On Wed, Feb 19, 2025 at 10:52:04AM -0800, Kalesh Singh wrote:
On Wed, Feb 19, 2025 at 1:17 AM Lorenzo Stoakes lorenzo.stoakes@oracle.com wrote:
On Wed, Feb 19, 2025 at 10:15:47AM +0100, David Hildenbrand wrote:
On 19.02.25 10:03, Lorenzo Stoakes wrote:
On Wed, Feb 19, 2025 at 12:25:51AM -0800, Kalesh Singh wrote:
On Thu, Feb 13, 2025 at 10:18 AM Lorenzo Stoakes lorenzo.stoakes@oracle.com wrote:
The guard regions feature was initially implemented to support anonymous mappings only, excluding shmem.
This was done such as to introduce the feature carefully and incrementally and to be conservative when considering the various caveats and corner cases that are applicable to file-backed mappings but not to anonymous ones.
Now this feature has landed in 6.13, it is time to revisit this and to extend this functionality to file-backed and shmem mappings.
In order to make this maximally useful, and since one may map file-backed mappings read-only (for instance ELF images), we also remove the restriction on read-only mappings and permit the establishment of guard regions in any non-hugetlb, non-mlock()'d mapping.
Hi Lorenzo,
Thank you for your work on this.
You're welcome.
Have we thought about how guard regions are represented in /proc/*/[s]maps?
This is off-topic here but... Yes, extensively. No they do not appear there.
I thought you had attended LPC and my talk where I mentioned this purposefully as a drawback?
I went out of my way to advertise this limitation at the LPC talk, in the original series, etc. so it's a little disappointing that this is being brought up so late, but nobody else has raised objections to this issue so I think in general it's not a limitation that matters in practice.
Sorry for raising this now, yes at the time I believe we discussed reducing the vma slab memory usage for the PROT_NONE mappings. I didn't imagine that apps could have dependencies on the mapped ELF ranges in /proc/self/[s]maps until recent breakages from a similar feature. Android itself doesn't depend on this but what I've seen is banking apps and apps that have obfuscation to prevent reverse engineering (the particulars of such obfuscation are a black box).
Ack ok fair enough, sorry, but obviously you can understand it's frustrating when I went to great lengths to advertise this not only at the talk but in the original series.
Really important to have these discussions early. Not that really we can do much about this, as inherently this feature cannot give you what you need.
Is it _only_ banking apps that do this? And do they exclusively read /proc/$pid/maps? I mean there's nothing we can do about that, sorry. If that's immutable, then unless you do your own very, very, very slow custom android maps implementation (that will absolutely break the /proc/$pid/maps scalability efforts atm) this is just a no-go.
In the field, I've found that many applications read the ranges from /proc/self/[s]maps to determine what they can access (usually related to obfuscation techniques). If they don't know of the guard regions it would cause them to crash; I think that we'll need similar entries to PROT_NONE (---p) for these, and generally to maintain consistency between the behavior and what is being said from /proc/*/[s]maps.
No, we cannot have these, sorry.
Firstly /proc/$pid/[s]maps describes VMAs. The entire purpose of this feature is to avoid having to accumulate VMAs for regions which are not intended to be accessible.
Secondly, there is no practical means for this to be accomplished in /proc/$pid/maps in _any_ way - as no metadata relating to a VMA indicates they have guard regions.
This is intentional, because setting such metadata is simply not practical
- why? Because when you try to split the VMA, how do you know which bit
gets the metadata and which doesn't? You can't without _reading page tables_.
Yeah the splitting becomes complicated with any vm flags for this... meaning any attempt to expose this in /proc/*/maps have to unconditionally walk the page tables :(
It's not really complicated, it's _impossible_ unless you made literally all VMA code walk page tables for every single operation. Which we are emphatically not going to do :)
And no, /proc/$pid/maps is _never_ going to walk page tables. For obvious performance reasons.
/proc/$pid/smaps _does_ read page tables, but we can't start pretending VMAs exist when they don't, this would be completely inaccurate, would break assumptions for things like mremap (which require a single VMA) and would be unworkable.
The best that _could_ be achieved is to have a marker in /proc/$pid/smaps saying 'hey this region has guard regions somewhere'.
And then simply expose it in /proc/$pid/pagemap, which is a better interface for this pte-level information inside of VMAs. We should still have a spare bit for that purpose in the pagemap entries.
Ah yeah thanks David forgot about that!
This is also a possibility if that'd solve your problems Kalesh?
I'm not sure what is the correct interface to advertise these. Maybe smaps as you suggested since we already walk the page tables there? and pagemap bit for the exact pages as well? It won't solve this particular issue, as 1000s of in field apps do look at this through /proc/*/maps. But maybe we have to live with that...
I mean why are we even considering this if you can't change this anywhere? Confused by that.
I'm afraid upstream can't radically change interfaces to suit this scenario.
We also can't change smaps in the way you want, it _has_ to still give output per VMA information.
The proposed change that would be there would be a flag or something indicating that the VMA has guard regions _SOMEWHERE_ in it.
Since this doesn't solve your problem, adds complexity, and nobody else seems to need it, I would suggest this is not worthwhile and I'd rather not do this.
Therefore for your needs there are literally only two choices here:
1. Add a bit to /proc/$pid/pagemap OR 2. a new interface.
I am not in favour of a new interface here, if we can just extend pagemap.
What you'd have to do is:
1. Find virtual ranges via /proc/$pid/maps 2. iterate through /proc/$pid/pagemaps to retrieve state for all ranges.
Since anything that would retrieve guard region state would need to walk page tables, any approach would be slow and I don't think this would be any less slow than any other interface.
This way you'd be able to find all guard regions all the time.
This is just the trade-off for this feature unfortunately - its whole design ethos is to allow modification of -faulting- behaviour without having to modify -VMA- behaviour.
But if it's banking apps whose code you can't control (surprised you don't lock down these interfaces), I mean is this even useful to you?
If your requirement is 'you have to change /proc/$pid/maps to show guard regions' I mean the answer is that we can't.
We can argue that such apps are broken since they may trip on the SIGBUS off the end of the file -- usually this isn't the case for the ELF segment mappings.
Or tearing of the maps interface, or things getting unmapped or or or... It's really not a sane thing to do.
This is still useful for other cases, I just wanted to get some ideas if this can be extended to further use cases.
Well I'm glad that you guys find it useful for _something_ ;)
Again this wasn't written only for you (it is broadly a good feature for upstream), but I did have your use case in mind, so I'm a little disappointed that it doesn't help, as I like to solve problems.
But I'm glad it solves at least some for you...
Thanks, Kalesh
This bit will be fought over haha
-- Cheers,
David / dhildenb