| Date | Tue, 28 Jul 2020 13:05:12 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 8/8] x86/kaslr: Don't use 64-bit mem_vector for 32-bit kernel |
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* Arvind Sankar <nivedita@alum.mit.edu> wrote:
> Commit > f28442497b5c ("x86/boot: Fix KASLR and memmap= collision") > converted mem_vector type to use 64-bit on the 32-bit kernel as well, > based on Thomas's review [0]. However: > - the code still doesn't consistently use 64-bit types. For instance, > mem_avoid_overlap uses 32-bit types when checking for overlaps. This > is actually okay, as the passed in memory range will have been clipped > to below 4G, but it is difficult to reason about the safety of the > code. > - KASLR on 32-bit can't use memory above 4G anyway, so it's pointless > to keep track of ranges above 4G. > > Switch the type back to unsigned long, and use a helper function to clip > regions to 4G on 32-bit, when storing mem_avoid, immovable_mem, EFI, > E820 and command-line memory regions. > > [0] https://lore.kernel.org/linux-nvdimm/alpine.DEB.2.20.1701111246400.3579@nanos/
So why not just standardize all the calculations around u64, and for 32-bit kernels add in a debug check that double checks that it all remained within the first 32 bits?
It's much easier to argue about this if we don't have to worry about 32-bit overflows and if the types are simple and bitness invariant.
Yes, technically it's slightly bloated on 32-bit kernels, but maintainability of 32-bit kernels is the primary concern now ...
Thanks,
Ingo
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