On Sat, Oct 11, 2014 at 03:34:29AM -0700, H. Peter Anvin wrote: > On 10/10/2014 08:14 PM, Baoquan He wrote: > >On 10/08/14 at 03:27pm, Vivek Goyal wrote: > >>On Wed, Oct 08, 2014 at 08:09:59AM -0700, H. Peter Anvin wrote: > > > >>>Sorry... this makes no sense. > >>> > >>>For x86-64, there is no direct connection between the physical and > >>>virtual address spaces that the kernel runs in... > >> > >>I am sorry I did not understand this one. I thought that initial > >>relocatable kernel implementaion did not have any direct connection > >>between virtual and physical address. One could load kernel anywhere > >>and kernel virtual address will not change and we will just adjust > >>page tables to map virtual address to right physical address. > >> > >>Now handle_relocation() stuff seems to introduce a close coupling > >>between physical and virtual address. So if kernel shifts by 16MB > >>in physical address space, then it will shift by equal amount > >>in virtual address space. So there seems to be a direct connection > >>between virtual and physical address space in this case. > > > >Yeah, it's exactly as Vivek said. > > > >Before kaslr was introduced, x86_64 kernel can be put anywhere, and > >always _text is 0xffffffff81000000. Meanwhile phys_base contains the > >offset between the compiled addr (namely 0x1000000) and kernel loaded > >addr. After kaslr implementation was added, as long as kernel loaded > >addr is different 0x1000000, it will call handle_relocations(). The > >offset now is added onto each symbols including _text and phys_base > >becomes 0. > > > >It's clearly showing that by checking /proc/kallsyms and value of > >phys_base. > > > > This really shouldn't have happened this way on x86-64. It has to happen > this way on i386, but I worry that this may be a serious misdesign in kaslr > on x86-64. I'm also wondering if there is any other fallout of this? I agree. On x86_64, we should stick to previous design and this new logic of performing relocations does not sound very clean and makes things very confusing. I am wondering that why couldn't we simply adjust page tables in case of kaslr on x86_64, instead of performing relocations. Thanks Vivek
