On Fri, 7 Oct 2022 at 15:00, Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx> wrote: > > First of all, thanks Ard for the historical explanation! > > > On 07/10/2022 06:11, Ard Biesheuvel wrote: > > [...] > >> I think it'd be great to make it configurable! Ard, do you have any > >> sense of what the max/min, etc, should be here? > >> > > > > Given that dbx on an arbitrary EFI system with secure boot enabled is > > already almost 4k, that seems like a reasonable default. As for the > > upper bound, there is no way to know what weird firmware bugs you > > might tickle by choosing highly unusual values here. > > > > If you need to store lots of data, you might want to look at [0] for > > some work done in the past on using capsule update for preserving data > > across a reboot. In the general case, this is not as useful, as the > > capsule is only delivered to the firmware after invoking the > > ResetSystem() EFI runtime service (as opposed to SetVariable() calls > > taking effect immediately). However, if you need to capture large > > amounts of data, and can tolerate the uncertainty involved in the > > capsule approach, it might be a reasonable option. > > > > [0] https://lore.kernel.org/all/20200312011335.70750-1-qiuxu.zhuo@xxxxxxxxx/ > > So, you mean 4K as the default? I can change, but I would try to not > mess with the current users, is there a case you can imagine something > like 4k would fail? Maybe 2K is safer? > Reducing the number of writes 4x on systems that can support this has its own advantages, so I am willing to risk it. > As for the maximum, I've tested with many values, and when it's larger > than ~30000 for edk2/ovmf, it fails with EFI_OUT_OF_RESOURCES and > doesn't collect the log; other than that, no issues observed. > OVMF has OvmfPkg/OvmfPkgX64.dsc: gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x2000 OvmfPkg/OvmfPkgX64.dsc: gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x8400 where the first one is without secure boot and the second with secure boot. Interestingly, the default is gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x400 so this is probably where this 1k number comes from. So perhaps it is better to leave it at 1k after all :-( > When set to ~24000, the interesting is that we have fewer big logs in > /sys/fs/pstore, so it's nice to see compared to the bunch of 1K files heheh > > Anyway, let's agree on the default and then I can resubmit that, I'm > glad you both consider that it's a good idea =) > > Thanks, > > > Guilherme
