From: Bodo S. <bst...@fu...> - 2004-11-15 19:05:09
|
Blaisorblade wrote: > On Friday 12 November 2004 15:10, Bodo Stroesser wrote: > >>From: Bodo Stroesser <bst...@fu...> >> >>The patch needs some small corrections: >>1) local_using_sysemu must be sampled *before* the next >> ptrace(PTRACE_SYSEMU/SYSCALL) and must stay the same until do_syscall() >> has been done. Currently it is sampled before do_syscall() and is used >> after this for ptrace(PTRACE_SYSEMU/SYSCALL). Even if no problem is >> visible to the UML user, a single syscall could be executed on the host >> when switching on sysemu. The result of this then is overwritten by the >> syscall execution in UML. > > >> Since the first event the tracer has to handle is not a syscall, it's >> enough to initialize local_using_sysemu to 0; > > > Sorry, what happens if the first signal it gets is a SIGTRAP, and so > local_using_sysemu is not yet set? If this is impossible, please add a > comment in the code for this. However, it seems that it can get to the > SIGTRAP case with tracing == 1. When beginning the procedure, it is 0, but it > can be changed with the value from is_tracing(task). I've not checked if that > is zeroed on process creation (i.e. by do_fork() calling copy_thread()), but > just note that in the code. OK: Let's summarize: 1) tracer() is started exactly once. 2) The first this it does, is starting the first ptraced-process via clone(). 3) Then it waits until the new process stops. 4) Since the process will run start_kernel() in kernel space, it is resumed with PTRACE_CONT. Thus, before having any syscall interception, the process has to stop itself with a SIGUSR1, giving the tracer an OP_TRACE_OP request. After this local_using_sysemu will be set and the process will be resumed with PTRACE_SYSCALL or PTRACE_SYSEMU. > > >>2) Even if the host never *does* a syscall in SYSEMU, we have to write the >> syscall number with -1, to not have the host doing syscall restarting. >> This would happen only with an invalid syscall number equal to one of >> the -ERESTART values. > > >> But to be perfect ... > > > Yes, but shouldn't this be handled on the host? Restarting a syscall which has > never been done does not seem something that SYSEMU should allow... I don't > want anybody to need going through the code and checking that this is safe. Yes. It should. So I exactly added this to the "advanced sysemu". But despite this UML should work on an older sysemu host, too. So I add this now, and with the SYSEMU_SINGLESTEP-patches a skip of writing -1 is inserted, if the new sysemu is in use. |