Hi guys, (CC: +Steve, +Kristina) "What's the best way of letting user-space know the MMU config when 52-bit VA and pointer-auth may be in use?" On 13/02/2019 19:52, Kazuhito Hagio wrote: > On 2/13/2019 1:22 PM, James Morse wrote: >> On 13/02/2019 11:15, Dave Young wrote: >>> On 02/12/19 at 11:03pm, Kazuhito Hagio wrote: >>>> On 2/12/2019 2:59 PM, Bhupesh Sharma wrote: >>>>> BTW, in the makedumpfile enablement patch thread for ARMv8.2 LVA >>>>> (which I sent out for 52-bit User space VA enablement) (see [0]), Kazu >>>>> mentioned that the changes look necessary. >>>>> >>>>> [0]. http://lists.infradead.org/pipermail/kexec/2019-February/022431.html >>>> >>>>>>> The increased 'PTRS_PER_PGD' value for such cases needs to be then >>>>>>> calculated as is done by the underlying kernel >> >> Aha! Nothing to do with which-bits-are-pfn in the tables... >> >> You need to know if the top level PGD is 512bytes or bigger. As we use a >> kmem-cache the adjacent data could be some else's page tables. >> >> Is this really a problem though? You can't pull the user-space pgd pointers out >> of no-where, you must have walked some task_struct and struct_mm's to find them. >> In which case you would have the VMAs on hand to tell you if its in the mapped >> user range. >> >> It would be good to avoid putting something arch-specific in here if we can at >> all help it. >>>>>>> (see >>>>>>> 'arch/arm64/include/asm/pgtable-hwdef.h' for details): >>>>>>> >>>>>>> #define PTRS_PER_PGD (1 << (MAX_USER_VA_BITS - PGDIR_SHIFT)) >>>> >>>> Yes, this is the reason why makedumpfile needs the MAX_USER_VA_BITS. >>>> It is used for pgd_index() also in makedumpfile to walk page tables. >>>> >>>> /* to find an entry in a page-table-directory */ >>>> #define pgd_index(addr) (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1)) >>> >>> Since Dave mentioned crash tool does not need it, but crash should also >>> travel the pg tables. > > The crash utility is always invoked with vmlinux, so it can read the > vabits_user variable directly from vmcore, but makedumpfile can not. (This sounds fragile. That symbol's name may change, it may disappear completely! ... but I guess crash changes with every kernel release anyway) >>> If this is really necessary it would be good to describe what will >>> happen without the patch, eg. some user visible error from an actual test etc. >> >> Yes please, it would really help if there was a specific example we could discuss. > > With 52-bit user space and 48-bit kernel space configuration, > makedumpfile will not be able to convert a virtual kernel address > to a physical address, and fail to capture a dumpfile, because the > pgd_index() will return a wrong index. Got it, thanks! (all this user stuff had me thinking it was user-space you were trying to walk). Yes, this is because of commit e842dfb5a2d3 ("arm64: mm: Offset TTBR1 to allow 52-bit PTRS_PER_PGD"). The kernel has offset the ttbr1 value, if you try and walk it without knowing the offset you get junk. Ideally we tell you the offset with some 'ttbr1_offset=' in vmcoreinfo, but if the offsetting code disappears, the kernel would still have to provide 'ttbr1_offset=0' for user-space to keep working. I'd like to find something future-proof that always has an unambiguous meaning, and isn't a problem if the kernel variable/symbol/kconfig names change. With pointer-auth in use too you can't guess which bits are address and which bits are data. Taking arch-specific to its extreme, we could expose TCR_EL1, but this is a problem if we ever switch that per task (some new bits may turn up with a new feature). Some of those bits vary per cpu too, so we'd have to mask them out in case user-space tries to conclude something from them. My current best suggestion is to export: from core code: * USER_MMAP_END, the maximum value a user-space can try and mmap(). This would normally be TASK_SIZE, but x86 and powerpc also have support for larger VA space, and its plumbed into mm slightly differently. We should have one arch-independent property that covers all these. On arm64 this would be the runtime va bits for user-space's TTBR. (This assumes the value isn't per-task) arch specific: * ARM64_TCR.T1SZ, the va bits mapped by the kernel's TTBR. (We can assume we'll never flip user/kernel space). This has to be arch specific, it will always have a value and its meaning comes from the ARM-ARM (so linux can't change it in the future). It should be the same on every CPU. * ARM64_TTBR1.BADDR, the pa of the kernel page tables, which implicitly has the offset. Again this always has a value, and its meaning comes from the ARM-ARM. If we ever get clever with different page-tables/TCR values on different CPUs, these two should come from the same CPU. I think this gives you what you need if user/kernel may both be using pointer-auth and both may be using 52-bit va. I'm pretty sure the 48:52 bits can be picked at boot time depending on the kernel kconfig and the hardware support. Does anyone have a better idea? (or a corner where this won't work?) Thanks, James _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
