On Mon, Jun 07, 2021 at 12:25:38PM +0100, Dave P Martin wrote: > On Thu, Jun 03, 2021 at 07:04:31PM +0100, Catalin Marinas via Libc-alpha wrote: > > On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote: > > > On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote: > > > > Do we know how libcs will detect that they don't need to do the > > > > mprotect() calls? Do we need a detection mechanism at all? > > > > > > > > Ignoring certain errors from mprotect() when ld.so is trying to set > > > > PROT_BTI on the main executable's code pages is probably a reasonable, > > > > backwards-compatible compromise here, but it seems a bit wasteful. > > > > > > I think the theory was that they would just do the mprotect() calls and > > > ignore any errors as they currently do, or declare that they depend on a > > > new enough kernel version I guess (not an option for glibc but might be > > > for others which didn't do BTI yet). > > > > I think we discussed the possibility of an AT_FLAGS bit. Until recently, > > this field was 0 but it gained a new bit now. If we are to expose this > > to arch-specific things, it may need some reservations. Anyway, that's > > an optimisation that can be added subsequently. > > I suppose so, but AT_FLAGS doesn't seem appropriate somehow. > > I wonder why we suddenly start considering adding a flag to AT_FLAGS > every few months, when it had sat empty for decades. This may say > something about the current health of the kernel ABI, but I'm not sure > exactly what. > > I think having mprotect() fail in a predictable way may be preferable > for now: glibc still only needs to probe with a single call and could > cache the knowledge after that. Code outside libc / ld.so seems quite > unlikely to care about this. I think that's the expected approach for now. If anyone complains about an extra syscall, we can look into options but I wouldn't rush on doing something. > Any ideas on how we would document this behaviour? The kernel and libc > behaviour are 100% clear: you _are_ allowed to twiddle PROT_BTI on > executable mappings, and there is no legitimate (or even useful) reason > to disallow this. It's only systemd deliberately breaking the API that > causes the behaviour seem by "userspace" to vary. I don't think we can document all the filters that can be added on top various syscalls, so I'd leave it undocumented (or part of the systemd documentation). It was a user space program (systemd) breaking another user space program (well, anything with a new enough glibc). The kernel ABI was still valid when /sbin/init started ;). -- Catalin
