On Thu, Sep 27, 2018 at 05:00:31PM -0600, Chris Murphy wrote: > On Thu, Sep 27, 2018 at 10:38 AM, Daniel Kiper <dki...@net-space.pl> wrote: > > On Wed, Sep 26, 2018 at 04:45:51PM -0600, Chris Murphy wrote: > >> On Fri, Sep 21, 2018 at 12:35 PM, Daniel Kiper <dki...@net-space.pl> wrote: > >> > On Tue, Sep 18, 2018 at 01:40:06PM -0600, Chris Murphy wrote: > >> >> On Mon, Sep 17, 2018 at 10:59 PM, Chris Murphy > >> >> <li...@colorremedies.com> wrote: > >> >> > Hi, > >> >> > > >> >> > GRUB code can certainly read files that are on Btrfs, md devices, > >> >> > LUKS, LVM, and so on. But GRUB code can also write to the physical > >> >> > block for grubenv - but are there safe guards that prevent it from > >> >> > doing so if grubenv is on something like Btrfs, mdadm raid5, LUKS? > >> >> > > >> >> > And also what about XFS? This used to be safe, but now with reflink > >> >> > support, grubenv could be reflink copied, meaning any overwrite is > >> >> > disallowed and must be COW'd. How is that handled? > >> >> > > >> >> > I'm pretty sure on Btrfs GRUB knows is can't write to grubenv, I'm > >> >> > just curious about the other cases. > >> >> > >> >> OK so it allows me to create a grubenv on Btrfs without any complaint. > >> >> Will the bootloader actually try to write to this if grub.cfg contains > >> >> save_env? > >> >> > >> >> $ sudo grub2-editenv --verbose grubenv create > >> >> [sudo] password for chris: > >> >> [chris@f29h ~]$ ll > >> >> -rw-r--r--. 1 root root 1024 Sep 18 13:37 grubenv > >> >> [chris@f29h ~]$ stat -f grubenv > >> >> File: "grubenv" > >> >> ID: ac9ba8ecdce5b017 Namelen: 255 Type: btrfs > >> >> Block size: 4096 Fundamental block size: 4096 > >> >> Blocks: Total: 46661632 Free: 37479747 Available: 37422535 > >> >> Inodes: Total: 0 Free: 0 > >> >> [chris@f29h ~]$ > >> > > >> > There two things here. grub2-editenv should create grubenv properly > >> > and double check that space on disk is reserved. If it is not then > >> > it should complain. GRUB2 during boot should check was grubenv file > >> > properly created. If it was not it should not update grubenv and > >> > complain. > >> > > >> > I am not sure is anything like that implemented in GRUB2. If does > >> > not I am happy to see and review the patches. > >> > >> When I create a grubenv on Btrfs is does succeed and it's an inline > >> extent, so no mattter what it's checksummed. There is a message on the > >> next boot: > >> > >> error: ../../grub-core/commands/loadenv.c:215:sparse file not allowed. > >> > >> And the grubenv is not overwritten even though the grub.cfg is using > >> save_env and when this same grub.cfg is used on ext4 it does overwrite > >> grubenv. So... It appears loadenv.c must have some inhibitor for > >> writing to Btrfs, which is a good thing. > > > > Great! That is in line with what I said earlier. > > > >> I'm uncertain whether GRUB avoids writing to any other case (LUKS, md > >> RAID, LVM). In particular, XFS, because XFS now supports reflinks, so > >> strictly speaking even if overwriting 2 sectors does not cause > >> corruption today (no inline extent support yet), it probably should > >> refuse to write to XFS as well. > > > > Yep! > > > >> Anyway, I've got a couple of different opinions from XFS devel@ and > >> ext4 devel@ about this. My understanding is each file system (ext4, > >> XFS, Btrfs at least) have reserve areas that can reliably be written > >> to by the bootloader (pre-boot), and it seems like those need to be > >> used instead of depending on grubenv. > >> > >> https://www.spinics.net/lists/linux-ext4/msg62364.html > >> > >> https://www.spinics.net/lists/linux-xfs/msg21902.html > > > > If something like that exits I am happy to use it. However, I would not > > change user interface in any way. Everything should happen auto-magically > > from user POV. > > > > The user space interface could remain unchange, i.e. grub-editenv > behavior. However, the grubenv file itself is also user facing, isn't
Great! > it? The grubenv file probably can't be entirely deprecated because > some file systems don't have reserve areas for bootloaders. Whereas on I am OK with that. > a file system like Btrfs, there's at least two reserve areas, and > they're fairly large compared to other file systems (big enough to > embed a GRUB core.img) - so in that case would grubenv exist? I do not think that it is needed. Even it may make more confusion in such cases. So, probably no for btrfs and other filesystems which have reserved areas for bootloader. However, I think that docs also should be updated properly. > grub-editenv alone can properly negotiate this. Perfect! Daniel _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
