1//===- SafeStackLayout.cpp - SafeStack frame layout -----------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "SafeStackLayout.h"
10#include "llvm/IR/Value.h"
11#include "llvm/Support/CommandLine.h"
12#include "llvm/Support/Compiler.h"
13#include "llvm/Support/Debug.h"
14#include "llvm/Support/raw_ostream.h"
15#include <algorithm>
16#include <cassert>
17
18using namespace llvm;
19using namespace llvm::safestack;
20
21#define DEBUG_TYPE "safestacklayout"
22
23static cl::opt<bool> ClLayout("safe-stack-layout",
24 cl::desc("enable safe stack layout"), cl::Hidden,
25 cl::init(Val: true));
26
27LLVM_DUMP_METHOD void StackLayout::print(raw_ostream &OS) {
28 OS << "Stack regions:\n";
29 for (unsigned i = 0; i < Regions.size(); ++i) {
30 OS << " " << i << ": [" << Regions[i].Start << ", " << Regions[i].End
31 << "), range " << Regions[i].Range << "\n";
32 }
33 OS << "Stack objects:\n";
34 for (auto &IT : ObjectOffsets) {
35 OS << " at " << IT.getSecond() << ": " << *IT.getFirst() << "\n";
36 }
37}
38
39void StackLayout::addObject(const Value *V, unsigned Size, Align Alignment,
40 const StackLifetime::LiveRange &Range) {
41 StackObjects.push_back(Elt: {.Handle: V, .Size: Size, .Alignment: Alignment, .Range: Range});
42 ObjectAlignments[V] = Alignment;
43 MaxAlignment = std::max(a: MaxAlignment, b: Alignment);
44}
45
46static unsigned AdjustStackOffset(unsigned Offset, unsigned Size,
47 Align Alignment) {
48 return alignTo(Size: Offset + Size, A: Alignment) - Size;
49}
50
51void StackLayout::layoutObject(StackObject &Obj) {
52 if (!ClLayout) {
53 // If layout is disabled, just grab the next aligned address.
54 // This effectively disables stack coloring as well.
55 unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
56 unsigned Start = AdjustStackOffset(Offset: LastRegionEnd, Size: Obj.Size, Alignment: Obj.Alignment);
57 unsigned End = Start + Obj.Size;
58 Regions.emplace_back(Args&: Start, Args&: End, Args&: Obj.Range);
59 ObjectOffsets[Obj.Handle] = End;
60 return;
61 }
62
63 LLVM_DEBUG(dbgs() << "Layout: size " << Obj.Size << ", align "
64 << Obj.Alignment.value() << ", range " << Obj.Range
65 << "\n");
66 assert(Obj.Alignment <= MaxAlignment);
67 unsigned Start = AdjustStackOffset(Offset: 0, Size: Obj.Size, Alignment: Obj.Alignment);
68 unsigned End = Start + Obj.Size;
69 LLVM_DEBUG(dbgs() << " First candidate: " << Start << " .. " << End << "\n");
70 for (const StackRegion &R : Regions) {
71 LLVM_DEBUG(dbgs() << " Examining region: " << R.Start << " .. " << R.End
72 << ", range " << R.Range << "\n");
73 assert(End >= R.Start);
74 if (Start >= R.End) {
75 LLVM_DEBUG(dbgs() << " Does not intersect, skip.\n");
76 continue;
77 }
78 if (Obj.Range.overlaps(Other: R.Range)) {
79 // Find the next appropriate location.
80 Start = AdjustStackOffset(Offset: R.End, Size: Obj.Size, Alignment: Obj.Alignment);
81 End = Start + Obj.Size;
82 LLVM_DEBUG(dbgs() << " Overlaps. Next candidate: " << Start << " .. "
83 << End << "\n");
84 continue;
85 }
86 if (End <= R.End) {
87 LLVM_DEBUG(dbgs() << " Reusing region(s).\n");
88 break;
89 }
90 }
91
92 unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
93 if (End > LastRegionEnd) {
94 // Insert a new region at the end. Maybe two.
95 if (Start > LastRegionEnd) {
96 LLVM_DEBUG(dbgs() << " Creating gap region: " << LastRegionEnd << " .. "
97 << Start << "\n");
98 Regions.emplace_back(Args&: LastRegionEnd, Args&: Start, Args: StackLifetime::LiveRange(0));
99 LastRegionEnd = Start;
100 }
101 LLVM_DEBUG(dbgs() << " Creating new region: " << LastRegionEnd << " .. "
102 << End << ", range " << Obj.Range << "\n");
103 Regions.emplace_back(Args&: LastRegionEnd, Args&: End, Args&: Obj.Range);
104 LastRegionEnd = End;
105 }
106
107 // Split starting and ending regions if necessary.
108 for (unsigned i = 0; i < Regions.size(); ++i) {
109 StackRegion &R = Regions[i];
110 if (Start > R.Start && Start < R.End) {
111 StackRegion R0 = R;
112 R.Start = R0.End = Start;
113 Regions.insert(I: &R, Elt: R0);
114 continue;
115 }
116 if (End > R.Start && End < R.End) {
117 StackRegion R0 = R;
118 R0.End = R.Start = End;
119 Regions.insert(I: &R, Elt: R0);
120 break;
121 }
122 }
123
124 // Update live ranges for all affected regions.
125 for (StackRegion &R : Regions) {
126 if (Start < R.End && End > R.Start)
127 R.Range.join(Other: Obj.Range);
128 if (End <= R.End)
129 break;
130 }
131
132 ObjectOffsets[Obj.Handle] = End;
133}
134
135void StackLayout::computeLayout() {
136 // Simple greedy algorithm.
137 // If this is replaced with something smarter, it must preserve the property
138 // that the first object is always at the offset 0 in the stack frame (for
139 // StackProtectorSlot), or handle stack protector in some other way.
140
141 // Sort objects by size (largest first) to reduce fragmentation.
142 if (StackObjects.size() > 2)
143 llvm::stable_sort(Range: drop_begin(RangeOrContainer&: StackObjects),
144 C: [](const StackObject &a, const StackObject &b) {
145 return a.Size > b.Size;
146 });
147
148 for (auto &Obj : StackObjects)
149 layoutObject(Obj);
150
151 LLVM_DEBUG(print(dbgs()));
152}
153