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From some book I have, I have in my notes that the Linux kernel resides at 0xc00000000 on 32-bit systems, and is mapped into user space for performance reasons.

Is this accurate ? How can I verify this ?

Also, where does the kernel reside on 64-bit systems? Is it still at 0xc00000000, or somewhere else ?

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  • This might help: users.nccs.gov/~fwang2/linux/lk_addressing.txt
    – Nykakin
    Jan 10, 2013 at 12:36
  • I read somewhere that OS:s in general are placed low in memory, and this has to do with the placement of the interrupt vector. Jan 11, 2013 at 22:11
  • @EmanuelBerg, Where the interrupt vector is doesn't really matter.
    – vonbrand
    Jan 22, 2013 at 0:38
  • @vonbrand: No, I agree... I'll find the source for that and be back at you, would be interesting to know. Perhaps for historical reasons, when memory access wasn't "random"? Jan 22, 2013 at 1:15

2 Answers 2

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Physical memory

In physical memory the kernel usually resides at a random offset above 0x1000000. This depends on the kernel configuration options CONFIG_PHYSICAL_START, CONFIG_PHYSICAL_ALIGN and CONFIG_RANDOMIZE_BASE and the kernel command line options kaslr/nokaslr as described in the linux-insides book.

Virtual memory (32-bit)

On 32-bit architectures the virtual address space split depends on the kernel configuration option CONFIG_PAGE_OFFSET which is on many architectures set implicitly by the CONFIG_VMSPLIT_* options. The configuration for your kernel is usually put alongside the kernel image into /boot/config-<version>, so you can check for the offset where the kernel area in virtual memory begins with grep PAGE_OFFSET "/boot/config-$(uname -r)". The usual configuration for x86 is CONFIG_VMSPLIT_3G which sets the PAGE_OFFSET to 0xC0000000, reserving the upper 1 GiB of the address space for the kernel. For performance reasons this mapping is kept the same in all virtual address spaces, including all userspace processes.

The exact address where the kernel code starts is somewhere in this area and may be randomized on boot. That depends on the same configuration and kernel command line settings as the physical address randomization.

Virtual memory (64-bit)

On 64-bit architectures the situation is different because of the vastly larger address space.

The details are architecture-dependent but the kernel resides at a memory range at the top of the address space by convention.

x86-64

On x86-64 (a.k.a. AMD64) the memory layout is documented quite well. There are two possible configurations depending on the hardware support and the configuration option CONFIG_X86_5LEVEL:

Start (4-level paging) Size (4-level p.) Start (5-level paging) Size (5-level p.) Description
0xFFFF800000000000 8 TiB 0xFF00000000000000 4 PiB guard hole, also reserved for hypervisor
0xFFFF880000000000 0.5 TiB 0xFF10000000000000 0.25 PiB LDT remap for PTI
0xFFFF888000000000 64 TiB 0xFF11000000000000 32 PiB direct mapping of all physical memory
0xFFFFC90000000000 32 TB 0xFFA0000000000000 12.5 PiB vmalloc/ioremap space
0xFFFFEA0000000000 1 TB 0xFFD4000000000000 0.5 PiB virtual memory map
0xFFFFEC0000000000 16 TB 0xFFDF000000000000 8 PiB KASAN shadow memory
idential layout from here on
0xFFFFFE0000000000 0.5 TB cpu_entry_area mapping
0xFFFFFF0000000000 0.5 TB %esp fixup stacks
0xFFFFFFEF00000000 64 GB EFI region mapping space
0xFFFFFFFF80000000 512 MB kernel text mapping, mapped to physical address 0
0xFFFFFFFFA0000000 1520 MB module mapping space
FIXADDR_START ~0.5 MB kernel-internal fixmap range, variable size and offset
0xFFFFFFFFFF600000 4 kB legacy vsyscall ABI

So, the kernel area starts either at 0xFFFF800000000000 or 0xFF00000000000000 and the kernel code resides (usually at a randomized offset) in the area starting at 0xFFFFFFFF80000000.

ARM64

On 64-bit ARM the memory layout is somewhat similar to the layout on x86-64. The exact split depends on the kernel configuration (4kB/64kB pages, 2/3/4-level paging):

Page size Paging levels kernel area starting address
4 kB 3 0xFFFFFF8000000000
4 kB 4 0xFFFF000000000000
64 kB 2 0xFFFFFC0000000000
64 kB 3 0xFFFF000000000000
0

Right now it resides at a semi random location thanks to aslr (assuming you have that compiled in and with a 64 bit system there's no reason not to and since it's 2016 you should be running a 64 bit system.... if you are low on memory just run a 64 bit kernel + 32 bit userland)

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    You seem to imply aslr is dependent upon a 64-bit architecture, which is incorrect. Wrt running a 64-bit system, "should" is a bit strong, too - there are plenty of 32-bit SoCs available, or stocked up by manufacturers of long-lived programs. This isn't the Ubuntu SE. (Commenting in 2020 because it floated up to the top of the questions list for some reason this morning.)
    – Rich
    Jan 13, 2020 at 16:36

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