Re: [PATCH] bpf: say why a module BTF mismatch keeps the module from loading

[Alan Maguire]

On 02/09/2026 23:58, Paul Menzel wrote:
> With CONFIG_MODULE_ALLOW_BTF_MISMATCH=n – the default, and what Debian

Seems like a good change might be to change that to default y, given the
severity of the failure (module fails to load), along with the better 
diagnostics you mention below?

> ships – btf_module_notify() turns a split BTF mismatch into a fatal
> -EINVAL at MODULE_STATE_COMING, so the module does not load at all.
> All the user is told is:
> 
>     typec_displayport: module verification failed: signature and/or required key missing - tainting kernel
>     BPF: [124962] TYPEDEF
>     BPF: type_id=124988
>     BPF:
>     BPF: Invalid name
>     BPF:
>     failed to validate module [typec_displayport] BTF: -22
>     BPF: [124944] ENUM (anon)
>     BPF: size=4 vlen=32
>     BPF:
>     BPF: Invalid name
>     BPF:
>     failed to validate module [usb_storage] BTF: -22
>     BPF: [125157] TYPEDEF
>     BPF: type_id=125185
>     BPF:
>     BPF: Invalid name
>     BPF:
>     failed to validate module [usbhid] BTF: -22
> 
> That message names neither the consequence nor the cause. The report this
> comes from is a Dell XPS 13 9370 on Debian experimental where an apt run
> replaced /lib/modules/7.2-amd64/ underneath the running 7.2~rc7 kernel;
> experimental carries no ABI number, so the new build overwrites the tree
> of the running one. usbhid then refused to load and the keyboard and
> mouse behind a USB-C hub stayed dead, which reads as a USB regression
> rather than as stale modules. mii failing takes r8152 with it, and
> usb_storage takes USB mass storage, widening the confusion.
> 
> Note, the debug info is all that is stale here: nothing about the module
> code is wrong, and dropping the BTF would have let the machine carry on.
> Say so, and point at the escape hatch.
> 
> State the consequence on the per-module line, and add a once-only
> explanation naming the likely cause and CONFIG_MODULE_ALLOW_BTF_MISMATCH.
> Restrict the explanation to -EINVAL so that an -ENOMEM from
> btf_parse_module() is not blamed on stale modules. Keep it a
> pr_warn_once() because a single hotplug event produced 28 of these lines.
> 
> Assisted-by: claude-opus-5 (Claude Code)
> Signed-off-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>
> ---
>  kernel/bpf/btf.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index da36d4b9d31ab..720c56e8ae498 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -8525,9 +8525,11 @@ static int btf_module_notify(struct notifier_block *nb, unsigned long op,
>  		if (IS_ERR(btf)) {
>  			kfree(btf_mod);
>  			if (!IS_ENABLED(CONFIG_MODULE_ALLOW_BTF_MISMATCH)) {
> -				pr_warn("failed to validate module [%s] BTF: %ld\n",
> -					mod->name, PTR_ERR(btf));
>  				err = PTR_ERR(btf);
> +				pr_warn("failed to validate module [%s] BTF: %d; refusing to load it\n",
> +					mod->name, err);
> +				if (err == -EINVAL)
> +					pr_warn_once("module BTF does not match this kernel build; the modules on disk are most likely from a different build than the running kernel (kernel package upgraded without rebooting?). Build with CONFIG_MODULE_ALLOW_BTF_MISMATCH=y to load such modules without their BTF.\n");

A message suggesting actions is a great idea. I would mention a few possible paths forward, though admittedly it is a bit verbose. 
Something like

"module BTF does not match kernel vmlinux BTF; modules may have been built against a different kernel. Either build/use a kernel with CONFIG_MODULE_ALLOW_BTF_MISMATCH=y or rebuild the module against the current kernel or a closely-related kernel using 'make -C path/to/module' (this will ensure it includes a .BTF.base section which aids module BTF loading where some vmlinux/module mismatches occur)."


When built as an "out-of-tree" module we automatically add a .BTF.base section [1] that describes the base BTF types
the module relies on, then at load time the BTF gets relocated so it refers to those types in the running kernel.
It's designed to handle reasonably small deltas between kernel and module, so if the disparity is large it may not help.

[1] https://docs.kernel.org/bpf/btf.html#btf-base-section


>  			} else {
>  				pr_warn_once("Kernel module BTF mismatch detected, BTF debug info may be unavailable for some modules\n");
>  			}