PRIVATE));
+
+  // Remainder of this test executes in a subprocess to ensure that if mremap
+  // incorrectly removes src, it is not remapped by another thread.
+  const auto rest = [&] {
+    void* ptr = mremap(src.ptr(), 0, kPageSize, MREMAP_MAYMOVE | MREMAP_FIXED,
+                       dst.ptr());
+    MaybeSave();
+    TEST_PCHECK_MSG(ptr != MAP_FAILED, "mremap failed");
+    TEST_CHECK(ptr == dst.ptr());
+    TEST_CHECK(IsMapped(src.addr()));
+    TEST_CHECK(IsMapped(dst.addr()));
+  };
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+}
+
+void ExpectAllBytesAre(absl::string_view v, char c) {
+  for (size_t i = 0; i < v.size(); i++) {
+    ASSERT_EQ(v[i], c) << "at offset " << i;
+  }
+}
+
+TEST(MremapTest, ExpansionPreservesCOWPagesAndExposesNewFilePages) {
+  // Create a file with 3 pages. The first is filled with 'a', the second is
+  // filled with 'b', and the third is filled with 'c'.
+  TempPath const file = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFile());
+  const FileDescriptor fd =
+      ASSERT_NO_ERRNO_AND_VALUE(Open(file.path(), O_RDWR));
+  ASSERT_THAT(WriteFd(fd.get(), std::string(kPageSize, 'a').c_str(), kPageSize),
+              SyscallSucceedsWithValue(kPageSize));
+  ASSERT_THAT(WriteFd(fd.get(), std::string(kPageSize, 'b').c_str(), kPageSize),
+              SyscallSucceedsWithValue(kPageSize));
+  ASSERT_THAT(WriteFd(fd.get(), std::string(kPageSize, 'c').c_str(), kPageSize),
+              SyscallSucceedsWithValue(kPageSize));
+
+  // Create a private mapping of the first 2 pages, and fill the second page
+  // with 'd'.
+  Mapping const src = ASSERT_NO_ERRNO_AND_VALUE(Mmap(nullptr, 2 * kPageSize,
+                                                     PROT_READ | PROT_WRITE,
+                                                     MAP_PRIVATE, fd.get(), 0));
+  memset(reinterpret_cast<void*>(src.addr() + kPageSize), 'd', kPageSize);
+  MaybeSave();
+
+  // Move the mapping while expanding it to 3 pages. The resulting mapping
+  // should contain the original first page of the file (filled with 'a'),
+  // followed by the private copy of the second page (filled with 'd'), followed
+  // by the newly-mapped third page of the file (filled with 'c').
+  Mapping const dst = ASSERT_NO_ERRNO_AND_VALUE(
+      MmapAnon(3 * kPageSize, PROT_NONE, MAP_PRIVATE));
+  ASSERT_THAT(Mremap(src.ptr(), 2 * kPageSize, 3 * kPageSize,
+                     MREMAP_MAYMOVE | MREMAP_FIXED, dst.ptr()),
+              IsPosixErrorOkAndHolds(dst.ptr()));
+  auto const v = dst.view();
+  ExpectAllBytesAre(v.substr(0, kPageSize), 'a');
+  ExpectAllBytesAre(v.substr(kPageSize, kPageSize), 'd');
+  ExpectAllBytesAre(v.substr(2 * kPageSize, kPageSize), 'c');
+}
+
+TEST(MremapDeathTest, SharedAnon) {
+  SetupGvisorDeathTest();
+
+  // Reserve 4 pages of address space.
+  Mapping const reserved = ASSERT_NO_ERRNO_AND_VALUE(
+      MmapAnon(4 * kPageSize, PROT_NONE, MAP_PRIVATE));
+
+  // Create a 2-page shared anonymous mapping at the beginning of the
+  // reservation. Fill the first page with 'a' and the second with 'b'.
+  Mapping const m = ASSERT_NO_ERRNO_AND_VALUE(
+      Mmap(reserved.ptr(), 2 * kPageSize, PROT_READ | PROT_WRITE,
+           MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED, -1, 0));
+  memset(m.ptr(), 'a', kPageSize);
+  memset(reinterpret_cast<void*>(m.addr() + kPageSize), 'b', kPageSize);
+  MaybeSave();
+
+  // Shrink the mapping to 1 page in-place.
+  ASSERT_THAT(Mremap(m.ptr(), 2 * kPageSize, kPageSize, 0, m.ptr()),
+              IsPosixErrorOkAndHolds(m.ptr()));
+
+  // Expand the mapping to 3 pages, moving it forward by 1 page in the process
+  // since the old and new mappings can't overlap.
+  void* const new_m = reinterpret_cast<void*>(m.addr() + kPageSize);
+  ASSERT_THAT(Mremap(m.ptr(), kPageSize, 3 * kPageSize,
+                     MREMAP_MAYMOVE | MREMAP_FIXED, new_m),
+              IsPosixErrorOkAndHolds(new_m));
+
+  // The first 2 pages of the mapping should still contain the data we wrote
+  // (i.e. shrinking should not have discarded the second page's data), while
+  // touching the third page should raise SIGBUS.
+  auto const v =
+      absl::string_view(static_cast<char const*>(new_m), 3 * kPageSize);
+  ExpectAllBytesAre(v.substr(0, kPageSize), 'a');
+  ExpectAllBytesAre(v.substr(kPageSize, kPageSize), 'b');
+  EXPECT_EXIT(ExpectAllBytesAre(v.substr(2 * kPageSize, kPageSize), '\0'),
+              ::testing::KilledBySignal(SIGBUS), "");
+}
+
+}  // namespace
+
+}  // namespace testing
+}  // namespace gvisor