On Fri, 2016-08-19 at 13:42 +0100, Leif Lindholm wrote: > On Fri, Aug 19, 2016 at 12:50:49PM +0100, Ben Hutchings wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > everything using mozilla-js). > > > > [...] > > > > > > > > Could we possibly work around that by reducing > > > > CONFIG_ARCH_MMAP_RND_BITS_MAX? (That's not directly configurable; it > > > > requires patching arch/arm64/Kconfig.) > > > > > > I think this would be opening up a real can of worms. Not all sizes > > > are supported by the architecture, and only certain VA_BITS/pagesize > > > combinations work in the kernel. > > > > > > We could switch to 42-bit VA, but that would require switching to 64K > > > pagesize, which would be an even huger can. > > > > I'm not suggesting using any unusual page table configuration. Just > > reducing the ASLR range that is currently implied by a 48-bit VA. > > But would that help anything? > Even if you don't allocate to the top bits, if they're used for > tagging, you'll still segfault. I seem to remember that AArch64 has the ill-advised rule that VA bits outside the range of the current page table format are ignored, so presumably you're concerned that the code relies on this. But since other 64-bit architectures (at least x86, PowerPC and SPARC) behave otherwise, I would expect semi-portable code to mask out the tag bits. Ben. -- Ben Hutchings It's easier to fight for one's principles than to live up to them.
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