Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol

[Will Deacon]

On Thu, May 23, 2019 at 09:41:40AM +0100, Ard Biesheuvel wrote:
> 
> 
> On 5/22/19 5:28 PM, Ard Biesheuvel wrote:
> > 
> > 
> > On 5/22/19 4:02 PM, Ard Biesheuvel wrote:
> > > The following commit
> > > 
> > >    7290d5809571 ("module: use relative references for __ksymtab entries")
> > > 
> > > updated the ksymtab handling of some KASLR capable architectures
> > > so that ksymtab entries are emitted as pairs of 32-bit relative
> > > references. This reduces the size of the entries, but more
> > > importantly, it gets rid of statically assigned absolute
> > > addresses, which require fixing up at boot time if the kernel
> > > is self relocating (which takes a 24 byte RELA entry for each
> > > member of the ksymtab struct).
> > > 
> > > Since ksymtab entries are always part of the same module as the
> > > symbol they export (or of the core kernel), it was assumed at the
> > > time that a 32-bit relative reference is always sufficient to
> > > capture the offset between a ksymtab entry and its target symbol.
> > > 
> > > Unfortunately, this is not always true: in the case of per-CPU
> > > variables, a per-CPU variable's base address (which usually differs
> > > from the actual address of any of its per-CPU copies) could be at
> > > an arbitrary offset from the ksymtab entry, and so it may be out
> > > of range for a 32-bit relative reference.
> > > 
> 
> (Apologies for the 3-act monologue)

Exposition, development and recapitulation ;)

> This turns out to be incorrect. The symbol address of per-CPU variables
> exported by modules is always in the vicinity of __per_cpu_start, and so it
> is simply a matter of making sure that the core kernel is in range for
> module ksymtab entries containing 32-bit relative references.
> 
> When running the arm64 with kaslr enabled, we currently randomize the module
> space based on the range of ADRP/ADD instruction pairs, which have a -/+ 4
> GB range rather than the -/+ 2 GB range of 32-bit place relative data
> relocations. So we can fix this by simply reducing the randomization window
> to 2 GB.

Makes sense. Do you see the need for an option to disable PREL relocs
altogether in case somebody wants the additional randomization range?

Will