On Sat, Jan 17, 2015 at 01:08:11AM +0900, Roman Peniaev wrote: > On Sat, Jan 17, 2015 at 12:59 AM, Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxxx> wrote: > > On Sat, Jan 17, 2015 at 12:57:02AM +0900, Roman Peniaev wrote: > >> On Fri, Jan 16, 2015 at 7:54 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > >> > One interesting thing I noticed (which is unchanged by this series), > >> > but pulling ARM_r7 during the seccomp ptrace event shows __NR_poll, > >> > not __NR_restart_syscall, even though it was a __NR_restart_syscall > >> > trap from seccomp. Is there a better place to see the actual syscall? > >> > >> As I understand we do not push new r7 to the stack, and ptrace uses the > >> old value. > > > > And why should we push r7 to the stack? ptrace should be using the > > recorded system call number, rather than poking about on the stack > > itself. > > Probably we should not, but the behaviour comparing arm to x86 is different. We definitely should not, because changing the stacked value changes the value in r7 after the syscall has returned. We have guaranteed that the value will be preserved across syscalls for years, so we really should not be changing that. > Also there is no any way from userspace to figure out what syscall was > restarted, if you do not trace each syscall enter and exit from the > very beginning. Thinking about ptrace, that's been true for years. It really depends whether you consider the restart syscall a userspace thing or a kernelspace thing. When you consider that the vast majority of syscall restarts are done internally in the kernel, and we just re-issue the syscall, it immediately brings up the question "why is the restart block method different?" and "should the restart block method be visible to userspace?" IMHO, it is prudent not to expose kernel internals to userspace unless there is a real reason to, otherwise they become part of the userspace API. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html