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[WebAssembly] Add --stack-first option which places the shadow stack at start of linear memory 

Fixes https://bugs.llvm.org/show_bug.cgi?id=37181

Differential Revision: https://reviews.llvm.org/D46141

llvm-svn: 331467
This commit is contained in:
Sam Clegg 2018-05-03 17:21:53 +00:00
parent 3c14663740
commit a0f095ebd7
5 changed files with 86 additions and 20 deletions
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42
lld/test/wasm/stack-first.test Normal file
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@ -0,0 +1,42 @@
; Test that the --stack-first option places the stack at the start of linear
; memory. In this case the --stack-first option is being passed along with a
; stack size of 512. This means (since the stack grows down) the stack pointer
; global should be initialized to 512.
RUN: llc -filetype=obj %p/Inputs/start.ll -o %t.o
RUN: wasm-ld --check-signatures -z stack-size=512 --stack-first --allow-undefined -o %t.wasm %t.o
RUN: obj2yaml %t.wasm | FileCheck %s
CHECK: - Type: GLOBAL
CHECK-NEXT: Globals:
CHECK-NEXT: - Index: 0
CHECK-NEXT: Type: I32
CHECK-NEXT: Mutable: true
CHECK-NEXT: InitExpr:
CHECK-NEXT: Opcode: I32_CONST
CHECK-NEXT: Value: 512
CHECK-NEXT: - Index: 1
CHECK-NEXT: Type: I32
CHECK-NEXT: Mutable: false
CHECK-NEXT: InitExpr:
CHECK-NEXT: Opcode: I32_CONST
CHECK-NEXT: Value: 512
CHECK-NEXT: - Index: 2
CHECK-NEXT: Type: I32
CHECK-NEXT: Mutable: false
CHECK-NEXT: InitExpr:
CHECK-NEXT: Opcode: I32_CONST
CHECK-NEXT: Value: 512
CHECK-NEXT: - Type: EXPORT
CHECK-NEXT: Exports:
CHECK-NEXT: - Name: memory
CHECK-NEXT: Kind: MEMORY
CHECK-NEXT: Index: 0
CHECK-NEXT: - Name: __heap_base
CHECK-NEXT: Kind: GLOBAL
CHECK-NEXT: Index: 1
CHECK-NEXT: - Name: __data_end
CHECK-NEXT: Kind: GLOBAL
CHECK-NEXT: Index: 2

1
lld/wasm/Config.h
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@ -29,6 +29,7 @@ struct Configuration {
bool Relocatable;
bool StripAll;
bool StripDebug;
bool StackFirst;
uint32_t GlobalBase;
uint32_t InitialMemory;
uint32_t MaxMemory;

1
lld/wasm/Driver.cpp
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@ -298,6 +298,7 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
Config->SearchPaths = args::getStrings(Args, OPT_L);
Config->StripAll = Args.hasArg(OPT_strip_all);
Config->StripDebug = Args.hasArg(OPT_strip_debug);
Config->StackFirst = Args.hasArg(OPT_stack_first);
errorHandler().Verbose = Args.hasArg(OPT_verbose);
ThreadsEnabled = Args.hasFlag(OPT_threads, OPT_no_threads, true);

3
lld/wasm/Options.td
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@ -124,6 +124,9 @@ def max_memory: J<"max-memory=">,
def no_entry: F<"no-entry">,
HelpText<"Do not output any entry point">;
def stack_first: F<"stack-first">,
HelpText<"Place stack at start of linear memory rather than after data">;
// Aliases
def alias_entry_e: JoinedOrSeparate<["-"], "e">, Alias<entry>;
def alias_entry_entry: J<"entry=">, Alias<entry>;

59
lld/wasm/Writer.cpp
View File

@ -580,22 +580,48 @@ void Writer::writeSections() {
// Fix the memory layout of the output binary. This assigns memory offsets
// to each of the input data sections as well as the explicit stack region.
// The memory layout is as follows, from low to high.
// The default memory layout is as follows, from low to high.
//
// - initialized data (starting at Config->GlobalBase)
// - BSS data (not currently implemented in llvm)
// - explicit stack (Config->ZStackSize)
// - heap start / unallocated
//
// The --stack-first option means that stack is placed before any static data.
// This can be useful since it means that stack overflow traps immediately rather
// than overwriting global data, but also increases code size since all static
// data loads and stores requires larger offsets.
void Writer::layoutMemory() {
uint32_t MemoryPtr = 0;
MemoryPtr = Config->GlobalBase;
log("mem: global base = " + Twine(Config->GlobalBase));
createOutputSegments();
uint32_t MemoryPtr = 0;
auto PlaceStack = [&]() {
if (Config->Relocatable)
return;
MemoryPtr = alignTo(MemoryPtr, kStackAlignment);
if (Config->ZStackSize != alignTo(Config->ZStackSize, kStackAlignment))
error("stack size must be " + Twine(kStackAlignment) + "-byte aligned");
log("mem: stack size = " + Twine(Config->ZStackSize));
log("mem: stack base = " + Twine(MemoryPtr));
MemoryPtr += Config->ZStackSize;
WasmSym::StackPointer->Global->Global.InitExpr.Value.Int32 = MemoryPtr;
log("mem: stack top = " + Twine(MemoryPtr));
};
if (Config->StackFirst) {
PlaceStack();
} else {
MemoryPtr = Config->GlobalBase;
log("mem: global base = " + Twine(Config->GlobalBase));
}
uint32_t DataStart = MemoryPtr;
// Arbitrarily set __dso_handle handle to point to the start of the data
// segments.
if (WasmSym::DsoHandle)
WasmSym::DsoHandle->setVirtualAddress(MemoryPtr);
WasmSym::DsoHandle->setVirtualAddress(DataStart);
for (OutputSegment *Seg : Segments) {
MemoryPtr = alignTo(MemoryPtr, Seg->Alignment);
@ -609,22 +635,15 @@ void Writer::layoutMemory() {
if (WasmSym::DataEnd)
WasmSym::DataEnd->setVirtualAddress(MemoryPtr);
log("mem: static data = " + Twine(MemoryPtr - Config->GlobalBase));
log("mem: static data = " + Twine(MemoryPtr - DataStart));
// Stack comes after static data and bss
if (!Config->StackFirst)
PlaceStack();
// Set `__heap_base` to directly follow the end of the stack or global data.
// The fact that this comes last means that a malloc/brk implementation
// can grow the heap at runtime.
if (!Config->Relocatable) {
MemoryPtr = alignTo(MemoryPtr, kStackAlignment);
if (Config->ZStackSize != alignTo(Config->ZStackSize, kStackAlignment))
error("stack size must be " + Twine(kStackAlignment) + "-byte aligned");
log("mem: stack size = " + Twine(Config->ZStackSize));
log("mem: stack base = " + Twine(MemoryPtr));
MemoryPtr += Config->ZStackSize;
WasmSym::StackPointer->Global->Global.InitExpr.Value.Int32 = MemoryPtr;
log("mem: stack top = " + Twine(MemoryPtr));
// Set `__heap_base` to directly follow the end of the stack. We don't
// allocate any heap memory up front, but instead really on the malloc/brk
// implementation growing the memory at runtime.
WasmSym::HeapBase->setVirtualAddress(MemoryPtr);
log("mem: heap base = " + Twine(MemoryPtr));
}