Christian Brauner
4684df8f97
Kernels before v6.16 (missing commit 1271a40eeafa "bpf: Allow access to const void pointer arguments in tracing programs") have a bug in btf_ctx_access() where const void * parameters in LSM hook signatures are not recognized as void pointers. The function checks t->type == 0 to detect void *, but for const void * the BTF chain is PTR -> CONST -> void, so t->type points to the CONST node rather than directly to type_id 0. This causes the verifier to reject any BPF program that reads the const void *value argument of bdev_setintegrity: func 'bpf_lsm_bdev_setintegrity' arg2 type UNKNOWN is not a struct invalid bpf_context access off=16 size=8 Work around this by providing a compat variant of the bdev_setintegrity BPF program that avoids reading the const void *value argument entirely. Instead it reads the size argument (a scalar integer) directly from the raw BPF context (ctx[3]), which is not subject to the broken type check. This is safe because dm-verity guarantees that value and size are always in lockstep: both NULL/0 for unsigned devices, both non-zero for signed devices. The loader tries the full version first (which reads both value and size for defense-in-depth) and falls back to the compat variant if loading fails. bpf_program__set_autoload(false) disables whichever variant is not needed so the verifier never sees it. This compat logic can be removed once the minimum kernel baseline includes the 1271a40eeafa fix. Signed-off-by: Christian Brauner <brauner@kernel.org>
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