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. jeff
