On Wed, Mar 19, 2014 at 03:05:51PM +0800, WANG Chao wrote: > On 03/17/14 at 08:56am, Vivek Goyal wrote: > > On Fri, Mar 14, 2014 at 04:20:18PM +0800, WANG Chao wrote: > > > With kASLR enabled (CONFIG_RANDOMIZED_BASE=y), kernel virtual address > > > base is PAGE_OFFSET plus a randomized offset from 0 to 1G. > > > > > > Current kexec-tools gets kernel vaddr and size from /proc/kcore. It > > > assumes kernel vaddr start/end is within the range [0,512M). If kaslr > > > enabled, kernel vaddr start/end will stay at [0+offset, 512M+offset). > > NACK this patch myself. > > There are several mistakes I made. I misunderstood some concepts. Then I > realize this kASLR issue is not trivial to fix. > > I think if kexec-tools needs to get kernel text mapping from kcore in > kALSR case, the max base offset (CONFIG_RANDOMIZE_MAX_BASE_OFFSET) of > the kernel text area must be exposed to userspace some where. We can use > that value to determine which area is for kernel text mapping and which > is for modules text mapping. How about looking at /proc/kallsyms and look at address of one of the symbols say _text. And search for the ELF header in kcore which contains _text address and that's ELF header representing kernel text mapping. That way you don't have to worry about the value of CONFIG_RANDOMIZE_MAX_BASE_OFFSET. > > > > > Hi Chao, > > > > Documentation/x86/x86_64/mm.txt still says that kernel text mapping area > > is 512MB. > > > > ffffffff80000000 - ffffffffa0000000 (=512 MB) kernel text mapping, from > > phys 0 > > > > So has that changed now due to kASLR. > > Yes, with kASLR enabled, kernel text mapping is as following > > ffffffff80000000 - (ffffffff80000000+CONFIG_RANDOMIZE_BASE_MAX_OFFSET) > > That said, if using CONFIG_RANDOMIZE_BASE_MAX_OFFSET=0x40000000 by > default, the kernel text mapping is as following: > > ffffffff80000000 - ffffffffc0000000 > Agreed. help text for CONFIG_RANDOMIZE_BASE_MAX_OFFSET says following. On 64-bit this is limited by how the kernel fixmap page table is positioned, so this cannot be larger than 1GiB currently. Without RANDOMIZE_BASE, there is a 512MiB to 1.5GiB split between kernel and modules. When RANDOMIZE_BASE_MAX_OFFSET is above 512MiB, the modules area will shrink to compensate, up to the current maximum 1GiB to 1GiB split. The default is 1GiB. Thanks Vivek
