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SubjectRe: [PATCH 3/3] x86_64: Use relative per-cpu offsets


On Sun, Nov 14, 2021, at 10:29 AM, Brian Gerst wrote:
> On Sun, Nov 14, 2021 at 6:03 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> On Sat, Nov 13, 2021 at 11:54:19PM -0500, Brian Gerst wrote:
>> > On Sat, Nov 13, 2021 at 8:18 PM Andy Lutomirski <luto@kernel.org> wrote:
>> > >
>> > >
>> > >
>> > > On Sat, Nov 13, 2021, at 4:40 AM, Brian Gerst wrote:
>> > > > The per-cpu section is currently linked at virtual address 0, because
>> > > > older compilers hardcoded the stack protector canary value at a fixed
>> > > > offset from the start of the GS segment. Use a standard relative offset
>> > > > as the GS base when the stack protector is disabled, or a newer compiler
>> > > > is used that supports a configurable location for the stack canary.
>> > >
>> > > Can you explain the benefit? Also, I think we should consider dropping support for the fixed model like we did on x86_32.
>> >
>> > This patch probably makes more sense if we drop the fixed model, as
>> > that gets rid of alot of code that works around having to link the
>> > percpu section differently.
>>
>> Can someone spell out these benefits please? To me having per-cpu start
>> at 0 makes perfect sense, how does not having that make things better?
>
> The best reason is that the percpu section is currently not subject to
> KASLR. It actually needs extra support to counter the effects of
> relocation. There have also been a number of linker bugs over the
> years that have had to be worked around.
>
> If we were to decide to drop the fixed stack protector the diffstat
> would look something like:
>
> arch/x86/Makefile | 19 ++--
> arch/x86/boot/compressed/misc.c | 12 ---
> arch/x86/entry/entry_64.S | 2 +-
> arch/x86/include/asm/percpu.h | 22 -----
> arch/x86/include/asm/processor.h | 24 ++---
> arch/x86/include/asm/stackprotector.h | 13 +--
> arch/x86/kernel/asm-offsets_64.c | 6 --
> arch/x86/kernel/cpu/common.c | 8 +-
> arch/x86/kernel/head_64.S | 11 ++-
> arch/x86/kernel/irq_64.c | 1 -
> arch/x86/kernel/vmlinux.lds.S | 33 -------
> arch/x86/tools/relocs.c | 143 +-----------------------------
> arch/x86/xen/xen-head.S | 10 +--
> scripts/gcc-x86_64-has-stack-protector.sh | 2 +-
> 14 files changed, 35 insertions(+), 271 deletions(-)
>

Ack. We did this for 32-bit and got few complaints. Let’s finish the job.

> --
> Brian Gerst

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