On Thu, Aug 20, 2020 at 06:47:53PM +0200, Brendan Jackman wrote: > From: Paul Renauld <renauld@xxxxxxxxxx> > > LSMs have high overhead due to indirect function calls through > retpolines. This RPC proposes to replace these with static calls [1] typo: RFC > instead. Yay! :) > [...] > This overhead prevents the adoption of bpf LSM on performance critical > systems, and also, in general, slows down all LSMs. I'd be curious to see other workloads too. (Your measurements are a bit synthetic, mostly showing "worst case": one short syscall in a tight loop. I'm curious how much performance gain can be had -- we should still do it, it'll be a direct performance improvement, but I'm curious about "real world" impact too.) > [...] > Previously, the code for this hook would have looked like this: > > ret = DEFAULT_RET; > > for each cb in [A, B, C]: > ret = cb(args); <--- costly indirect call here > if ret != 0: > break; > > return ret; > > Static calls are defined at build time and are initially empty (NOP > instructions). When the LSMs are initialized, the slots are filled as > follows: > > slot idx content > |-----------| > 0 | | > |-----------| > 1 | | > |-----------| > 2 | call A | <-- base_slot_idx = 2 > |-----------| > 3 | call B | > |-----------| > 4 | call C | > |-----------| > > The generated code will unroll the foreach loop to have a static call for > each possible LSM: > > ret = DEFAULT_RET; > switch(base_slot_idx): > > case 0: > NOP > if ret != 0: > break; > // fallthrough > case 1: > NOP > if ret != 0: > break; > // fallthrough > case 2: > ret = A(args); <--- direct call, no retpoline > if ret != 0: > break; > // fallthrough > case 3: > ret = B(args); <--- direct call, no retpoline > if ret != 0: > break; > // fallthrough > > [...] > > default: > break; > > return ret; > > A similar logic is applied for void hooks. > > Why this trick with a switch statement? The table of static call is defined > at compile time. The number of hook callbacks that will be defined is > unknown at that time, and the table cannot be resized at runtime. Static > calls do not define a conditional execution for a non-void function, so the > executed slots must be non-empty. With this use of the table and the > switch, it is possible to jump directly to the first used slot and execute > all of the slots after. This essentially makes the entry point of the table > dynamic. Instead, it would also be possible to start from 0 and break after > the final populated slot, but that would require an additional conditional > after each slot. Instead of just "NOP", having the static branches perform a jump would solve this pretty cleanly, yes? Something like: ret = DEFAULT_RET; ret = A(args); <--- direct call, no retpoline if ret != 0: goto out; ret = B(args); <--- direct call, no retpoline if ret != 0: goto out; goto out; if ret != 0: goto out; out: return ret; > [...] > The number of available slots for each LSM hook is currently fixed at > 11 (the number of LSMs in the kernel). Ideally, it should automatically > adapt to the number of LSMs compiled into the kernel. Seems like a reasonable thing to do and could be a separate patch. > If there’s no practical way to implement such automatic adaptation, an > option instead would be to remove the panic call by falling-back to the old > linked-list mechanism, which is still present anyway (see below). > > A few special cases of LSM don't use the macro call_[int/void]_hook but > have their own calling logic. The linked-lists are kept as a possible slow > path fallback for them. I assume you mean the integrity subsystem? That just needs to be fixed correctly. If we switch to this, let's ditch the linked list entirely. Fixing integrity's stacking can be a separate patch too. > [...] > Signed-off-by: Paul Renauld <renauld@xxxxxxxxxx> > Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx> > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx> This implies a maintainership chain, with Paul as the sole author. If you mean all of you worked on the patch, include Co-developed-by: as needed[1]. -Kees [1] https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by -- Kees Cook
