> On May 30, 2020, at 5:26 PM, Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> wrote: > > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > >>>> On May 29, 2020, at 11:00 PM, Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> wrote: >>> >>> Modern Windows applications are executing system call instructions >>> directly from the application's code without going through the WinAPI. >>> This breaks Wine emulation, because it doesn't have a chance to >>> intercept and emulate these syscalls before they are submitted to Linux. >>> >>> In addition, we cannot simply trap every system call of the application >>> to userspace using PTRACE_SYSEMU, because performance would suffer, >>> since our main use case is to run Windows games over Linux. Therefore, >>> we need some in-kernel filtering to decide whether the syscall was >>> issued by the wine code or by the windows application. >> >> Do you really need in-kernel filtering? What if you could have >> efficient userspace filtering instead? That is, set something up so >> that all syscalls, except those from a special address, are translated >> to CALL thunk where the thunk is configured per task. Then the thunk >> can do whatever emulation is needed. > > Hi, > > I suggested something similar to my customer, by using > libsyscall-intercept. The idea would be overwritting the syscall > instruction with a call to the entry point. I'm not a specialist on the > specifics of Windows games, (cc'ed Paul Gofman, who can provide more > details on that side), but as far as I understand, the reason why that > is not feasible is that the anti-cheat protection in games will abort > execution if the binary region was modified either on-disk or in-memory. > > Is there some mechanism to do that without modiyfing the application? I’m suggesting that the kernel learn how to help you, maybe like this: prctl(PR_SET_SYSCALL_THUNK, target, address_of_unredirected_syscall, 0, 0, 0, 0); This would be inherited on clone/fork and cleared on execve. > >> Getting the details and especially the interaction with any seccomp >> filters that may be installed right could be tricky, but the performance >> should be decent, at least on non-PTI systems. >> >> (If we go this route, I suspect that the correct interaction with >> seccomp is that this type of redirection takes precedence over seccomp >> and seccomp filters are not invoked for redirected syscalls. After all, >> a redirected syscall is, functionally, not a syscall at all.) >> > > > -- > Gabriel Krisman Bertazi
