systrap: don't restart the task after replacing a syscall with a function call

PiperOrigin-RevId: 627556852

pkg/sentry/platform/systrap/systrap.go

@@ -186,10 +186,7 @@ restart:
 		return nil, hostarch.NoAccess, err
 	}
 	if needPatch {
-		restart, _ := s.usertrap.PatchSyscall(ctx, ac, mm)
-		if restart {
-			goto restart
-		}
+		s.usertrap.PatchSyscall(ctx, ac, mm)
 	}
 	if !isSyscall && linux.Signal(c.signalInfo.Signo) == linux.SIGILL {
 		err := s.usertrap.HandleFault(ctx, ac, mm)

pkg/sentry/platform/systrap/usertrap/usertrap_amd64.go

@@ -187,12 +187,10 @@ func loadUsertrap(ctx context.Context, mm memoryManager, addr hostarch.Addr) err
 }
 
 // PatchSyscall changes the syscall instruction into a function call.
-//
-// Returns true if the thread has to be restarted.
-func (s *State) PatchSyscall(ctx context.Context, ac *arch.Context64, mm memoryManager) (bool, error) {
+func (s *State) PatchSyscall(ctx context.Context, ac *arch.Context64, mm memoryManager) error {
 	task := kernel.TaskFromContext(ctx)
 	if task == nil {
-		return false, fmt.Errorf("no task found")
+		return fmt.Errorf("no task found")
 	}
 
 	s.mu.Lock()
@@ -203,7 +201,7 @@ func (s *State) PatchSyscall(ctx context.Context, ac *arch.Context64, mm memoryM
 
 	prevCode := make([]uint8, len(jmpInst))
 	if _, err := primitive.CopyUint8SliceIn(task.OwnCopyContext(usermem.IOOpts{AddressSpaceActive: false}), hostarch.Addr(patchAddr), prevCode); err != nil {
-		return false, err
+		return err
 	}
 
 	// Check that another thread has not patched this syscall yet.
@@ -214,7 +212,7 @@ func (s *State) PatchSyscall(ctx context.Context, ac *arch.Context64, mm memoryM
 		trapAddr, err := s.addTrapLocked(ctx, ac, mm, uint32(sysno))
 		if trapAddr == 0 || err != nil {
 			ctx.Warningf("Failed to add a new trap: %v", err)
-			return false, nil
+			return nil
 		}
 
 		// Replace "mov sysno, %eax; syscall" with "jmp trapAddr".
@@ -258,24 +256,22 @@ func (s *State) PatchSyscall(ctx context.Context, ac *arch.Context64, mm memoryM
 		// the invalid instruction and restart a patched code.
 		faultInstB := primitive.ByteSlice(faultInst[:])
 		if _, err := faultInstB.CopyOut(ignorePermContext, hostarch.Addr(patchAddr+faultInstOffset)); err != nil {
-			return false, err
+			return err
 		}
 		// The second step is to replace all bytes except the first one
 		// which is the opcode of the mov instruction, so that the first
 		// five bytes remain "mov XXX, %rax".
 		if _, err := primitive.CopyUint8SliceOut(ignorePermContext, hostarch.Addr(patchAddr+1), newCode[1:]); err != nil {
-			return false, err
+			return err
 		}
 		// The final step is to replace the first byte of the patch.
 		// After this point, all threads will read the valid jmp
 		// instruction.
 		if _, err := primitive.CopyUint8SliceOut(ignorePermContext, hostarch.Addr(patchAddr), newCode[0:1]); err != nil {
-			return false, err
+			return err
 		}
 	}
-	ac.RestartSyscall()
-	ac.SetIP(patchAddr)
-	return true, nil
+	return nil
 }
 
 // HandleFault handles a fault on a patched syscall instruction.