Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

[David Hildenbrand]

On 16.07.25 19:38, Lorenzo Stoakes wrote:
> The madvise() logic is inexplicably performed in mm/mseal.c - this ought
> to be located in mm/madvise.c.
>
> Additionally can_modify_vma_madv() is inconsistently named and, in
> combination with is_ro_anon(), is very confusing logic.
>
> Put a static function in mm/madvise.c instead - can_madvise_modify() -
> that spells out exactly what's happening.  Also explicitly check for an
> anon VMA.
>
> Also add commentary to explain what's going on.
>
> Essentially - we disallow discarding of data in mseal()'d mappings in
> instances where the user couldn't otherwise write to that data.
>
> Shared mappings are always backed, so no discard will actually truly
> discard the data.  Read-only anonymous and MAP_PRIVATE file-backed
> mappings are the ones we are interested in.
>
> We make a change to the logic here to correct a mistake - we must disallow
> discard of read-only MAP_PRIVATE file-backed mappings, which previously we
> were not.
>
> The justification for this change is to account for the case where:
>
> 1. A MAP_PRIVATE R/W file-backed mapping is established.
> 2. The mapping is written to, which backs it with anonymous memory.
> 3. The mapping is mprotect()'d read-only.
> 4. The mapping is mseal()'d.

Thinking about this a bit (should have realized this implication 
earlier) ... assuming we have:

1. A MAP_PRIVATE R/O file-backed mapping.
2. The mapping is mseal()'d.

We only really have anon folios in there with things like (a) uprobe (b) 
debugger access (c) similarly weird FOLL_FORCE stuff.

Now, most executables/libraries are mapped that way. If someone would 
rely on MADV_DONTNEED to zap pages in there (to free up memory), that 
would get rejected.

Does something like that rely on MADV_DONTNEED working? Good question.

Checking for anon_vma in addition, ad mentioned in the other thread, 
would be a "cheap" check to rule out that there are currently anon vmas 
in there.

Well, not 100% reliable, because MADV_DONTNEED can race with page faults ...

--
Cheers,

David / dhildenb