On Fri, Feb 14, 2025 at 5:33 PM Harry (Hyeonggon) Yoo
[off-list ref] wrote:

On Thu, Feb 13, 2025 at 11:20:22AM +0800, Huacai Chen wrote:

quoted

Hi, Harry,

On Wed, Feb 12, 2025 at 11:39 PM Harry (Hyeonggon) Yoo
[off-list ref] wrote:

quoted

On Wed, Feb 12, 2025 at 11:17 PM Huacai Chen [off-list ref] wrote:

quoted

Hibernation assumes the memory layout after resume be the same as that
before sleep, but CONFIG_RANDOM_KMALLOC_CACHES breaks this assumption.

Could you please elaborate what do you mean by
hibernation assumes 'the memory layout' after resume be the same as that
before sleep?

I don't understand how updating random_kmalloc_seed breaks resuming from
hibernation. Changing random_kmalloc_seed affects which kmalloc caches
newly allocated objects are from, but it should not affect the objects that are
already allocated (before hibernation).

When resuming, the booting kernel should switch to the target kernel,
if the address of switch code (from the booting kernel) is the
effective data of the target kernel, then the switch code may be
overwritten.

Hmm... I'm still missing some pieces.
How is the kernel binary overwritten when slab allocations are randomized?

Also, I'm not sure if it's even safe to assume that the memory layout is the
same across boots. But I'm not an expert on swsusp anyway...

It'd be really helpful for linux-pm folks to clarify 1) what are the
(architecture-independent) assumptions are for swsusp to work, and
2) how architectures dealt with other randomization features like kASLR...

I'm sorry to confuse you. Binary overwriting is indeed caused by
kASLR, so at least on LoongArch we should disable kASLR for
hibernation.

Random kmalloc is another story, on LoongArch it breaks smpboot when
resuming, the details are:
1, LoongArch uses kmalloc() family to allocate idle_task's
stack/thread_info and other data structures.
2, If random kmalloc is enabled, idle_task's stack in the booting
kernel may be other things in the target kernel.
3, When CPU0 executes the switch code, other CPUs are executing
idle_task, and their stacks may be corrupted by the switch code.

So in experiments we can fix hibernation only by moving
random_kmalloc_seed initialization after smp_init(). But obviously,
moving it after all initcalls is harmless and safer.


Huacai

quoted

For LoongArch there is an additional problem: the regular kernel
function uses absolute address to call exception handlers, this means
the code calls to exception handlers should at the same address for
booting kernel and target kernel.

--
Harry