* Topi Miettinen: > +3 Additionally enable full randomization of memory mappings created > + with mmap(NULL, ...). With 2, the base of the VMA used for such > + mappings is random, but the mappings are created in predictable > + places within the VMA and in sequential order. With 3, new VMAs > + are created to fully randomize the mappings. > + > + Also mremap(..., MREMAP_MAYMOVE) will move the mappings even if > + not necessary and the location of stack and vdso are also > + randomized. > + > + On 32 bit systems this may cause problems due to increased VM > + fragmentation if the address space gets crowded.
Isn't this a bit of an understatement? I think you'll have to restrict this randomization to a subregion of the entire address space, otherwise the reduction in maximum mapping size due to fragmentation will be a problem on 64-bit architectures as well (which generally do not support the full 64 bits for user-space addresses). > + On all systems, it will reduce performance and increase memory > + usage due to less efficient use of page tables and inability to > + merge adjacent VMAs with compatible attributes. In the worst case, > + additional page table entries of up to 4 pages are created for > + each mapping, so with small mappings there's considerable penalty. The number 4 is architecture-specific, right? Thanks, Florian -- Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn, Commercial register: Amtsgericht Muenchen, HRB 153243, Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill
