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/MathExtras.h"
15 #include "llvm/Support/raw_ostream.h"
16 #include <algorithm>
17 #include <cassert>
18
19 using namespace llvm;
20 using namespace llvm::safestack;
21
22 #define DEBUG_TYPE "safestacklayout"
23
24 static cl::opt<bool> ClLayout("safe-stack-layout",
25 cl::desc("enable safe stack layout"), cl::Hidden,
26 cl::init(true));
27
print(raw_ostream & OS)28 LLVM_DUMP_METHOD void StackLayout::print(raw_ostream &OS) {
29 OS << "Stack regions:\n";
30 for (unsigned i = 0; i < Regions.size(); ++i) {
31 OS << " " << i << ": [" << Regions[i].Start << ", " << Regions[i].End
32 << "), range " << Regions[i].Range << "\n";
33 }
34 OS << "Stack objects:\n";
35 for (auto &IT : ObjectOffsets) {
36 OS << " at " << IT.getSecond() << ": " << *IT.getFirst() << "\n";
37 }
38 }
39
addObject(const Value * V,unsigned Size,unsigned Alignment,const StackLifetime::LiveRange & Range)40 void StackLayout::addObject(const Value *V, unsigned Size, unsigned Alignment,
41 const StackLifetime::LiveRange &Range) {
42 StackObjects.push_back({V, Size, Alignment, Range});
43 ObjectAlignments[V] = Alignment;
44 MaxAlignment = std::max(MaxAlignment, Alignment);
45 }
46
AdjustStackOffset(unsigned Offset,unsigned Size,unsigned Alignment)47 static unsigned AdjustStackOffset(unsigned Offset, unsigned Size,
48 unsigned Alignment) {
49 return alignTo(Offset + Size, Alignment) - Size;
50 }
51
layoutObject(StackObject & Obj)52 void StackLayout::layoutObject(StackObject &Obj) {
53 if (!ClLayout) {
54 // If layout is disabled, just grab the next aligned address.
55 // This effectively disables stack coloring as well.
56 unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
57 unsigned Start = AdjustStackOffset(LastRegionEnd, Obj.Size, Obj.Alignment);
58 unsigned End = Start + Obj.Size;
59 Regions.emplace_back(Start, End, Obj.Range);
60 ObjectOffsets[Obj.Handle] = End;
61 return;
62 }
63
64 LLVM_DEBUG(dbgs() << "Layout: size " << Obj.Size << ", align "
65 << Obj.Alignment << ", range " << Obj.Range << "\n");
66 assert(Obj.Alignment <= MaxAlignment);
67 unsigned Start = AdjustStackOffset(0, Obj.Size, 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(R.Range)) {
79 // Find the next appropriate location.
80 Start = AdjustStackOffset(R.End, Obj.Size, 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(LastRegionEnd, Start, 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(LastRegionEnd, End, 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(&R, 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(&R, 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(Obj.Range);
128 if (End <= R.End)
129 break;
130 }
131
132 ObjectOffsets[Obj.Handle] = End;
133 }
134
computeLayout()135 void 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 std::stable_sort(StackObjects.begin() + 1, StackObjects.end(),
144 [](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