On 17.02.2017 02:08, Kees Cook wrote: > On Thu, Feb 16, 2017 at 2:25 PM, Pavel Machek <pavel@xxxxxx> wrote: >> Hi! >> >>> >>> -config DEBUG_RODATA >>> +config STRICT_KERNEL_RWX >>> bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX >>> depends on ARCH_HAS_STRICT_KERNEL_RWX >>> default !ARCH_OPTIONAL_KERNEL_RWX || >> >> Debug features are expected to have runtime cost, so kconfig help is >> silent about those. But there are runtime costs, right? It would be >> nice to mention them in the help text... > > It depends on the architecture. The prior help text for arm said: > > The tradeoff is that each region is padded to section-size (1MiB) > boundaries (because their permissions are different and splitting > the 1M pages into 4K ones causes TLB performance problems), which > can waste memory. > > parisc (somewhat inaccurately) said: > > This option may have a slight performance impact because a > portion of the kernel code won't be covered by a TLB anymore. The logic on parisc is actually: If huge page support is enabled, we map 1MB pages (and behave like arm wrt alignments). If huge page support is disabled we stay at 4k/PAGE_SIZE pages (without 1M alignment). > IIUC, arm64 does what parisc is hinting at: mappings at the end are > broken down to PAGE_SIZE. On parisc we never implemented that. > On x86, IIUC, there's actually no change to > TLB performance due to how the mappings are already set up. > > I'm not sure the best way to express this in the new help text. Do you > have some suggestions on wording? Personally, I don't really think > it's worth mentioning this in Kconfig help, I agree on this. > which, in theory, is > supposed to limit how technical it gets. And I think the performance > impact is almost entirely negligible compared to the risks addressed. Helge -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
