Support for game DRM which overwrite the GS segment selector

[Alessandro Pignotti]

Hi everyone,

I'm trying to get a specific game which employs a seemingly custom
protection scheme to work. The DRM does various bad things as usual, but
a very bad one is manipulating to GS segment selector and setting it to
a NULL segment. The GS segment is used by libc though in various ways
(stack protection and syscall support, and probably others).

I managed to get the activation procedure to go further and further by
enclosing each offending syscall using the following 2 macros.

#define SAFE_GS_START \
    do { \
        wine_set_gs(ntdll_get_thread_data()->gs); \
        do

#define SAFE_GS_END \
        while(0); \
    } while(0)

Still, this method is very cumbersome since system calls happens in many
places even outside of ntdll. Fixing the GS is also needed to support
sigsetjmp which is used by wine's exception handling.

I'd like to ask for feedback about what would be a sane way of
supporting this application. A possible solution would be to modify
wine's segfault handler to check if the instruction has a GS prefix
(0x65 IIRC) and try to execute the instruction again after fixing the
GS.

Please keep me in CC since I'm not subscribed to the ML.

Regards,
Alessandro Pignotti