Re: [PATCH v2] scripts/link-vmlinux.sh: Add alias to duplicate symbols for kallsyms

Nick Alcock
2023-07-20T13:01:23Z

On 19 Jul 2023, Alexander Lobakin verbalised:

> From: Nick Alcock <nick.alcock@oracle.com>
> Date: Wed, 19 Jul 2023 12:12:06 +0100
>>> Yes, please coordinate with Nick and review each other's work, now we
>>> have two separate efforts with different reasons but hopefully we'll
>
> Three efforts[0] :D Mine went unnoticed unfortunately, so I switched to
> other projects then.

It's odd, nobody seems to have noticed these until recently and now
suddenly people are crawling out of the woodwork wanting unique
addresses :) maybe the ambiguous ones are just getting commonplace
enough that they're biting people more often?

> My idea was to give relative path from the kernel root to the objfile,
> as we have a good bunch of non-unique "filename + symbol name" pairs.

I considered that, but unfortunately that has two problems to a raging
perfectionist like me:

- the objfile probably won't exist except if you're actually doing
kernel development, since kernel build trees are big enough that a
lot of people delete them after building or ship kernels to other
machines: if someone else built your kernel (overwhelmingly common
among non-kernel-devs) the objfiles are sure to be absent. (But an
option to not truncate the names when you know they won't be absent
might be a good idea, though this pushes space requirements up by
hundreds of kilobytes so it should probably be off by default.)

- even giving a path to the kernel module on disk (much lower
resolution and vulnerable to ambiguity again) is unreliable because
there's absolutely no guarantee that any given process can see any of
them: they might be in a different fs namespace or the modules might
only be present in an initramfs (hell, I even know setups which
*compile* the modules needed for rootfs mounting in the initramfs!
Yes this is borderline insane, yes it happens). More commonly, they
might be compressed using any of a number of compressors, changing
the name, and the kernel has no idea which compressor might have been
used (not unless you want it to go and look, and, well, wandering
around over the fs hunting down .ko.* files from kernelspace to get
their names right is *not* my idea of a good time! It's hard enough
to get that right from userspace, honestly, even with kmod helping.)

The most you could do would be to provide a key you could use with
kmod to dig the real modules out from userspace. Partial names are as
good as anything for that :)

So all the objfile names are, when it comes down to it, is names with no
intrinsic meaning: even if they're filenames of some kind, tools can't
rely on being able to access those files. (For my most common use case,
using a tracer on an enterprise-built production kernel, they'd almost
never be able to.)

So you might as well treat the objfile names as arbitrary string keys
that might be a memory-jogger for humans, which means you can chop
boring bits off them to save space :)