kvm: avoid siginfo allocations.

PiperOrigin-RevId: 219492587
Change-Id: I47f6fc0b74a4907ab0aff03d5f26453bdb983bb5

3 files changed, 89 insertions, 71 deletions

diff --git a/pkg/sentry/platform/kvm/context.go b/pkg/sentry/platform/kvm/context.go
index be902be88..c75a4b415 100644
--- a/pkg/sentry/platform/kvm/context.go
+++ b/pkg/sentry/platform/kvm/context.go
@@ -29,6 +29,9 @@ type context struct {
 	// machine is the parent machine, and is immutable.
 	machine *machine
 
+	// info is the arch.SignalInfo cached for this context.
+	info arch.SignalInfo
+
 	// interrupt is the interrupt context.
 	interrupt interrupt.Forwarder
 }
@@ -65,7 +68,7 @@ func (c *context) Switch(as platform.AddressSpace, ac arch.Context, _ int32) (*a
 	}
 
 	// Take the blue pill.
-	si, at, err := cpu.SwitchToUser(switchOpts)
+	at, err := cpu.SwitchToUser(switchOpts, &c.info)
 
 	// Clear the address space.
 	cpu.active.set(nil)
@@ -75,7 +78,7 @@ func (c *context) Switch(as platform.AddressSpace, ac arch.Context, _ int32) (*a
 
 	// All done.
 	c.interrupt.Disable()
-	return si, at, err
+	return &c.info, at, err
 }
 
 // Interrupt interrupts the running context.
diff --git a/pkg/sentry/platform/kvm/kvm_test.go b/pkg/sentry/platform/kvm/kvm_test.go
index 45eeb96ff..fff463a6e 100644
--- a/pkg/sentry/platform/kvm/kvm_test.go
+++ b/pkg/sentry/platform/kvm/kvm_test.go
@@ -156,12 +156,13 @@ func applicationTest(t testHarness, useHostMappings bool, target func(), fn func
 
 func TestApplicationSyscall(t *testing.T) {
 	applicationTest(t, true, testutil.SyscallLoop, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
-		if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
 			FullRestore:        true,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			return true // Retry.
 		} else if err != nil {
 			t.Errorf("application syscall with full restore failed: %v", err)
@@ -169,11 +170,12 @@ func TestApplicationSyscall(t *testing.T) {
 		return false
 	})
 	applicationTest(t, true, testutil.SyscallLoop, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
-		if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			return true // Retry.
 		} else if err != nil {
 			t.Errorf("application syscall with partial restore failed: %v", err)
@@ -185,27 +187,29 @@ func TestApplicationSyscall(t *testing.T) {
 func TestApplicationFault(t *testing.T) {
 	applicationTest(t, true, testutil.Touch, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
 		testutil.SetTouchTarget(regs, nil) // Cause fault.
-		if si, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
 			FullRestore:        true,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			return true // Retry.
-		} else if err != platform.ErrContextSignal || (si != nil && si.Signo != int32(syscall.SIGSEGV)) {
+		} else if err != platform.ErrContextSignal || si.Signo != int32(syscall.SIGSEGV) {
 			t.Errorf("application fault with full restore got (%v, %v), expected (%v, SIGSEGV)", err, si, platform.ErrContextSignal)
 		}
 		return false
 	})
 	applicationTest(t, true, testutil.Touch, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
 		testutil.SetTouchTarget(regs, nil) // Cause fault.
-		if si, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			return true // Retry.
-		} else if err != platform.ErrContextSignal || (si != nil && si.Signo != int32(syscall.SIGSEGV)) {
+		} else if err != platform.ErrContextSignal || si.Signo != int32(syscall.SIGSEGV) {
 			t.Errorf("application fault with partial restore got (%v, %v), expected (%v, SIGSEGV)", err, si, platform.ErrContextSignal)
 		}
 		return false
@@ -216,11 +220,12 @@ func TestRegistersSyscall(t *testing.T) {
 	applicationTest(t, true, testutil.TwiddleRegsSyscall, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
 		testutil.SetTestRegs(regs) // Fill values for all registers.
 		for {
-			if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+			var si arch.SignalInfo
+			if _, err := c.SwitchToUser(ring0.SwitchOpts{
 				Registers:          regs,
 				FloatingPointState: dummyFPState,
 				PageTables:         pt,
-			}); err == platform.ErrContextInterrupt {
+			}, &si); err == platform.ErrContextInterrupt {
 				continue // Retry.
 			} else if err != nil {
 				t.Errorf("application register check with partial restore got unexpected error: %v", err)
@@ -238,12 +243,13 @@ func TestRegistersFault(t *testing.T) {
 	applicationTest(t, true, testutil.TwiddleRegsFault, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
 		testutil.SetTestRegs(regs) // Fill values for all registers.
 		for {
-			if si, _, err := c.SwitchToUser(ring0.SwitchOpts{
+			var si arch.SignalInfo
+			if _, err := c.SwitchToUser(ring0.SwitchOpts{
 				Registers:          regs,
 				FloatingPointState: dummyFPState,
 				PageTables:         pt,
 				FullRestore:        true,
-			}); err == platform.ErrContextInterrupt {
+			}, &si); err == platform.ErrContextInterrupt {
 				continue // Retry.
 			} else if err != platform.ErrContextSignal || si.Signo != int32(syscall.SIGSEGV) {
 				t.Errorf("application register check with full restore got unexpected error: %v", err)
@@ -261,12 +267,13 @@ func TestSegments(t *testing.T) {
 	applicationTest(t, true, testutil.TwiddleSegments, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
 		testutil.SetTestSegments(regs)
 		for {
-			if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+			var si arch.SignalInfo
+			if _, err := c.SwitchToUser(ring0.SwitchOpts{
 				Registers:          regs,
 				FloatingPointState: dummyFPState,
 				PageTables:         pt,
 				FullRestore:        true,
-			}); err == platform.ErrContextInterrupt {
+			}, &si); err == platform.ErrContextInterrupt {
 				continue // Retry.
 			} else if err != nil {
 				t.Errorf("application segment check with full restore got unexpected error: %v", err)
@@ -286,11 +293,12 @@ func TestBounce(t *testing.T) {
 			time.Sleep(time.Millisecond)
 			c.BounceToKernel()
 		}()
-		if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
-		}); err != platform.ErrContextInterrupt {
+		}, &si); err != platform.ErrContextInterrupt {
 			t.Errorf("application partial restore: got %v, wanted %v", err, platform.ErrContextInterrupt)
 		}
 		return false
@@ -300,12 +308,13 @@ func TestBounce(t *testing.T) {
 			time.Sleep(time.Millisecond)
 			c.BounceToKernel()
 		}()
-		if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
 			FullRestore:        true,
-		}); err != platform.ErrContextInterrupt {
+		}, &si); err != platform.ErrContextInterrupt {
 			t.Errorf("application full restore: got %v, wanted %v", err, platform.ErrContextInterrupt)
 		}
 		return false
@@ -331,11 +340,12 @@ func TestBounceStress(t *testing.T) {
 				c.BounceToKernel()
 			}()
 			randomSleep()
-			if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+			var si arch.SignalInfo
+			if _, err := c.SwitchToUser(ring0.SwitchOpts{
 				Registers:          regs,
 				FloatingPointState: dummyFPState,
 				PageTables:         pt,
-			}); err != platform.ErrContextInterrupt {
+			}, &si); err != platform.ErrContextInterrupt {
 				t.Errorf("application partial restore: got %v, wanted %v", err, platform.ErrContextInterrupt)
 			}
 			c.unlock()
@@ -351,11 +361,12 @@ func TestInvalidate(t *testing.T) {
 	applicationTest(t, true, testutil.Touch, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
 		testutil.SetTouchTarget(regs, &data) // Read legitimate value.
 		for {
-			if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+			var si arch.SignalInfo
+			if _, err := c.SwitchToUser(ring0.SwitchOpts{
 				Registers:          regs,
 				FloatingPointState: dummyFPState,
 				PageTables:         pt,
-			}); err == platform.ErrContextInterrupt {
+			}, &si); err == platform.ErrContextInterrupt {
 				continue // Retry.
 			} else if err != nil {
 				t.Errorf("application partial restore: got %v, wanted nil", err)
@@ -365,12 +376,13 @@ func TestInvalidate(t *testing.T) {
 		// Unmap the page containing data & invalidate.
 		pt.Unmap(usermem.Addr(reflect.ValueOf(&data).Pointer() & ^uintptr(usermem.PageSize-1)), usermem.PageSize)
 		for {
-			if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+			var si arch.SignalInfo
+			if _, err := c.SwitchToUser(ring0.SwitchOpts{
 				Registers:          regs,
 				FloatingPointState: dummyFPState,
 				PageTables:         pt,
 				Flush:              true,
-			}); err == platform.ErrContextInterrupt {
+			}, &si); err == platform.ErrContextInterrupt {
 				continue // Retry.
 			} else if err != platform.ErrContextSignal {
 				t.Errorf("application partial restore: got %v, wanted %v", err, platform.ErrContextSignal)
@@ -388,27 +400,29 @@ func IsFault(err error, si *arch.SignalInfo) bool {
 
 func TestEmptyAddressSpace(t *testing.T) {
 	applicationTest(t, false, testutil.SyscallLoop, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
-		if si, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			return true // Retry.
-		} else if !IsFault(err, si) {
+		} else if !IsFault(err, &si) {
 			t.Errorf("first fault with partial restore failed got %v", err)
 			t.Logf("registers: %#v", &regs)
 		}
 		return false
 	})
 	applicationTest(t, false, testutil.SyscallLoop, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
-		if si, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
 			FullRestore:        true,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			return true // Retry.
-		} else if !IsFault(err, si) {
+		} else if !IsFault(err, &si) {
 			t.Errorf("first fault with full restore failed got %v", err)
 			t.Logf("registers: %#v", &regs)
 		}
@@ -459,11 +473,12 @@ func BenchmarkApplicationSyscall(b *testing.B) {
 		a int // Count for ErrContextInterrupt.
 	)
 	applicationTest(b, true, testutil.SyscallLoop, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
-		if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			a++
 			return true // Ignore.
 		} else if err != nil {
@@ -495,11 +510,12 @@ func BenchmarkWorldSwitchToUserRoundtrip(b *testing.B) {
 		a int
 	)
 	applicationTest(b, true, testutil.SyscallLoop, func(c *vCPU, regs *syscall.PtraceRegs, pt *pagetables.PageTables) bool {
-		if _, _, err := c.SwitchToUser(ring0.SwitchOpts{
+		var si arch.SignalInfo
+		if _, err := c.SwitchToUser(ring0.SwitchOpts{
 			Registers:          regs,
 			FloatingPointState: dummyFPState,
 			PageTables:         pt,
-		}); err == platform.ErrContextInterrupt {
+		}, &si); err == platform.ErrContextInterrupt {
 			a++
 			return true // Ignore.
 		} else if err != nil {
diff --git a/pkg/sentry/platform/kvm/machine_amd64.go b/pkg/sentry/platform/kvm/machine_amd64.go
index e0aec42b8..c03792a1b 100644
--- a/pkg/sentry/platform/kvm/machine_amd64.go
+++ b/pkg/sentry/platform/kvm/machine_amd64.go
@@ -156,19 +156,19 @@ func (c *vCPU) initArchState() error {
 // nonCanonical generates a canonical address return.
 //
 //go:nosplit
-func nonCanonical(addr uint64, signal int32) (*arch.SignalInfo, usermem.AccessType, error) {
-	info := &arch.SignalInfo{
+func nonCanonical(addr uint64, signal int32, info *arch.SignalInfo) (usermem.AccessType, error) {
+	*info = arch.SignalInfo{
 		Signo: signal,
 		Code:  arch.SignalInfoKernel,
 	}
 	info.SetAddr(addr) // Include address.
-	return info, usermem.NoAccess, platform.ErrContextSignal
+	return usermem.NoAccess, platform.ErrContextSignal
 }
 
 // fault generates an appropriate fault return.
 //
 //go:nosplit
-func (c *vCPU) fault(signal int32) (*arch.SignalInfo, usermem.AccessType, error) {
+func (c *vCPU) fault(signal int32, info *arch.SignalInfo) (usermem.AccessType, error) {
 	bluepill(c) // Probably no-op, but may not be.
 	faultAddr := ring0.ReadCR2()
 	code, user := c.ErrorCode()
@@ -176,11 +176,10 @@ func (c *vCPU) fault(signal int32) (*arch.SignalInfo, usermem.AccessType, error)
 		// The last fault serviced by this CPU was not a user
 		// fault, so we can't reliably trust the faultAddr or
 		// the code provided here. We need to re-execute.
-		return nil, usermem.NoAccess, platform.ErrContextInterrupt
-	}
-	info := &arch.SignalInfo{
-		Signo: signal,
+		return usermem.NoAccess, platform.ErrContextInterrupt
 	}
+	// Reset the pointed SignalInfo.
+	*info = arch.SignalInfo{Signo: signal}
 	info.SetAddr(uint64(faultAddr))
 	accessType := usermem.AccessType{
 		Read:    code&(1<<1) == 0,
@@ -192,20 +191,20 @@ func (c *vCPU) fault(signal int32) (*arch.SignalInfo, usermem.AccessType, error)
 	} else {
 		info.Code = 2 // SEGV_ACCERR.
 	}
-	return info, accessType, platform.ErrContextSignal
+	return accessType, platform.ErrContextSignal
 }
 
 // SwitchToUser unpacks architectural-details.
-func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts) (*arch.SignalInfo, usermem.AccessType, error) {
+func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts, info *arch.SignalInfo) (usermem.AccessType, error) {
 	// Check for canonical addresses.
 	if regs := switchOpts.Registers; !ring0.IsCanonical(regs.Rip) {
-		return nonCanonical(regs.Rip, int32(syscall.SIGSEGV))
+		return nonCanonical(regs.Rip, int32(syscall.SIGSEGV), info)
 	} else if !ring0.IsCanonical(regs.Rsp) {
-		return nonCanonical(regs.Rsp, int32(syscall.SIGBUS))
+		return nonCanonical(regs.Rsp, int32(syscall.SIGBUS), info)
 	} else if !ring0.IsCanonical(regs.Fs_base) {
-		return nonCanonical(regs.Fs_base, int32(syscall.SIGBUS))
+		return nonCanonical(regs.Fs_base, int32(syscall.SIGBUS), info)
 	} else if !ring0.IsCanonical(regs.Gs_base) {
-		return nonCanonical(regs.Gs_base, int32(syscall.SIGBUS))
+		return nonCanonical(regs.Gs_base, int32(syscall.SIGBUS), info)
 	}
 
 	// Assign PCIDs.
@@ -231,25 +230,25 @@ func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts) (*arch.SignalInfo, user
 	switch vector {
 	case ring0.Syscall, ring0.SyscallInt80:
 		// Fast path: system call executed.
-		return nil, usermem.NoAccess, nil
+		return usermem.NoAccess, nil
 
 	case ring0.PageFault:
-		return c.fault(int32(syscall.SIGSEGV))
+		return c.fault(int32(syscall.SIGSEGV), info)
 
 	case ring0.Debug, ring0.Breakpoint:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGTRAP),
 			Code:  1, // TRAP_BRKPT (breakpoint).
 		}
 		info.SetAddr(switchOpts.Registers.Rip) // Include address.
-		return info, usermem.AccessType{}, platform.ErrContextSignal
+		return usermem.AccessType{}, platform.ErrContextSignal
 
 	case ring0.GeneralProtectionFault,
 		ring0.SegmentNotPresent,
 		ring0.BoundRangeExceeded,
 		ring0.InvalidTSS,
 		ring0.StackSegmentFault:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGSEGV),
 			Code:  arch.SignalInfoKernel,
 		}
@@ -258,52 +257,52 @@ func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts) (*arch.SignalInfo, user
 			// When CPUID faulting is enabled, we will generate a #GP(0) when
 			// userspace executes a CPUID instruction. This is handled above,
 			// because we need to be able to map and read user memory.
-			return info, usermem.AccessType{}, platform.ErrContextSignalCPUID
+			return usermem.AccessType{}, platform.ErrContextSignalCPUID
 		}
-		return info, usermem.AccessType{}, platform.ErrContextSignal
+		return usermem.AccessType{}, platform.ErrContextSignal
 
 	case ring0.InvalidOpcode:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGILL),
 			Code:  1, // ILL_ILLOPC (illegal opcode).
 		}
 		info.SetAddr(switchOpts.Registers.Rip) // Include address.
-		return info, usermem.AccessType{}, platform.ErrContextSignal
+		return usermem.AccessType{}, platform.ErrContextSignal
 
 	case ring0.DivideByZero:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGFPE),
 			Code:  1, // FPE_INTDIV (divide by zero).
 		}
 		info.SetAddr(switchOpts.Registers.Rip) // Include address.
-		return info, usermem.AccessType{}, platform.ErrContextSignal
+		return usermem.AccessType{}, platform.ErrContextSignal
 
 	case ring0.Overflow:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGFPE),
 			Code:  1, // FPE_INTOVF (integer overflow).
 		}
 		info.SetAddr(switchOpts.Registers.Rip) // Include address.
-		return info, usermem.AccessType{}, platform.ErrContextSignal
+		return usermem.AccessType{}, platform.ErrContextSignal
 
 	case ring0.X87FloatingPointException,
 		ring0.SIMDFloatingPointException:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGFPE),
 			Code:  7, // FPE_FLTINV (invalid operation).
 		}
 		info.SetAddr(switchOpts.Registers.Rip) // Include address.
-		return info, usermem.AccessType{}, platform.ErrContextSignal
+		return usermem.AccessType{}, platform.ErrContextSignal
 
 	case ring0.Vector(bounce): // ring0.VirtualizationException
-		return nil, usermem.NoAccess, platform.ErrContextInterrupt
+		return usermem.NoAccess, platform.ErrContextInterrupt
 
 	case ring0.AlignmentCheck:
-		info := &arch.SignalInfo{
+		*info = arch.SignalInfo{
 			Signo: int32(syscall.SIGBUS),
 			Code:  2, // BUS_ADRERR (physical address does not exist).
 		}
-		return info, usermem.NoAccess, platform.ErrContextSignal
+		return usermem.NoAccess, platform.ErrContextSignal
 
 	case ring0.NMI:
 		// An NMI is generated only when a fault is not servicable by
@@ -311,7 +310,7 @@ func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts) (*arch.SignalInfo, user
 		// really not. This could happen, e.g. if some file is
 		// truncated (and would generate a SIGBUS) and we map it
 		// directly into the instance.
-		return c.fault(int32(syscall.SIGBUS))
+		return c.fault(int32(syscall.SIGBUS), info)
 
 	case ring0.DeviceNotAvailable,
 		ring0.DoubleFault,