Implement madvise(MADV_DONTFORK)

PiperOrigin-RevId: 254253777

pkg/sentry/mm/lifecycle.go

@@ -86,10 +86,22 @@ func (mm *MemoryManager) Fork(ctx context.Context) (*MemoryManager, error) {
 	}
 
 	// Copy vmas.
+	dontforks := false
 	dstvgap := mm2.vmas.FirstGap()
 	for srcvseg := mm.vmas.FirstSegment(); srcvseg.Ok(); srcvseg = srcvseg.NextSegment() {
 		vma := srcvseg.Value() // makes a copy of the vma
 		vmaAR := srcvseg.Range()
+
+		if vma.dontfork {
+			length := uint64(vmaAR.Length())
+			mm2.usageAS -= length
+			if vma.isPrivateDataLocked() {
+				mm2.dataAS -= length
+			}
+			dontforks = true
+			continue
+		}
+
 		// Inform the Mappable, if any, of the new mapping.
 		if vma.mappable != nil {
 			if err := vma.mappable.AddMapping(ctx, mm2, vmaAR, vma.off, vma.canWriteMappableLocked()); err != nil {
@@ -118,6 +130,10 @@ func (mm *MemoryManager) Fork(ctx context.Context) (*MemoryManager, error) {
 	defer mm2.activeMu.Unlock()
 	mm.activeMu.Lock()
 	defer mm.activeMu.Unlock()
+	if dontforks {
+		defer mm.pmas.MergeRange(mm.applicationAddrRange())
+	}
+	srcvseg := mm.vmas.FirstSegment()
 	dstpgap := mm2.pmas.FirstGap()
 	var unmapAR usermem.AddrRange
 	for srcpseg := mm.pmas.FirstSegment(); srcpseg.Ok(); srcpseg = srcpseg.NextSegment() {
@@ -125,6 +141,27 @@ func (mm *MemoryManager) Fork(ctx context.Context) (*MemoryManager, error) {
 		if !pma.private {
 			continue
 		}
+
+		if dontforks {
+			// Find the 'vma' that contains the starting address
+			// associated with the 'pma' (there must be one).
+			srcvseg = srcvseg.seekNextLowerBound(srcpseg.Start())
+			if checkInvariants {
+				if !srcvseg.Ok() {
+					panic(fmt.Sprintf("no vma covers pma range %v", srcpseg.Range()))
+				}
+				if srcpseg.Start() < srcvseg.Start() {
+					panic(fmt.Sprintf("vma %v ran ahead of pma %v", srcvseg.Range(), srcpseg.Range()))
+				}
+			}
+
+			srcpseg = mm.pmas.Isolate(srcpseg, srcvseg.Range())
+			if srcvseg.ValuePtr().dontfork {
+				continue
+			}
+			pma = srcpseg.ValuePtr()
+		}
+
 		if !pma.needCOW {
 			pma.needCOW = true
 			if pma.effectivePerms.Write {

pkg/sentry/mm/mm.go

@@ -274,6 +274,9 @@ type vma struct {
 	// metag, none of which we currently support.
 	growsDown bool `state:"manual"`
 
+	// dontfork is the MADV_DONTFORK setting for this vma configured by madvise().
+	dontfork bool
+
 	mlockMode memmap.MLockMode
 
 	// numaPolicy is the NUMA policy for this vma set by mbind().

pkg/sentry/mm/syscalls.go

@@ -1026,6 +1026,32 @@ func (mm *MemoryManager) SetNumaPolicy(addr usermem.Addr, length uint64, policy
 	}
 }
 
+// SetDontFork implements the semantics of madvise MADV_DONTFORK.
+func (mm *MemoryManager) SetDontFork(addr usermem.Addr, length uint64, dontfork bool) error {
+	ar, ok := addr.ToRange(length)
+	if !ok {
+		return syserror.EINVAL
+	}
+
+	mm.mappingMu.Lock()
+	defer mm.mappingMu.Unlock()
+	defer func() {
+		mm.vmas.MergeRange(ar)
+		mm.vmas.MergeAdjacent(ar)
+	}()
+
+	for vseg := mm.vmas.LowerBoundSegment(ar.Start); vseg.Ok() && vseg.Start() < ar.End; vseg = vseg.NextSegment() {
+		vseg = mm.vmas.Isolate(vseg, ar)
+		vma := vseg.ValuePtr()
+		vma.dontfork = dontfork
+	}
+
+	if mm.vmas.SpanRange(ar) != ar.Length() {
+		return syserror.ENOMEM
+	}
+	return nil
+}
+
 // Decommit implements the semantics of Linux's madvise(MADV_DONTNEED).
 func (mm *MemoryManager) Decommit(addr usermem.Addr, length uint64) error {
 	ar, ok := addr.ToRange(length)

pkg/sentry/mm/vma.go

@@ -439,6 +439,7 @@ func (vmaSetFunctions) Merge(ar1 usermem.AddrRange, vma1 vma, ar2 usermem.AddrRa
 		vma1.mlockMode != vma2.mlockMode ||
 		vma1.numaPolicy != vma2.numaPolicy ||
 		vma1.numaNodemask != vma2.numaNodemask ||
+		vma1.dontfork != vma2.dontfork ||
 		vma1.id != vma2.id ||
 		vma1.hint != vma2.hint {
 		return vma{}, false

pkg/sentry/syscalls/linux/sys_mmap.go

@@ -180,6 +180,10 @@ func Madvise(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysca
 	switch adv {
 	case linux.MADV_DONTNEED:
 		return 0, nil, t.MemoryManager().Decommit(addr, length)
+	case linux.MADV_DOFORK:
+		return 0, nil, t.MemoryManager().SetDontFork(addr, length, false)
+	case linux.MADV_DONTFORK:
+		return 0, nil, t.MemoryManager().SetDontFork(addr, length, true)
 	case linux.MADV_HUGEPAGE, linux.MADV_NOHUGEPAGE:
 		fallthrough
 	case linux.MADV_MERGEABLE, linux.MADV_UNMERGEABLE:
@@ -191,7 +195,7 @@ func Madvise(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysca
 	case linux.MADV_NORMAL, linux.MADV_RANDOM, linux.MADV_SEQUENTIAL, linux.MADV_WILLNEED:
 		// Do nothing, we totally ignore the suggestions above.
 		return 0, nil, nil
-	case linux.MADV_REMOVE, linux.MADV_DOFORK, linux.MADV_DONTFORK:
+	case linux.MADV_REMOVE:
 		// These "suggestions" have application-visible side effects, so we
 		// have to indicate that we don't support them.
 		return 0, nil, syserror.ENOSYS

test/syscalls/linux/BUILD

@@ -985,6 +985,7 @@ cc_binary(
         "//test/util:file_descriptor",
         "//test/util:logging",
         "//test/util:memory_util",
+        "//test/util:multiprocess_util",
         "//test/util:posix_error",
         "//test/util:temp_path",
         "//test/util:test_main",

test/syscalls/linux/madvise.cc

@@ -29,6 +29,7 @@
 #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"
@@ -136,6 +137,114 @@ TEST(MadviseDontneedTest, IgnoresPermissions) {
   EXPECT_THAT(madvise(m.ptr(), m.len(), MADV_DONTNEED), SyscallSucceeds());
 }
 
+TEST(MadviseDontforkTest, AddressLength) {
+  auto m =
+      ASSERT_NO_ERRNO_AND_VALUE(MmapAnon(kPageSize, PROT_NONE, MAP_PRIVATE));
+  char *addr = static_cast<char *>(m.ptr());
+
+  // Address must be page aligned.
+  EXPECT_THAT(madvise(addr + 1, kPageSize, MADV_DONTFORK),
+              SyscallFailsWithErrno(EINVAL));
+
+  // Zero length madvise always succeeds.
+  EXPECT_THAT(madvise(addr, 0, MADV_DONTFORK), SyscallSucceeds());
+
+  // Length must not roll over after rounding up.
+  size_t badlen = std::numeric_limits<std::size_t>::max() - (kPageSize / 2);
+  EXPECT_THAT(madvise(0, badlen, MADV_DONTFORK), SyscallFailsWithErrno(EINVAL));
+
+  // Length need not be page aligned - it is implicitly rounded up.
+  EXPECT_THAT(madvise(addr, 1, MADV_DONTFORK), SyscallSucceeds());
+  EXPECT_THAT(madvise(addr, kPageSize, MADV_DONTFORK), SyscallSucceeds());
+}
+
+TEST(MadviseDontforkTest, DontforkShared) {
+  // Mmap two shared file-backed pages and MADV_DONTFORK the second page.
+  TempPath f = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFileWith(
+      /* parent = */ GetAbsoluteTestTmpdir(),
+      /* content = */ std::string(kPageSize * 2, 2), TempPath::kDefaultFileMode));
+  FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(Open(f.path(), O_RDWR));
+
+  Mapping m = ASSERT_NO_ERRNO_AND_VALUE(Mmap(
+      nullptr, kPageSize * 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd.get(), 0));
+
+  const Mapping ms1 = Mapping(reinterpret_cast<void *>(m.addr()), kPageSize);
+  const Mapping ms2 =
+      Mapping(reinterpret_cast<void *>(m.addr() + kPageSize), kPageSize);
+  m.release();
+
+  ASSERT_THAT(madvise(ms2.ptr(), kPageSize, MADV_DONTFORK), SyscallSucceeds());
+
+  const auto rest = [&] {
+    // First page is mapped in child and modifications are visible to parent
+    // via the shared mapping.
+    TEST_CHECK(IsMapped(ms1.addr()));
+    ExpectAllMappingBytes(ms1, 2);
+    memset(ms1.ptr(), 1, kPageSize);
+    ExpectAllMappingBytes(ms1, 1);
+
+    // Second page must not be mapped in child.
+    TEST_CHECK(!IsMapped(ms2.addr()));
+  };
+
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+
+  ExpectAllMappingBytes(ms1, 1);  // page contents modified by child.
+  ExpectAllMappingBytes(ms2, 2);  // page contents unchanged.
+}
+
+TEST(MadviseDontforkTest, DontforkAnonPrivate) {
+  // Mmap three anonymous pages and MADV_DONTFORK the middle page.
+  Mapping m = ASSERT_NO_ERRNO_AND_VALUE(
+      MmapAnon(kPageSize * 3, PROT_READ | PROT_WRITE, MAP_PRIVATE));
+  const Mapping mp1 = Mapping(reinterpret_cast<void *>(m.addr()), kPageSize);
+  const Mapping mp2 =
+      Mapping(reinterpret_cast<void *>(m.addr() + kPageSize), kPageSize);
+  const Mapping mp3 =
+      Mapping(reinterpret_cast<void *>(m.addr() + 2 * kPageSize), kPageSize);
+  m.release();
+
+  ASSERT_THAT(madvise(mp2.ptr(), kPageSize, MADV_DONTFORK), SyscallSucceeds());
+
+  // Verify that all pages are zeroed and memset the first, second and third
+  // pages to 1, 2, and 3 respectively.
+  ExpectAllMappingBytes(mp1, 0);
+  memset(mp1.ptr(), 1, kPageSize);
+
+  ExpectAllMappingBytes(mp2, 0);
+  memset(mp2.ptr(), 2, kPageSize);
+
+  ExpectAllMappingBytes(mp3, 0);
+  memset(mp3.ptr(), 3, kPageSize);
+
+  const auto rest = [&] {
+    // Verify first page is mapped, verify its contents and then modify the
+    // page. The mapping is private so the modifications are not visible to
+    // the parent.
+    TEST_CHECK(IsMapped(mp1.addr()));
+    ExpectAllMappingBytes(mp1, 1);
+    memset(mp1.ptr(), 11, kPageSize);
+    ExpectAllMappingBytes(mp1, 11);
+
+    // Verify second page is not mapped.
+    TEST_CHECK(!IsMapped(mp2.addr()));
+
+    // Verify third page is mapped, verify its contents and then modify the
+    // page. The mapping is private so the modifications are not visible to
+    // the parent.
+    TEST_CHECK(IsMapped(mp3.addr()));
+    ExpectAllMappingBytes(mp3, 3);
+    memset(mp3.ptr(), 13, kPageSize);
+    ExpectAllMappingBytes(mp3, 13);
+  };
+  EXPECT_THAT(InForkedProcess(rest), IsPosixErrorOkAndHolds(0));
+
+  // The fork and COW by child should not affect the parent mappings.
+  ExpectAllMappingBytes(mp1, 1);
+  ExpectAllMappingBytes(mp2, 2);
+  ExpectAllMappingBytes(mp3, 3);
+}
+
 }  // namespace
 
 }  // namespace testing

test/syscalls/linux/mremap.cc

@@ -46,17 +46,6 @@ PosixErrorOr<void*> Mremap(void* old_address, size_t old_size, size_t new_size,
   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/util/memory_util.h

@@ -118,6 +118,17 @@ inline PosixErrorOr<Mapping> MmapAnon(size_t length, int prot, int flags) {
   return Mmap(nullptr, length, prot, flags | MAP_ANONYMOUS, -1, 0);
 }
 
+// Returns true if the page containing addr is mapped.
+inline 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;
+}
+
 }  // namespace testing
 }  // namespace gvisor