Re: bpf kernel code leaks internal error codes to userspace

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, May 29, 2024 at 12:08:34PM +0200, Lennart Poettering wrote:
> Hi!
> 
> It seems that the bpf code in the kernel sometimes leaks
> kernel-internal error codes, i.e. those from include/linux/errno.h
> into userspace (as opposed to those from
> include/uapi/asm-generic/errno.h which are public userspace facing
> API).
> 
> According to the comments from that internal header file: "These
> should never be seen by user programs."
> 
> Specifically, this is about ENOTSUPP, which userspace simply cannot
> handle, there's no error 524 defined in glibc or anywhere else.
> 
> We ran into this in systemd recently:
> 
> https://github.com/systemd/systemd/issues/32170#issuecomment-2076928761
> 
> (a google search reveals others were hit by this too)
> 
> We commited a work-around for this for now:
> 
> https://github.com/systemd/systemd/pull/33067
> 
> But it really sucks to work around this in userspace, this is a kernel
> internal definition after all, conflicting with userspace (where
> ENOTSUPP is just an alias for EOPNOTSUPP), hence not really fixable.
> 
> ENOSUPP is kinda useless anyway, since EOPNOTSUPP is pretty much
> equally expressive, and something userspace can actually handle.
> 
> Various kernel subsystems have been fixed over the years in similar
> situations. For example:
> 
> https://patchwork.kernel.org/project/linux-wireless/patch/20231211085121.3841b71c867d.Idf2ad01d9dfe8d6d6c352bf02deb06e49701ad1d@changeid/
> 
> or
> 
> https://patchwork.kernel.org/project/linux-media/patch/af5b2e8ac6695383111328267a689bcf1c0ecdb1.1702369869.git.sean@mess.org/
> 
> or
> 
> https://patchwork.ozlabs.org/project/linux-mtd/patch/20231129064311.272422-1-acelan.kao@canonical.com/
> 
> I think BPF should really fix that, too.

hm, I don't think we can change that, user space already depends
on those values and we'd break it with new value

it's unfortunate, but I don't think we can do much about that,
other than enforcing EOPNOTSUPP for new code

jirka

> 
> Or if you insist on leaking internals to userspace then please move
> the definition to the public "uapi" headers, but yikes you are in for
> some pain then given the conflicting userspace definitions.
> 
> Lennart
>