src/runtime/debugcall.go

   1 // Copyright 2018 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 //go:build amd64 || arm64
   6
   7 package runtime
   8
   9 import (
  10         "internal/abi"
  11         "unsafe"
  12 )
  13
  14 const (
  15         debugCallSystemStack = "executing on Go runtime stack"
  16         debugCallUnknownFunc = "call from unknown function"
  17         debugCallRuntime     = "call from within the Go runtime"
  18         debugCallUnsafePoint = "call not at safe point"
  19 )
  20
  21 func debugCallV2()
  22 func debugCallPanicked(val any)
  23
  24 // debugCallCheck checks whether it is safe to inject a debugger
  25 // function call with return PC pc. If not, it returns a string
  26 // explaining why.
  27 //
  28 //go:nosplit
  29 func debugCallCheck(pc uintptr) string {
  30         // No user calls from the system stack.
  31         if getg() != getg().m.curg {
  32                 return debugCallSystemStack
  33         }
  34         if sp := getcallersp(); !(getg().stack.lo < sp && sp <= getg().stack.hi) {
  35                 // Fast syscalls (nanotime) and racecall switch to the
  36                 // g0 stack without switching g. We can't safely make
  37                 // a call in this state. (We can't even safely
  38                 // systemstack.)
  39                 return debugCallSystemStack
  40         }
  41
  42         // Switch to the system stack to avoid overflowing the user
  43         // stack.
  44         var ret string
  45         systemstack(func() {
  46                 f := findfunc(pc)
  47                 if !f.valid() {
  48                         ret = debugCallUnknownFunc
  49                         return
  50                 }
  51
  52                 name := funcname(f)
  53
  54                 switch name {
  55                 case "debugCall32",
  56                         "debugCall64",
  57                         "debugCall128",
  58                         "debugCall256",
  59                         "debugCall512",
  60                         "debugCall1024",
  61                         "debugCall2048",
  62                         "debugCall4096",
  63                         "debugCall8192",
  64                         "debugCall16384",
  65                         "debugCall32768",
  66                         "debugCall65536":
  67                         // These functions are allowed so that the debugger can initiate multiple function calls.
  68                         // See: https://golang.org/cl/161137/
  69                         return
  70                 }
  71
  72                 // Disallow calls from the runtime. We could
  73                 // potentially make this condition tighter (e.g., not
  74                 // when locks are held), but there are enough tightly
  75                 // coded sequences (e.g., defer handling) that it's
  76                 // better to play it safe.
  77                 if pfx := "runtime."; len(name) > len(pfx) && name[:len(pfx)] == pfx {
  78                         ret = debugCallRuntime
  79                         return
  80                 }
  81
  82                 // Check that this isn't an unsafe-point.
  83                 if pc != f.entry() {
  84                         pc--
  85                 }
  86                 up := pcdatavalue(f, abi.PCDATA_UnsafePoint, pc)
  87                 if up != abi.UnsafePointSafe {
  88                         // Not at a safe point.
  89                         ret = debugCallUnsafePoint
  90                 }
  91         })
  92         return ret
  93 }
  94
  95 // debugCallWrap starts a new goroutine to run a debug call and blocks
  96 // the calling goroutine. On the goroutine, it prepares to recover
  97 // panics from the debug call, and then calls the call dispatching
  98 // function at PC dispatch.
  99 //
 100 // This must be deeply nosplit because there are untyped values on the
 101 // stack from debugCallV2.
 102 //
 103 //go:nosplit
 104 func debugCallWrap(dispatch uintptr) {
 105         var lockedm bool
 106         var lockedExt uint32
 107         callerpc := getcallerpc()
 108         gp := getg()
 109
 110         // Create a new goroutine to execute the call on. Run this on
 111         // the system stack to avoid growing our stack.
 112         systemstack(func() {
 113                 // TODO(mknyszek): It would be nice to wrap these arguments in an allocated
 114                 // closure and start the goroutine with that closure, but the compiler disallows
 115                 // implicit closure allocation in the runtime.
 116                 fn := debugCallWrap1
 117                 newg := newproc1(*(**funcval)(unsafe.Pointer(&fn)), gp, callerpc)
 118                 args := &debugCallWrapArgs{
 119                         dispatch: dispatch,
 120                         callingG: gp,
 121                 }
 122                 newg.param = unsafe.Pointer(args)
 123
 124                 // If the current G is locked, then transfer that
 125                 // locked-ness to the new goroutine.
 126                 if gp.lockedm != 0 {
 127                         // Save lock state to restore later.
 128                         mp := gp.m
 129                         if mp != gp.lockedm.ptr() {
 130                                 throw("inconsistent lockedm")
 131                         }
 132
 133                         lockedm = true
 134                         lockedExt = mp.lockedExt
 135
 136                         // Transfer external lock count to internal so
 137                         // it can't be unlocked from the debug call.
 138                         mp.lockedInt++
 139                         mp.lockedExt = 0
 140
 141                         mp.lockedg.set(newg)
 142                         newg.lockedm.set(mp)
 143                         gp.lockedm = 0
 144                 }
 145
 146                 // Mark the calling goroutine as being at an async
 147                 // safe-point, since it has a few conservative frames
 148                 // at the bottom of the stack. This also prevents
 149                 // stack shrinks.
 150                 gp.asyncSafePoint = true
 151
 152                 // Stash newg away so we can execute it below (mcall's
 153                 // closure can't capture anything).
 154                 gp.schedlink.set(newg)
 155         })
 156
 157         // Switch to the new goroutine.
 158         mcall(func(gp *g) {
 159                 // Get newg.
 160                 newg := gp.schedlink.ptr()
 161                 gp.schedlink = 0
 162
 163                 // Park the calling goroutine.
 164                 if traceEnabled() {
 165                         traceGoPark(traceBlockDebugCall, 1)
 166                 }
 167                 casGToWaiting(gp, _Grunning, waitReasonDebugCall)
 168                 dropg()
 169
 170                 // Directly execute the new goroutine. The debug
 171                 // protocol will continue on the new goroutine, so
 172                 // it's important we not just let the scheduler do
 173                 // this or it may resume a different goroutine.
 174                 execute(newg, true)
 175         })
 176
 177         // We'll resume here when the call returns.
 178
 179         // Restore locked state.
 180         if lockedm {
 181                 mp := gp.m
 182                 mp.lockedExt = lockedExt
 183                 mp.lockedInt--
 184                 mp.lockedg.set(gp)
 185                 gp.lockedm.set(mp)
 186         }
 187
 188         gp.asyncSafePoint = false
 189 }
 190
 191 type debugCallWrapArgs struct {
 192         dispatch uintptr
 193         callingG *g
 194 }
 195
 196 // debugCallWrap1 is the continuation of debugCallWrap on the callee
 197 // goroutine.
 198 func debugCallWrap1() {
 199         gp := getg()
 200         args := (*debugCallWrapArgs)(gp.param)
 201         dispatch, callingG := args.dispatch, args.callingG
 202         gp.param = nil
 203
 204         // Dispatch call and trap panics.
 205         debugCallWrap2(dispatch)
 206
 207         // Resume the caller goroutine.
 208         getg().schedlink.set(callingG)
 209         mcall(func(gp *g) {
 210                 callingG := gp.schedlink.ptr()
 211                 gp.schedlink = 0
 212
 213                 // Unlock this goroutine from the M if necessary. The
 214                 // calling G will relock.
 215                 if gp.lockedm != 0 {
 216                         gp.lockedm = 0
 217                         gp.m.lockedg = 0
 218                 }
 219
 220                 // Switch back to the calling goroutine. At some point
 221                 // the scheduler will schedule us again and we'll
 222                 // finish exiting.
 223                 if traceEnabled() {
 224                         traceGoSched()
 225                 }
 226                 casgstatus(gp, _Grunning, _Grunnable)
 227                 dropg()
 228                 lock(&sched.lock)
 229                 globrunqput(gp)
 230                 unlock(&sched.lock)
 231
 232                 if traceEnabled() {
 233                         traceGoUnpark(callingG, 0)
 234                 }
 235                 casgstatus(callingG, _Gwaiting, _Grunnable)
 236                 execute(callingG, true)
 237         })
 238 }
 239
 240 func debugCallWrap2(dispatch uintptr) {
 241         // Call the dispatch function and trap panics.
 242         var dispatchF func()
 243         dispatchFV := funcval{dispatch}
 244         *(*unsafe.Pointer)(unsafe.Pointer(&dispatchF)) = noescape(unsafe.Pointer(&dispatchFV))
 245
 246         var ok bool
 247         defer func() {
 248                 if !ok {
 249                         err := recover()
 250                         debugCallPanicked(err)
 251                 }
 252         }()
 253         dispatchF()
 254         ok = true
 255 }