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Stage2: wasm-linker - Place stack at the beginning of the linear memory #10572

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merged 2 commits into from
Jan 12, 2022
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Stage2: wasm-linker - Place stack at the beginning of the linear memory #10572

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f767f8e
wasm: Place the stack at the start of memory
Luukdegram Jan 11, 2022
975049e
wasm-ld: Append `--stack-first` by default
Luukdegram Jan 11, 2022
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38 changes: 26 additions & 12 deletions src/link/Wasm.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -494,9 +494,21 @@ fn setupMemory(self: *Wasm) !void {
log.debug("Setting up memory layout", .{});
const page_size = 64 * 1024;
const stack_size = self.base.options.stack_size_override orelse page_size * 1;
const stack_alignment = 16;
var memory_ptr: u64 = self.base.options.global_base orelse 1024;
memory_ptr = std.mem.alignForwardGeneric(u64, memory_ptr, stack_alignment);
const stack_alignment = 16; // wasm's stack alignment as specified by tool-convention
// Always place the stack at the start by default
// unless the user specified the global-base flag
var place_stack_first = true;
var memory_ptr: u64 = if (self.base.options.global_base) |base| blk: {
place_stack_first = false;
break :blk base;
} else 0;

if (place_stack_first) {
memory_ptr = std.mem.alignForwardGeneric(u64, memory_ptr, stack_alignment);
memory_ptr += stack_size;
// We always put the stack pointer global at index 0
self.globals.items[0].init.i32_const = @bitCast(i32, @intCast(u32, memory_ptr));
}

var offset: u32 = @intCast(u32, memory_ptr);
for (self.segments.items) |*segment, i| {
Expand All @@ -510,8 +522,11 @@ fn setupMemory(self: *Wasm) !void {
offset += segment.size;
}

memory_ptr = std.mem.alignForwardGeneric(u64, memory_ptr, stack_alignment);
memory_ptr += stack_size;
if (!place_stack_first) {
memory_ptr = std.mem.alignForwardGeneric(u64, memory_ptr, stack_alignment);
memory_ptr += stack_size;
self.globals.items[0].init.i32_const = @bitCast(i32, @intCast(u32, memory_ptr));
}

// Setup the max amount of pages
// For now we only support wasm32 by setting the maximum allowed memory size 2^32-1
Expand Down Expand Up @@ -554,9 +569,6 @@ fn setupMemory(self: *Wasm) !void {
self.memories.limits.max = @intCast(u32, max_memory / page_size);
log.debug("Maximum memory pages: {d}", .{self.memories.limits.max});
}

// We always put the stack pointer global at index 0
self.globals.items[0].init.i32_const = @bitCast(i32, @intCast(u32, memory_ptr));
}

fn resetState(self: *Wasm) void {
Expand Down Expand Up @@ -1231,6 +1243,12 @@ fn linkWithLLD(self: *Wasm, comp: *Compilation) !void {
if (self.base.options.global_base) |global_base| {
const arg = try std.fmt.allocPrint(arena, "--global-base={d}", .{global_base});
try argv.append(arg);
} else {
// We prepend it by default, so when a stack overflow happens the runtime will trap correctly,
// rather than silently overwrite all global declarations. See https://github.com/ziglang/zig/issues/4496
//
// The user can overwrite this behavior by setting the global-base
try argv.append("--stack-first");
}

var auto_export_symbols = true;
Expand Down Expand Up @@ -1282,10 +1300,6 @@ fn linkWithLLD(self: *Wasm, comp: *Compilation) !void {
const arg = try std.fmt.allocPrint(arena, "stack-size={d}", .{stack_size});
try argv.append(arg);

// Put stack before globals so that stack overflow results in segfault immediately
// before corrupting globals. See https://github.com/ziglang/zig/issues/4496
try argv.append("--stack-first");

if (self.base.options.wasi_exec_model == .reactor) {
// Reactor execution model does not have _start so lld doesn't look for it.
try argv.append("--no-entry");
Expand Down