linux-kernel - Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

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Message-ID: <e84a82b8-b788-499c-be79-e6dcb64ac969@redhat.com>
Date: Thu, 25 Apr 2024 21:56:02 +0200
From: David Hildenbrand <david@...hat.com>
To: Guillaume Morin <guillaume@...infr.org>
Cc: oleg@...hat.com, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, muchun.song@...ux.dev
Subject: Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

On 25.04.24 17:19, Guillaume Morin wrote:
> On 24 Apr 23:00, David Hildenbrand wrote:
>>> One issue here is that FOLL_FORCE|FOLL_WRITE is not implemented for
>>> hugetlb mappings. However this was also on my TODO and I have a draft
>>> patch that implements it.
>>
>> Yes, I documented it back then and added sanity checks in GUP code to fence
>> it off. Shouldn't be too hard to implement (famous last words) and would be
>> the cleaner thing to use here once I manage to switch over to
>> FOLL_WRITE|FOLL_FORCE to break COW.
> 
> Yes, my patch seems to be working. The hugetlb code is pretty simple.
> And it allows ptrace and the proc pid mem file to work on the executable
> private hugetlb mappings.
> 
> There is one thing I am unclear about though. hugetlb enforces that
> huge_pte_write() is true on FOLL_WRITE in both the fault and
> follow_page_mask paths. I am not sure if we can simply assume in the
> hugetlb code that if the pte is not writable and this is a write fault
> then we're in the FOLL_FORCE|FOLL_WRITE case.  Or do we want to keep the
> checks simply not enforce it for FOLL_FORCE|FOLL_WRITE?
> 
> The latter is more complicated in the fault path because there is no
> FAULT_FLAG_FORCE flag.
> 

I just pushed something to
	https://github.com/davidhildenbrand/linux/tree/uprobes_cow

Only very lightly tested so far. Expect the worst :)

I still detest having the zapping logic there, but to get it all right I 
don't see a clean way around that.


For hugetlb, we'd primarily have to implement the 
mm_walk_ops->hugetlb_entry() callback (well, and FOLL_FORCE).

Likely vaddr and PAGE_SIZE in uprobe_write_opcode() would have to be 
expanded to cover the full hugetlb page.

-- 
Cheers,

David / dhildenb