Advertise vsyscall support via /proc/<pid>/maps.

Also update test utilities for probing vsyscall support and add a
metric to see if vsyscalls are actually used in sandboxes.

PiperOrigin-RevId: 221698834
Change-Id: I57870ecc33ea8c864bd7437833f21aa1e8117477

pkg/sentry/kernel/BUILD

@@ -131,6 +131,7 @@ go_library(
         "//pkg/cpuid",
         "//pkg/eventchannel",
         "//pkg/log",
+        "//pkg/metric",
         "//pkg/refs",
         "//pkg/secio",
         "//pkg/sentry/arch",

pkg/sentry/kernel/task_syscall.go

@@ -21,6 +21,7 @@ import (
 
 	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
 	"gvisor.googlesource.com/gvisor/pkg/bits"
+	"gvisor.googlesource.com/gvisor/pkg/metric"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/memmap"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
@@ -60,6 +61,8 @@ const (
 	ERESTART_RESTARTBLOCK = SyscallRestartErrno(516)
 )
 
+var vsyscallCount = metric.MustCreateNewUint64Metric("/kernel/vsyscall_count", false /* sync */, "Number of times vsyscalls were invoked by the application")
+
 // Error implements error.Error.
 func (e SyscallRestartErrno) Error() string {
 	// Descriptions are borrowed from strace.
@@ -325,6 +328,8 @@ func (*runSyscallExit) execute(t *Task) taskRunState {
 // indicated by an execution fault at address addr. doVsyscall returns the
 // task's next run state.
 func (t *Task) doVsyscall(addr usermem.Addr, sysno uintptr) taskRunState {
+	vsyscallCount.Increment()
+
 	// Grab the caller up front, to make sure there's a sensible stack.
 	caller := t.Arch().Native(uintptr(0))
 	if _, err := t.CopyIn(usermem.Addr(t.Arch().Stack()), caller); err != nil {

pkg/sentry/mm/proc_pid_maps.go

@@ -53,6 +53,22 @@ func (mm *MemoryManager) ReadSeqFileData(ctx context.Context, handle seqfile.Seq
 			Handle: &vmaAddr,
 		})
 	}
+
+	// We always emulate vsyscall, so advertise it here. Everything about a
+	// vsyscall region is static, so just hard code the maps entry since we
+	// don't have a real vma backing it. The vsyscall region is at the end of
+	// the virtual address space so nothing should be mapped after it (if
+	// something is really mapped in the tiny ~10 MiB segment afterwards, we'll
+	// get the sorting on the maps file wrong at worst; but that's not possible
+	// on any current platform).
+	//
+	// Artifically adjust the seqfile handle so we only output vsyscall entry once.
+	if vsyscallEnd := usermem.Addr(0xffffffffff601000); start != vsyscallEnd {
+		data = append(data, seqfile.SeqData{
+			Buf:    []byte("ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]\n"),
+			Handle: &vsyscallEnd,
+		})
+	}
 	return data, 1
 }