On Thu, Jan 27, 2022 at 6:27 PM Jann Horn [off-list ref] wrote:

I like that idea... but this_cpu_dec()/this_cpu_inc() use GS-relative
addressing, at least on X86-64, so NULL might make things worse, I
think? /proc/kallsyms on my machine starts with:

0000000000000000 A fixed_percpu_data
0000000000000000 A __per_cpu_start
0000000000001000 A cpu_debug_store
0000000000002000 A irq_stack_backing_store
0000000000006000 A cpu_tss_rw
000000000000b000 A gdt_page
000000000000c000 A exception_stacks
0000000000010000 A entry_stack_storage
0000000000011000 A espfix_waddr

So we'd probably need some different placeholder instead of NULL to
actually crash...

Orthogonal problem, maybe we should make sure the first page of
per-cpu data is un-mapped.