Add /proc/sys/vm/max_map_count

Set it to int32 max because gVisor doesn't have a limit.

Fixes #2337

PiperOrigin-RevId: 378722230

pkg/sentry/fs/proc/sys.go

@@ -77,6 +77,27 @@ func (*overcommitMemory) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandl
 	}, 0
 }
 
+// +stateify savable
+type maxMapCount struct{}
+
+// NeedsUpdate implements seqfile.SeqSource.
+func (*maxMapCount) NeedsUpdate(int64) bool {
+	return true
+}
+
+// ReadSeqFileData implements seqfile.SeqSource.
+func (*maxMapCount) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]seqfile.SeqData, int64) {
+	if h != nil {
+		return nil, 0
+	}
+	return []seqfile.SeqData{
+		{
+			Buf:    []byte("2147483647\n"),
+			Handle: (*maxMapCount)(nil),
+		},
+	}, 0
+}
+
 func (p *proc) newKernelDir(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
 	h := hostname{
 		SimpleFileInode: *fsutil.NewSimpleFileInode(ctx, fs.RootOwner, fs.FilePermsFromMode(0444), linux.PROC_SUPER_MAGIC),
@@ -96,6 +117,7 @@ func (p *proc) newKernelDir(ctx context.Context, msrc *fs.MountSource) *fs.Inode
 
 func (p *proc) newVMDir(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
 	children := map[string]*fs.Inode{
+		"max_map_count":     seqfile.NewSeqFileInode(ctx, &maxMapCount{}, msrc),
 		"mmap_min_addr":     seqfile.NewSeqFileInode(ctx, &mmapMinAddrData{p.k}, msrc),
 		"overcommit_memory": seqfile.NewSeqFileInode(ctx, &overcommitMemory{}, msrc),
 	}

pkg/sentry/fsimpl/proc/tasks_sys.go

@@ -55,6 +55,7 @@ func (fs *filesystem) newSysDir(ctx context.Context, root *auth.Credentials, k *
 			}),
 		}),
 		"vm": fs.newStaticDir(ctx, root, map[string]kernfs.Inode{
+			"max_map_count":     fs.newInode(ctx, root, 0444, newStaticFile("2147483647\n")),
 			"mmap_min_addr":     fs.newInode(ctx, root, 0444, &mmapMinAddrData{k: k}),
 			"overcommit_memory": fs.newInode(ctx, root, 0444, newStaticFile("0\n")),
 		}),

test/syscalls/linux/proc.cc

@@ -2364,6 +2364,15 @@ TEST(ProcSysKernelHostname, MatchesUname) {
   EXPECT_EQ(procfs_hostname, hostname);
 }
 
+TEST(ProcSysVmMaxmapCount, HasNumericValue) {
+  const std::string val_str =
+      ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/sys/vm/max_map_count"));
+  int32_t val;
+  EXPECT_TRUE(absl::SimpleAtoi(val_str, &val))
+      << "/proc/sys/vm/max_map_count does not contain a numeric value: "
+      << val_str;
+}
+
 TEST(ProcSysVmMmapMinAddr, HasNumericValue) {
   const std::string mmap_min_addr_str =
       ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/sys/vm/mmap_min_addr"));