Implement mlock(), kind of.

Currently mlock() and friends do nothing whatsoever. However, mlocking
is directly application-visible in a number of ways; for example,
madvise(MADV_DONTNEED) and msync(MS_INVALIDATE) both fail on mlocked
regions. We handle this inconsistently: MADV_DONTNEED is too important
to not work, but MS_INVALIDATE is rejected.

Change MM to track mlocked regions in a manner consistent with Linux.
It still will not actually pin pages into host physical memory, but:

- mlock() will now cause sentry memory management to precommit mlocked
pages.

- MADV_DONTNEED and MS_INVALIDATE will interact with mlocked pages as
described above.

PiperOrigin-RevId: 225861605
Change-Id: Iee187204979ac9a4d15d0e037c152c0902c8d0ee

1 files changed, 5 insertions, 7 deletions

diff --git a/pkg/sentry/mm/address_space.go b/pkg/sentry/mm/address_space.go
index 7488f7c4a..e7aa24c69 100644
--- a/pkg/sentry/mm/address_space.go
+++ b/pkg/sentry/mm/address_space.go
@@ -149,7 +149,7 @@ func (mm *MemoryManager) Deactivate() {
 // for all addresses in ar should be precommitted.
 //
 // Preconditions: mm.activeMu must be locked. mm.as != nil. ar.Length() != 0.
-// ar must be page-aligned. pseg.Range().Contains(ar.Start).
+// ar must be page-aligned. pseg == mm.pmas.LowerBoundSegment(ar.Start).
 func (mm *MemoryManager) mapASLocked(pseg pmaIterator, ar usermem.AddrRange, precommit bool) error {
 	// By default, map entire pmas at a time, under the assumption that there
 	// is no cost to mapping more of a pma than necessary.
@@ -173,7 +173,9 @@ func (mm *MemoryManager) mapASLocked(pseg pmaIterator, ar usermem.AddrRange, pre
 		}
 	}
 
-	for {
+	// Since this checks ar.End and not mapAR.End, we will never map a pma that
+	// is not required.
+	for pseg.Ok() && pseg.Start() < ar.End {
 		pma := pseg.ValuePtr()
 		pmaAR := pseg.Range()
 		pmaMapAR := pmaAR.Intersect(mapAR)
@@ -184,13 +186,9 @@ func (mm *MemoryManager) mapASLocked(pseg pmaIterator, ar usermem.AddrRange, pre
 		if err := pma.file.MapInto(mm.as, pmaMapAR.Start, pseg.fileRangeOf(pmaMapAR), perms, precommit); err != nil {
 			return err
 		}
-		// Since this checks ar.End and not mapAR.End, we will never map a pma
-		// that is not required.
-		if ar.End <= pmaAR.End {
-			return nil
-		}
 		pseg = pseg.NextSegment()
 	}
+	return nil
 }
 
 // unmapASLocked removes all AddressSpace mappings for addresses in ar.