[Mageia-dev] Grub2 vs. Grub Legacy in M3

Frank Griffin ftg at roadrunner.com
Tue Jan 29 13:48:36 CET 2013

On 01/28/2013 06:44 PM, Barry Jackson wrote:
> My limited understanding is that the code in the 512 byte PBR has to 
> use block lists to find the core image in /boot, since it is too small 
> to understand filesystems. This is fragile in that filesystems and 
> file utilities may move files around on disk invalidating the block 
> lists, and for this reason the method is discouraged.
> A far as I know the same potential problem exists with grub legacy.

OK, we've come full circle.  This is why I was used to rerunning 
install.sh, because historically this has been true of most 
bootloaders.  They all have to fit in 512 bytes, and they all have to 
load the next stage of the boot without support for filesystems.

That's why I stick with chainloading.  PBRs don't move around at the 
whim of a filesystem, and if you do something to a root partition that 
repositions something critical, you just rebuild the PBR as part of it.

I wasn't aware of the MBR gap.  I guess you're saying that the core.img 
that fits in it *is* filesystem-aware ?  Otherwise, it seems like you've 
just pushed the locate-the-next-stage vulnerability from the MBR to the 
MBR gap, as chainloading pushes it from the MBR to the PBR.

Anyway, my point still stands: for anyone who just wants to have grub 
and grub2 partitons coexist on the same disk with either one in the MBR, 
chainloading will accomplish this without any downside that isn't 
already present in grub legacy.  Grub2 may be the way of the future, but 
to *require* it to own the MBR is just misleading.

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