On Mon, Sep 17, 2018 at 11:24 PM, Andrei Borzenkov <arvidj...@gmail.com> wrote: > 18.09.2018 07:21, Chris Murphy пишет: >> On Mon, Sep 17, 2018 at 9:44 PM, Chris Murphy <li...@colorremedies.com> >> wrote: >>> https://btrfs.wiki.kernel.org/index.php/FAQ#Does_grub_support_btrfs.3F >>> >>> Does anyone know if this is still a problem on Btrfs if grubenv has >>> xattr +C set? In which case it should be possible to overwrite and >>> there's no csums that are invalidated. >>> >>> I kinda wonder if in 2018 it's specious for, effectively out of tree >>> code, to be making modifications to the file system, outside of the >>> file system. >> >> a. The bootloader code (pre-boot, not user space setup stuff) would >> have to know how to read xattr and refuse to overwrite a grubenv >> lacking xattr +C. >> b. The bootloader code, would have to have sophisticated enough Btrfs >> knowledge to know if the grubenv has been reflinked or snapshot, >> because even if +C, it may not be valid to overwrite, and COW must >> still happen, and there's no way the code in GRUB can do full blow COW >> and update a bunch of metadata. >> >> So answering my own question, this isn't workable. And it seems the >> same problem for dm-thin. >> >> There are a couple of reserve locations in Btrfs at the start and I >> think after the first superblock, for bootloader embedding. Possibly >> one or both of those areas could be used for this so it's outside the >> file system. But other implementations are going to run into this >> problem too. >> > > That's what SUSE grub2 version does - it includes patches to redirect > writes on btrfs to reserved area. I am not sure how it behaves in case > of multi-device btrfs though.
The patches aren't upstream yet? Will they be? They redirect writes to grubenv specifically? Or do they use the reserved areas like a hidden and fixed location for what grubenv would contain? I guess the user space grub-editenv could write to grubenv, which even if COW, GRUB can pick up that change. But GRUB itself writes its changes to a reserved area. Hmmm. Complicated. -- Chris Murphy
