On 19/06/17 20:04, Jeff Law wrote: > On 06/19/2017 11:50 AM, Joseph Myers wrote: >> On Mon, 19 Jun 2017, Jeff Law wrote: >> >>> A key point to remember is that you can never have an allocation >>> (potentially using more than one allocation site) which is larger than a >>> page without probing the page. >> >> There's a platform ABI issue here. At least some kernel fixes for these >> stack issues, as I understand it, increase the size of the stack guard to >> more than a single page. It would be possible to define the ABI to >> require such a larger guard for protection and so reduce the number of >> (non-alloca/VLA-using) functions that need probes generated, depending on >> whether a goal is to achieve security on kernels without such a fix. >> (Thinking in terms of how to get to enabling such probes by default.) > On 32 bit platforms we don't have a lot of address space left, so we > have to be careful about creating too large of a guard. > > On 64 bit platforms we have a lot more freedom and I suspect larger > guards, mandated by the ABI would be useful, if for no other reason than > allowing us to allocate more stack without probing. A simple array of > PATH_MAX characters triggers probing right now. I suspect (but didn't > bother to confirm) that PATH_MAX array are what causes git to have so > many large stacks. > > Also if we look at something like ppc and aarch64, we've currently got > the PROBE_INTERVAL set to 4k. But in reality they're using much larger > page sizes. So we could improve things there as well. >
There are aarch64 linux systems using 4k pages for compatibility with existing aarch32 binaries. R. > > jeff >
