debugcall.go

Documentation: runtime

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