[Bug 47812] New: Cygwin's strace.exe crashes due to custom TLS corruption

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=47812

Bug ID: 47812
           Summary: Cygwin's strace.exe crashes due to custom TLS
                    corruption
           Product: Wine
           Version: 4.16
          Hardware: x86
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: -unknown
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

This is not (as far as I can see) a Wine bug, and it's certainly not worth
fixing, but I'm writing this down here so that someone doesn't go through the
same work to figure it out...

Cygwin, for whatever reason, reimplements TLS by storing things at a fixed
offset from the stack base (i.e. NtCurrentTeb()->Tib.StackBase). Somewhere
along the line this gets corrupted, specifically the _locale member.

I think the assumptions made at [1] with regard to alloca() are just bogus.
There's no guarantee that the returned memory will contain wholly the custom
TLS slot, and I'm not particularly sure why they expect it to remain untouched
after DLL_PROCESS_ATTACH returns.

[1] https://github.com/Alexpux/Cygwin/blob/master/winsup/cygwin/init.cc#L90