pstore/ftrace: Factor KASLR offset in the core kernel instruction addresses

The pstore ftrace frontend works by purely collecting the
instruction address, saving it on the persistent area through
the backend and when the log is read, on next boot for example,
the address is then resolved by using the regular printk symbol
lookup (%pS for example).

Problem: if we are running a relocatable kernel with KASLR enabled,
this is a recipe for failure in the symbol resolution on next boots,
since the addresses are offset'ed by the KASLR address. So, naturally
the way to go is factor the KASLR address out of instruction address
collection, and adding the fresh offset when resolving the symbol
on future boots.

Problem #2: modules also have varying addresses that float based
on module base address and potentially the module ordering in
memory, meaning factoring KASLR offset for them is useless.
So, let's hereby only take KASLR offset into account for core
kernel addresses, leaving module ones as is.

And we have yet a 3rd complexity: not necessarily the check range
for core kernel addresses holds true on future boots, since the
module base address will vary. With that, the choice was to mark
the addresses as being core vs module based on its MSB. And with
that...

...we have the 4th challenge here: for some "simple" architectures,
the CPU number is saved bit-encoded on the instruction pointer, to
allow bigger timestamps - this is set through the PSTORE_CPU_IN_IP
define for such architectures. Hence, the approach here is to skip
such architectures (at least in a first moment).

Finished? No. On top of all previous complexities, we have one
extra pain point: kaslr_offset() is inlined and fully "resolved"
at boot-time, after kernel decompression, through ELF relocation
mechanism. Once the offset is known, it's patched to the kernel
text area, wherever it is used. The mechanism, and its users, are
only built-in - incompatible with module usage. Though there are
possibly some hacks (as computing the offset using some kallsym
lookup), the choice here is to restrict this optimization to the
(hopefully common) case of CONFIG_PSTORE=y.

TL;DR: let's factor KASLR offsets on pstore/ftrace for core kernel
addresses, only when PSTORE is built-in and leaving module addresses
out, as well as architectures that define PSTORE_CPU_IN_IP.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Link: https://patch.msgid.link/20260410205848.2607169-1-gpiccoli@igalia.com
Signed-off-by: Kees Cook <kees@kernel.org>

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