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

[Ard Biesheuvel]

On 5/22/19 5:28 PM, Ard Biesheuvel wrote:
> On 5/22/19 4:02 PM, Ard Biesheuvel wrote:
> > The following commit
> >
> > ÂÂ 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)

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.

So please disregard this patch (and the followup one against arch/x86/tools)

--
Ard.