Messages in this thread | | | Subject | Re: [RFC PATCH v16 0/6] mm: security: ro protection for dynamic data | From | Igor Stoppa <> | Date | Wed, 21 Feb 2018 11:56:22 +0200 |
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On 21/02/18 03:36, Dave Chinner wrote: > On Tue, Feb 20, 2018 at 03:56:00PM -0800, Matthew Wilcox wrote: >> On Wed, Feb 21, 2018 at 08:36:04AM +1100, Dave Chinner wrote: >>> FWIW, I'm not wanting to use it to replace static variables. All the >>> structures are dynamically allocated right now, and get assigned to >>> other dynamically allocated pointers. I'd likely split the current >>> structures into a "ro after init"
I would prefer to use a different terminology, because, if I have understood the use case, this is not exactly the same as __ro_after_init
So, this is my understanding:
* "const" needs to be known at link time - there might be some adjustments later on, ex: patching of "const" pointers, after relocation has taken place - I am assuming we are not planning to patch const data The compiler can perform whatever optimization it feels like and it is allowed to do, on this.
* __ro_after_init is almost the same as a const, from a r/w perspective, but it will become effectively read_only after the completion of the init phase. The compiler cannot use it in any way to detect errors, AFAIK. The system will just generate a runtime error is someone tries to alter some __ro_after_init data, when it's read-only. The only trick available is to use, after the protection, a different type of handle, const.
* pmalloc pools can be protected (hence the "p") at any time, but they start as r/w. Also, they cannot be declared statically.
* data which is either const or __ro_after_init is placed into specific sections (on arm64 it's actually the same) and its pages are then marked as read-only.
>>> structure and rw structure, so >>> how does the "__ro_after_init" attribute work in that case? Is it >>> something like this? >>> >>> struct xfs_mount { >>> struct xfs_mount_ro{ >>> ....... >>> } *ro __ro_after_init; > ^^^^^^^^ > > pointer, not embedded structure....
I doubt this would work, because I think it's not possible to put a field of a structure into a separate section, afaik.
__ro_after_init would refer to the ro field, not to the memory it refers to.
>>> ...... >> >> No, you'd do: >> >> struct xfs_mount_ro { >> [...] >> };
is this something that is readonly from the beginning and then shared among mount points or is it specific to each mount point?
>> struct xfs_mount { >> const struct xfs_mount_ro *ro; >> [...] >> }; > > .... so that's pretty much the same thing :P
The "const" modifier is a nice way to catch errors through the compiler, iff the ro data will not be initialized through this handle, when it's still writable.
>>> Also, what compile time checks are in place to catch writes to >>> ro structure members? Is sparse going to be able to check this sort >>> of thing, like is does with endian-specific variables? >> >> Just labelling the pointer const should be enough for the compiler to >> catch unintended writes. > > Ok.
yes, anyway the first one trying to alter it at run time, is in for some surprise.
>>>> I'd be interested to have your review of the pmalloc API, if you think >>>> something is missing, once I send out the next revision. >>> >>> I'll look at it in more depth when it comes past again. :P >> >> I think the key question is whether you want a slab-style interface >> or whether you want a kmalloc-style interface. I'd been assuming >> the former, but Igor has implemented the latter already. > > Slabs are rally only useful when you have lots of a specific type of > object. I'm concerned mostly about one-off per-mount point > structures, of which there are relatively few. A heap-like pool per > mount is fine for this.
That was my same sentiment.
Actually it would be even possible to simulate caches with pools: each pool supports a granularity parameter, during creation. One could have multiple pools, each with different granularity, but it would probably lead to a proliferation of pools.
Instead, I preferred to have pmalloc as a drop-in replacement for the variants of k/v/kv malloc.
The only real issue was the - previous - inability of tracking the size of an allocation, given its address, but that is taken care of by the patch for the genalloc bitmap.
If I could have a pointer to a good candidate for the pmalloc treatment, I could come up with a patch, to show how it could be done.
Then it might be easier to discuss if the API needs to be modified and/or extended somehow.
-- igor
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