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runtime: scan gp._panic in stack scan
In runtime.gopanic, the _panic object p is stack allocated and referenced from gp._panic. With stack objects, p on stack is dead at the point preprintpanics runs. gp._panic points to p, but stack scan doesn't look at gp. Heap scan of gp does look at gp._panic, but it stops and ignores the pointer as it points to the stack. So whatever p points to may be collected and clobbered. We need to scan gp._panic explicitly during stack scan. To test it reliably, we introduce a GODEBUG mode "clobberfree", which clobbers the memory content when the GC frees an object. Fixes #30150. Change-Id: I11128298f03a89f817faa221421a9d332b41dced Reviewed-on: https://go-review.googlesource.com/c/161778 Run-TryBot: Cherry Zhang <cherryyz@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Austin Clements <austin@google.com>
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@ -728,3 +728,15 @@ func TestG0StackOverflow(t *testing.T) {
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runtime.G0StackOverflow()
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}
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// Test that panic message is not clobbered.
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// See issue 30150.
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func TestDoublePanic(t *testing.T) {
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output := runTestProg(t, "testprog", "DoublePanic", "GODEBUG=clobberfree=1")
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wants := []string{"panic: XXX", "panic: YYY"}
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for _, want := range wants {
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if !strings.Contains(output, want) {
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t.Errorf("output:\n%s\n\nwant output containing: %s", output, want)
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}
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}
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}
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@ -27,6 +27,10 @@ It is a comma-separated list of name=val pairs setting these named variables:
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allocfreetrace: setting allocfreetrace=1 causes every allocation to be
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profiled and a stack trace printed on each object's allocation and free.
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clobberfree: setting clobberfree=1 causes the garbage collector to
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clobber the memory content of an object with bad content when it frees
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the object.
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cgocheck: setting cgocheck=0 disables all checks for packages
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using cgo to incorrectly pass Go pointers to non-Go code.
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Setting cgocheck=1 (the default) enables relatively cheap
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@ -709,7 +709,13 @@ func scanstack(gp *g, gcw *gcWork) {
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return true
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}
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gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0)
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// Find additional pointers that point into the stack from the heap.
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// Currently this includes defers and panics. See also function copystack.
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tracebackdefers(gp, scanframe, nil)
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if gp._panic != nil {
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state.putPtr(uintptr(unsafe.Pointer(gp._panic)))
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}
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// Find and scan all reachable stack objects.
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state.buildIndex()
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@ -291,7 +291,7 @@ func (s *mspan) sweep(preserve bool) bool {
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}
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}
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if debug.allocfreetrace != 0 || raceenabled || msanenabled {
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if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled {
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// Find all newly freed objects. This doesn't have to
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// efficient; allocfreetrace has massive overhead.
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mbits := s.markBitsForBase()
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@ -302,6 +302,9 @@ func (s *mspan) sweep(preserve bool) bool {
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if debug.allocfreetrace != 0 {
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tracefree(unsafe.Pointer(x), size)
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}
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if debug.clobberfree != 0 {
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clobberfree(unsafe.Pointer(x), size)
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}
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if raceenabled {
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racefree(unsafe.Pointer(x), size)
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}
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@ -446,3 +449,12 @@ retry:
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traceGCSweepDone()
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}
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}
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// clobberfree sets the memory content at x to bad content, for debugging
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// purposes.
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func clobberfree(x unsafe.Pointer, size uintptr) {
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// size (span.elemsize) is always a multiple of 4.
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for i := uintptr(0); i < size; i += 4 {
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*(*uint32)(add(x, i)) = 0xdeadbeef
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}
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}
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@ -301,6 +301,7 @@ type dbgVar struct {
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var debug struct {
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allocfreetrace int32
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cgocheck int32
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clobberfree int32
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efence int32
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gccheckmark int32
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gcpacertrace int32
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@ -318,6 +319,7 @@ var debug struct {
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var dbgvars = []dbgVar{
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{"allocfreetrace", &debug.allocfreetrace},
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{"clobberfree", &debug.clobberfree},
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{"cgocheck", &debug.cgocheck},
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{"efence", &debug.efence},
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{"gccheckmark", &debug.gccheckmark},
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21
src/runtime/testdata/testprog/crash.go
vendored
21
src/runtime/testdata/testprog/crash.go
vendored
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@ -11,6 +11,7 @@ import (
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func init() {
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register("Crash", Crash)
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register("DoublePanic", DoublePanic)
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}
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func test(name string) {
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@ -43,3 +44,23 @@ func Crash() {
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testInNewThread("second-new-thread")
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test("main-again")
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}
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type P string
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func (p P) String() string {
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// Try to free the "YYY" string header when the "XXX"
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// panic is stringified.
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runtime.GC()
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runtime.GC()
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runtime.GC()
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return string(p)
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}
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// Test that panic message is not clobbered.
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// See issue 30150.
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func DoublePanic() {
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defer func() {
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panic(P("YYY"))
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}()
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panic(P("XXX"))
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}
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