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

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`__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).
This commit is contained in:
Alex Rønne Petersen 2024-08-03 20:18:14 +02:00
parent 1d8fca0060
commit cb1fffb29e
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1 changed files with 5 additions and 4 deletions
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9
lib/std/os/windows/tls.zig
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@ -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,
};