Hi again,
I've quickly implemented the aforementioned idea of fixing the segment
in the segfault handler when needed. I'm attaching my proposed patch.
Alessandro
Il giorno mer, 30/01/2013 alle 16.44 +0100, Alessandro Pignotti ha
scritto:
> 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
>From 35d5b875c7ca63845ddb68f1036f58d31ff4957e Mon Sep 17 00:00:00 2001
From: Alessandro Pignotti <a.pigno...@sssup.it>
Date: Wed, 30 Jan 2013 17:09:15 +0100
Subject: [PATCH] Restore the GS selector when crashing (if needed)
---
dlls/ntdll/signal_i386.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/dlls/ntdll/signal_i386.c b/dlls/ntdll/signal_i386.c
index 4a83a70..8e9e46b 100644
--- a/dlls/ntdll/signal_i386.c
+++ b/dlls/ntdll/signal_i386.c
@@ -1863,6 +1863,14 @@ static void segv_handler( int signal, siginfo_t *siginfo, void *sigcontext )
return;
}
+ /* special handling for segment faults of the GS segment */
+ unsigned char* opcode=(unsigned char*)EIP_sig(context);
+ if (get_trap_code(context) == TRAP_x86_PROTFLT && *opcode==0x65)
+ {
+ GS_sig(context)=ntdll_get_thread_data()->gs;
+ return;
+ }
+
rec = setup_exception_record( context, stack, fs, gs, raise_segv_exception );
if (rec->ExceptionCode == EXCEPTION_STACK_OVERFLOW) return;
--
1.7.10.4
