On Mon, Sep 12, 2022 at 4:25 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Mon, Sep 12, 2022, at 6:14 AM, Guo Ren wrote: > > On Mon, Sep 12, 2022 at 2:40 AM Arnd Bergmann <arnd@xxxxxxxx> wrote: > >> On Sun, Sep 11, 2022, at 1:35 AM, Guo Ren wrote: > >> > On Sun, Sep 11, 2022 at 12:07 AM Arnd Bergmann <arnd@xxxxxxxx> wrote: > >> >> > >> >> That sounds like a really bad idea, why would you want to risk > >> >> using such a small stack without CONFIG_VMAP_STACK? > >> >> > >> >> Are you worried about increased memory usage or something else? > >> > The requirement is from [1], and I think disabling CONFIG_VMAP_STACK > >> > would be the last step after serious testing. > >> > >> I still don't see why you need to turn off VMAP_STACK at all > >> if it works. The only downside I can see with using VMAP_STACK > >> on a 64-bit system is that it may expose bugs with device > >> drivers that do DMA to stack data. Once you have tested the > >> system successfully, you can also assume that you have no such > >> drivers. > > 1st, VMAP_STACK could be enabled&disabled in arch/Kconfig. If we don't > > force users to enable VMAP_STACK, why couldn't user adjust > > THREAD_SIZE? > > Turning off VMAP_STACK is harmless and may help debug issues > with VMAP_STACK itself. It's also required on architectures > that don't have KASAN_VMALLOC or something else that conflicts > with it. > > Changing THREAD_SIZE is also fine, as long as VMAP_STACK catches > the inevitable overflows. I would not object to having an > option that allows setting the stack size larger than the > default without VMAP_STACK, as long as setting it lower requires > using VMAP_STACK. That would however add a lot more complexity > and probably doesn't do what you want either. Thx for the detailed clarification, I agree with the point. I've put an EXPERT on config. > > > 2nd, VMAP_STACK is not free, we still need 1KB shadow_stack. > > The EXPERT is enough for your concern. > > It's actually more than the 1KB: you need both 1KB of shadow > stack and 4KB per CPU for the actual overflow_stack. If you > are micro-optimizing at this level, then a possible option > may be to change the handle_kernel_stack_overflow() function > to not preserve the task stack and just panic() without > showing the backtrace. That way you don't see which code > caused the issue, but at least you avoid corrupting random > data. Thx for the detailed explanation, the handle_kernel_stack_overflow() is a novel idea, which I will consider later. > > Arnd -- Best Regards Guo Ren
