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

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Message-ID: <f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.amperecomputing.com>
Date: Wed, 17 Sep 2025 10:21:17 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Ard Biesheuvel <ardb@...nel.org>, scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v7 0/6] arm64: support FEAT_BBM level 2 and large block
 mapping when rodata=full



On 9/17/25 9:28 AM, Ryan Roberts wrote:
> 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.

Yes, it doesn't call set_memory_*(). I spotted this in the earlier 
email. But it is actually RW, so it should be ok to miss the call. The 
later set_direct_map_invalid call in vfree() may fail, but 
set_direct_map_default call will set RW permission back. But I think it 
doesn't have to use execmem_alloc(), the plain vmalloc() should be good 
enough.

>
> 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.

IIUC, execmem_alloc_rw() is used to allocate memory for modules' text 
section and data section. The code will set mod->mem[type].is_rox 
according to the type of the section. It is true for text, false for 
data. Then set_memory_rox() will be called later if it is true *after* 
insns are copied to the memory. So it is still RW 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.

Yes, all call sites check the return value and bail out if 
set_memory_*() failed if I don't miss anything.

>
> 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?

Yes, it is true.

>
> 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.

Yeah, absolutely agreed.

>
>> 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.

I actually just had v8 ready in my tree. I removed pageattr_pgd_entry 
and pageattr_pud_entry in pageattr.c and fixed pmd_leaf/pud_leaf as you 
suggested. Is it the cleanup you are supposed to do? And I also rebased 
it on top of Shijie's series 
(https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=bfbbb0d3215f) 
which has been picked up by Will.

>
>> 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.

I can include it in v8 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.

Thank you. I can post v8 today.

Thanks,
Yang

>
> Thanks,
> Ryan
>
>> Thanks,
>> Yang
>>