Re: [PATCH v7 0/6] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

[Ryan Roberts <ryan.roberts@arm.com>]

Hi Yang,

Sorry for the slow reply; I'm just getting back to this...

On 11/09/2025 23:03, Yang Shi wrote:
> Hi Ryan & Catalin,
> 
> Any more concerns about this? 

I've been trying to convince myself that your assertion that all users that set
the VM_FLUSH_RESET_PERMS also call set_memory_*() for the entire range that was
returned my vmalloc. I agree that if that is the contract and everyone is
following it, then there is no problem here.

But I haven't been able to convince myself...

Some examples (these might intersect with examples you previously raised):

1. bpf_dispatcher_change_prog() -> bpf_jit_alloc_exec() -> execmem_alloc() ->
sets VM_FLUSH_RESET_PERMS. But I don't see it calling set_memory_*() for rw_image.

2. module_memory_alloc() -> execmem_alloc_rw() -> execmem_alloc() -> sets
VM_FLUSH_RESET_PERMS (note that execmem_force_rw() is nop for arm64).
set_memory_*() is not called until much later on in module_set_memory(). Another
error in the meantime could cause the memory to be vfreed before that point.

3. When set_vm_flush_reset_perms() is set for the range, it is called before
set_memory_*() which might then fail to split prior to vfree.

But I guess as long as set_memory_*() is never successfully called for a
*sub-range* of the vmalloc'ed region, then for all of the above issues, the
memory must still be RW at vfree-time, so this issue should be benign... I think?

In summary this all looks horribly fragile. But I *think* it works. It would be
good to clean it all up and have some clearly documented rules regardless. But I
think that could be a follow up series.

> Shall we move forward with v8? 

Yes; Do you wnat me to post that or would you prefer to do it? I'm happy to do
it; there are a few other tidy ups in pageattr.c I want to make which I spotted.

> We can include the
> fix to kprobes in v8 or I can send it separately, either is fine to me.

Post it on list, and I'll also incorporate into the series.

> Hopefully we can make v6.18.

It's probably getting a bit late now. Anyway, I'll aim to get v8 out tomorrow or
Friday and we will see what Will thinks.

Thanks,
Ryan

> 
> Thanks,
> Yang
>