Commit 975049e96e2245022a90368360fe7f3617e5f194 | zig

author	Luuk de Gram <luuk@degram.dev> 2022-01-11 20:35:44+01:00
committer	Luuk de Gram <luuk@degram.dev> 2022-01-11 20:35:44+01:00
log	975049e96e2245022a90368360fe7f3617e5f194
tree	3622e219db6c2ffa73f0541f4ec1fe101ab0cbb4
parent	f767f8e3dc158a74debdd288884458b860d2ebc9
signature	Commit is signed but in an unrecognized format.

wasm-ld: Append `--stack-first` by default

By placing the stack at the start of the memory section, we prevent the runtime
from silently overwriting the global declarations and instead trap.

We do however, allow users to overwrite this behavior by setting the global-base,
which puts the stack at the end of the memory section and the static data at the base that was specified.
The reason a user would want to do this, is when they are sure the stack will not overflow and they want
to decrease the binary size as the offsets to the static memory are generally smaller.
(Having the stack in front, means that accessing the memory after the stack has a bigger offset when loading/storing from memory).

1 files changed, 6 insertions(+), 4 deletions(-)

src
- link
  - Wasm.zig

src/link/Wasm.zig+6-4

 @@ -1243,6 +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;
 @@ -1294,10 +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");