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Diffstat (limited to 'src/runtime/stack2.go')
-rw-r--r-- | src/runtime/stack2.go | 106 |
1 files changed, 106 insertions, 0 deletions
diff --git a/src/runtime/stack2.go b/src/runtime/stack2.go new file mode 100644 index 000000000..c3718c205 --- /dev/null +++ b/src/runtime/stack2.go @@ -0,0 +1,106 @@ +// Copyright 2011 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package runtime + +/* +Stack layout parameters. +Included both by runtime (compiled via 6c) and linkers (compiled via gcc). + +The per-goroutine g->stackguard is set to point StackGuard bytes +above the bottom of the stack. Each function compares its stack +pointer against g->stackguard to check for overflow. To cut one +instruction from the check sequence for functions with tiny frames, +the stack is allowed to protrude StackSmall bytes below the stack +guard. Functions with large frames don't bother with the check and +always call morestack. The sequences are (for amd64, others are +similar): + + guard = g->stackguard + frame = function's stack frame size + argsize = size of function arguments (call + return) + + stack frame size <= StackSmall: + CMPQ guard, SP + JHI 3(PC) + MOVQ m->morearg, $(argsize << 32) + CALL morestack(SB) + + stack frame size > StackSmall but < StackBig + LEAQ (frame-StackSmall)(SP), R0 + CMPQ guard, R0 + JHI 3(PC) + MOVQ m->morearg, $(argsize << 32) + CALL morestack(SB) + + stack frame size >= StackBig: + MOVQ m->morearg, $((argsize << 32) | frame) + CALL morestack(SB) + +The bottom StackGuard - StackSmall bytes are important: there has +to be enough room to execute functions that refuse to check for +stack overflow, either because they need to be adjacent to the +actual caller's frame (deferproc) or because they handle the imminent +stack overflow (morestack). + +For example, deferproc might call malloc, which does one of the +above checks (without allocating a full frame), which might trigger +a call to morestack. This sequence needs to fit in the bottom +section of the stack. On amd64, morestack's frame is 40 bytes, and +deferproc's frame is 56 bytes. That fits well within the +StackGuard - StackSmall bytes at the bottom. +The linkers explore all possible call traces involving non-splitting +functions to make sure that this limit cannot be violated. +*/ + +const ( + // StackSystem is a number of additional bytes to add + // to each stack below the usual guard area for OS-specific + // purposes like signal handling. Used on Windows and on + // Plan 9 because they do not use a separate stack. + _StackSystem = _Windows*512*ptrSize + _Plan9*512 + + // The minimum size of stack used by Go code + _StackMin = 2048 + + // The minimum stack size to allocate. + // The hackery here rounds FixedStack0 up to a power of 2. + _FixedStack0 = _StackMin + _StackSystem + _FixedStack1 = _FixedStack0 - 1 + _FixedStack2 = _FixedStack1 | (_FixedStack1 >> 1) + _FixedStack3 = _FixedStack2 | (_FixedStack2 >> 2) + _FixedStack4 = _FixedStack3 | (_FixedStack3 >> 4) + _FixedStack5 = _FixedStack4 | (_FixedStack4 >> 8) + _FixedStack6 = _FixedStack5 | (_FixedStack5 >> 16) + _FixedStack = _FixedStack6 + 1 + + // Functions that need frames bigger than this use an extra + // instruction to do the stack split check, to avoid overflow + // in case SP - framesize wraps below zero. + // This value can be no bigger than the size of the unmapped + // space at zero. + _StackBig = 4096 + + // The stack guard is a pointer this many bytes above the + // bottom of the stack. + _StackGuard = 512 + _StackSystem + + // After a stack split check the SP is allowed to be this + // many bytes below the stack guard. This saves an instruction + // in the checking sequence for tiny frames. + _StackSmall = 128 + + // The maximum number of bytes that a chain of NOSPLIT + // functions can use. + _StackLimit = _StackGuard - _StackSystem - _StackSmall +) + +// Goroutine preemption request. +// Stored into g->stackguard0 to cause split stack check failure. +// Must be greater than any real sp. +// 0xfffffade in hex. +const ( + _StackPreempt = uintptrMask & -1314 + _StackFork = uintptrMask & -1234 +) |