On Sun, 24 Feb 2019 20:49:45 -0800 Andy Lutomirski <luto@xxxxxxxxxx> wrote: > On Sun, Feb 24, 2019 at 6:40 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > > On Sun, 24 Feb 2019 09:26:45 -0800 > > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > On Sun, Feb 24, 2019 at 7:18 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > > > > > > On Sat, 23 Feb 2019 20:38:03 -0800 > > > > Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > > > > > > > > > Can we just get rid of this might_sleep()? access_ok() doesn't sleep > > > > > as far as I know. > > > > > > > > Hmm, which might_sleep() would you pointed? What I talked was a > > > > WARN_ON_ONCE(!in_task()) in access_ok() on x86 (only!), and in_task() just > > > > checks preempt_count. > > > > > > So the in_task() check does kind of make sense. Using "access_ok()" > > > outside of task context is certainly an odd thing, for several > > > reasons. The main one being simply that outside of task context, the > > > whole "which task" question is open, and you don't know if the task is > > > the active one, and so it's not clear if whatever task you interrupt > > > might have done "set_fs()" or not. > > > > Ah I got it. Usual case access_ok() in IRQ handler is strange. > > > > > > > > So PeterZ isn't wrong: > > > > > > > I guess PeterZ assumed that access_ok() is used only with user space access > > > > APIs (e.g. copy_from_user) which can cause page-fault and locks mm (and might > > > > sleep :)), but now we are trying to use access_ok() with new functions which > > > > disables page-fault and just return -EFAULT. > > > > > > .. but in this case, if we do it all *within* code that saves and > > > restores the user access flag with get_fs/set_fs, access_ok() would be > > > ok and it doesn't have the above issue. > > > > > > So access_ok() in _general_ is absolutely not safe to do from > > > interrupts, but within the context of probing user memory from a > > > tracing event it just happens to be ok. > > > > Hmm, but user can specify user-memory access from the tracing event > > which is located in interrupt handler. So I understand that it is safe > > only if we correctly setup access flag with get_fs/set_fs, is that > > correct? > > > > > It would be lovely to have a special macro for this, and keep the > > > warning for the general case, but because this is a "every > > > architecture needs to build their own" it's probably too painful. > > > > Agreed. > > This should probably go with whatever effort makes nmi_uaccess_ok() > available on all architectures. That being said, how about just > making copy_from_user_nmi() work on all architectures, even if it just > fails unconditionally on some of them? I think even if we have copy_from_user_nmi(), we need something like nmi_uaccess_ok() because without it we can not correctly use __copy_from_user_inatomic()... Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
