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Re: [PATCH 0/2] aarch64: Enable pointer auth in PLT
On 24/06/2019 17:28, Florian Weimer wrote:
> * Szabolcs Nagy:
>
>> there is no easy way to mark binaries such that
>> old loader rejects them if it does not understand
>> the marking.
>
> There are symbol versions, new relocation types, and EI_ABIVERSION.
> For the latter, see libc-abis.
bumping EI_ABIVERSION may work for pac-plt, i'll look
into it (that will have to be documented in the elf abi).
an alternative is to leave the elf abi as is and solve this
in the bfd linker by -z pac-plt implying -u __pac_plt_supported
and adding __pac_plt_supported symbol to glibc.
(i.e. only solve this in the tooling, the magic symbol
ref is not required by the elf abi, just a safety net
provided by the binutils implementation, a huge hack,
but in practice it should be enough)
i think a new dynamic reloc requires too many changes
with non-trivial consequences.
>> the kernel api won't change since the architecture
>> does not make it easy to disable pauth per process.
>
> That's rather … strange. Do you know why things have to be this way?
> Since these instructions live in the NOP space, you end up with binaries
> that could not have been tested against hardware support for this
> feature for a long time. And without a per-process knob, you don't have
> a way to get this binaries running if a need for that arises.
i don't know the details, but the knob is in the sctlr_el1
system register that affects both EL1 and EL0 (kernel and
userspace) so if the kernel uses pauth then it cant just
disable it per process and there may be other reasons it
cannot be context switched efficiently. (in a kvm virtual
machine presumably you can turn it off)