Commit `cb1fffb29e`

Alex Rønne Petersen <alex@alexrp.com>

2024-08-03 20:18:14

std.os.windows.tls: Set `AddressOfCallBacks` to `&__xl_a + 1`.

`__xl_a` is just a global variable containing a null function pointer. There's nothing magical about it or its name at all. The section names used on `__xl_a` and `__xl_b` (`.CRT$XLA` and `.CRT$XLZ`) are the real magic here. The compiler emits TLS variables into `.CRT$XL<x>` sections, where `x` is an uppercase letter between A and Z (exclusive). The linker then sorts those sections alphabetically (due to the `$`), and the result is a neat array of TLS initialization callbacks between `__xl_a` and `__xl_z`. That array is null-terminated, though! Normally, `__xl_z` serves as the null terminator; however, by pointing `AddressesOfCallBacks` to `__xl_a`, which just contains a null function pointer, we've effectively made it so that the PE loader will just immediately stop invoking TLS callbacks. Fix that by pointing to the first actual TLS callback instead (or `__xl_z` if there are none).

master

1 parent 1d8fca0

Changed files (1)

lib

std

windows

tls.zig

@@ -20,8 +20,6 @@ comptime {
 }
 
 // TODO this is how I would like it to be expressed
-// TODO also note, ReactOS has a +1 on StartAddressOfRawData and AddressOfCallBacks. Investigate
-// why they do that.
 //export const _tls_used linksection(".rdata$T") = std.os.windows.IMAGE_TLS_DIRECTORY {
 //    .StartAddressOfRawData = @intFromPtr(&_tls_start),
 //    .EndAddressOfRawData = @intFromPtr(&_tls_end),
@@ -35,7 +33,7 @@ pub const IMAGE_TLS_DIRECTORY = extern struct {
     StartAddressOfRawData: *?*anyopaque,
     EndAddressOfRawData: *?*anyopaque,
     AddressOfIndex: *u32,
-    AddressOfCallBacks: *windows.PIMAGE_TLS_CALLBACK,
+    AddressOfCallBacks: [*:null]windows.PIMAGE_TLS_CALLBACK,
     SizeOfZeroFill: u32,
     Characteristics: u32,
 };
@@ -43,7 +41,10 @@ export const _tls_used linksection(".rdata$T") = IMAGE_TLS_DIRECTORY{
     .StartAddressOfRawData = &_tls_start,
     .EndAddressOfRawData = &_tls_end,
     .AddressOfIndex = &_tls_index,
-    .AddressOfCallBacks = &__xl_a,
+    // __xl_a is just a global variable containing a null pointer; the actual callbacks sit in
+    // between __xl_a and __xl_z. So we need to skip over __xl_a here. If there are no callbacks,
+    // this just means we point to __xl_z (the null terminator).
+    .AddressOfCallBacks = @as([*:null]windows.PIMAGE_TLS_CALLBACK, @ptrCast(&__xl_a)) + 1,
     .SizeOfZeroFill = 0,
     .Characteristics = 0,
 };

Commit cb1fffb29e

Commit `cb1fffb29e`