On Thu, Feb 1, 2024 at 12:45 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > * Jeff Xu <jeffxu@xxxxxxxxxxxx> [240131 20:27]: > > On Wed, Jan 31, 2024 at 11:34 AM Liam R. Howlett > > <Liam.Howlett@xxxxxxxxxx> wrote: > > > > > Having to opt-in to allowing mseal will probably not work well. I'm leaving the opt-in discussion in Linus's thread. > Initial library mappings happen in one huge chunk then it's cut up into > smaller VMAs, at least that's what I see with my maple tree tracing. If > you opt-in, then the entire library will have to opt-in and so the > 'discourage inadvertent sealing' argument is not very strong. > Regarding "The initial library mappings happen in one huge chunk then it is cut up into smaller VMAS", this is not a problem. As example of elf loading (fs/binfmt_elf.c), there is just a few places to pass in what type of memory to be allocated, e.g. MAP_PRIVATE, MAP_FIXED_NOREPLACE, we can add MAP_SEALABLE at those places. If glic does additional splitting on the memory range, by using mprotect(), then the MAP_SEALABLE is automatically applied after splitting. If glic uses mmap(MAP_FIXED), then it should use mmap(MAP_FIXED|MAP_SEALABLE). > It also makes a somewhat messy tracking of inheritance of the attribute > across splitting, MAP_FIXED replacement, vma_move, vma_copy. I think > most of this is forced on the user? > The inheritance is the same as other VMA flags. > It makes your call less flexible, it means you have to hope that the VMA > origin was blessed before you decide you want to mseal it. > > What if you want to ensure the library mapped by a parent or on launch > is mseal'ed? > > What about the initial relocated VMA (expand/shrink of VMA)? > > Creating something as "non-sealable" is pointless. If you don't want it > sealed, then don't mseal() that region. > > If your use case doesn't need it, then can we please drop the opt-in > behaviour and just have all VMAs treated the same? > > If it does need it, can you explain why? > > The glibc relocation/fixup will then work. glibc could mseal once it is > complete - or an application could bypass glibc support and use the > feature itself. Yes. That is the idea. > > If we proceed to remove the MAP_SEALABLE flag to mmap, then we have the > heap/stack concerns. We can either let people shoot their own feet off > or try to protect them. > > Right now, you seem to be trying to protect them. Keeping with that, I > guess we could either get the kernel to mark those VMAs or tell some > other way? I'd suggest a range, but people do very strange things with > these special VMAs [1]. I don't think you can predict enough crazy > actions to make a difference in trying to protect people. > > There are far fewer VMAs that should not be allowed to be mseal'ed than > should be, and the kernel creates those so it seems logical to only let > the kernel opt-out on those ones. > > I'd rather just let people shoot themselves and return an error. > > I also hope it reduces the complexity of this code while increasing the > flexibility of the feature. As stated before, we remove the dependency > of needing support from the initial loader. > > Merging VMAs > I can see this going Very Bad with brk + mseal. But, again, if someone > decides to mseal these VMAs then they should expect Bad Things to > happen (or maybe they know what they are doing even in some complex > situation?) > > vma_merge() can also expand a VMA. I think this is okay as it checks > for the same flags, so you will allow VMA expansion of two (or three) > vma areas to become one. Is this okay in your model? > > > > > > I mean, you specifically state that this is a 'very specific > > > requirement' in your cover letter. Does this mean even other browsers > > > have no use for it? > > > > > No, I don’t mean “other browsers have no use for it”. > > > > About specific requirements from Chrome, that refers to "The lifetime > > of those mappings are not tied to the lifetime of the process, which > > is not the case of libc" as in the cover letter. This addition to the > > cover letter was made in V3, thus, it might be beneficial to provide > > additional context to help answer the question. > > > > This patch series begins with multiple-bit approaches (v1,v2,v3), the > > rationale for this is that I am uncertain if Chrome's specific needs > > are common enough for other use cases. Consequently, I am unable to > > make this decision myself without input from the community. To > > accommodate this, multiple bits are selected initially due to their > > adaptability. > > > > Since V1, after hearing from the community, Chrome has changed its > > design (no longer relying on separating out mprotect), and Linus > > acknowledged the defect of madvise(DONOTNEED) [1]. With those inputs, > > today mseal() has a simple design that: > > - meet Chrome's specific needs. > > How many VMAs will chrome have that are mseal'ed? Is this a common > operation? > > PROT_SEAL seems like an extra flag we could drop. I don't expect we'll > be sealing enough VMAs that a hand full of extra syscalls would make a > difference? > > > - meet Libc's needs. > > What needs of libc are you referring to? I'm looking through the > version changelog and I guess you mean return EPERM? > I meant libc's sealing RO part of the elf binary, those memory's lifetime are associated with the lifetime of the process. > > - Chrome's specific need doesn't interfere with Libc's. > > > > [1] https://lore.kernel.org/all/CAHk-=wiVhHmnXviy1xqStLRozC4ziSugTk=1JOc8ORWd2_0h7g@xxxxxxxxxxxxxx/ > > Linus said he'd be happier if we made the change in general. > > > > > > I am very concerned this feature will land and have to be maintained by > > > the core mm people for the one user it was specifically targeting. > > > > > See above. This feature is not specifically targeting Chrome. > > > > > Can we also get some benchmarking on the impact of this feature? I > > > believe my answer in v7 removed the worst offender, but since there is > > > no benchmarking we really are guessing (educated or not, hard data would > > > help). We still have an extra loop in madvise, mprotect_pkey, mremap_to > > > (and mreamp syscall?). > > > > > Yes. There is an extra loop in mmap(FIXED), munmap(), > > madvise(DONOTNEED), mremap(), to emulate the VMAs for the given > > address range. I suspect the impact would be low, but having some hard > > data would be good. I will see what I can find to assist the perf > > testing. If you have a specific test suite in mind, I can also try it. > > You should look at mmtests [2]. But since you are adding loops across > VMA ranges, you need to test loops across several ranges of VMAs. That > is, it would be good to see what happens on 1, 3, 6, 12, 24 VMAs, or > some subset of small and large numbers to get an idea of complexity we > are adding. My hope is that the looping will be cache-hot in the maple > tree and have minimum effect. > > In my personal testing, I've seen munmap often do a single VMA, or 3, or > more rare 7 on x86_64. There should be some good starting points in > mmtests for the common operations. > Thanks. Will do. > [1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/mmapstress/mmapstress03.c > [2] https://github.com/gormanm/mmtests > > Thanks, > Liam
