On Fri, Sep 5, 2025 at 6:32 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>
> Currently, functions with 'union' arguments cannot be traced with
> fentry/fexit:
union-s passed _by value_.
It's an important detail.
>
> bpftrace -e 'fentry:release_pages { exit(); }' -v
> AST node count: 6
> Attaching 1 probe...
> ERROR: Error loading BPF program for fentry_vmlinux_release_pages_1.
> Kernel error log:
> The function release_pages arg0 type UNION is unsupported.
> processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
>
> ERROR: Loading BPF object(s) failed.
>
> The type of the 'release_pages' argument is defined as:
>
> typedef union {
> struct page **pages;
> struct folio **folios;
> struct encoded_page **encoded_pages;
> } release_pages_arg __attribute__ ((__transparent_union__));
>
> This patch relaxes the restriction by allowing function arguments of type
> 'union' to be traced.
>
> Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
> ---
> kernel/bpf/btf.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 64739308902f7..86883b3c97d20 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6762,7 +6762,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> /* skip modifiers */
> while (btf_type_is_modifier(t))
> t = btf_type_by_id(btf, t->type);
> - if (btf_type_is_small_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
> + if (btf_type_is_small_int(t) || btf_is_any_enum(t) || btf_type_is_struct(t))
> /* accessing a scalar */
> return true;
> if (!btf_type_is_ptr(t)) {
> @@ -7334,7 +7334,7 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
> if (btf_type_is_ptr(t))
> /* kernel size of pointer. Not BPF's size of pointer*/
> return sizeof(void *);
> - if (btf_type_is_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
> + if (btf_type_is_int(t) || btf_is_any_enum(t) || btf_type_is_struct(t))
> return t->size;
Did you look at
commit 720e6a435194 ("bpf: Allow struct argument in trampoline based programs")
that added support for accessing struct passed by value?
Study it and figure out what part of the verifier you forgot
to update while adding this support for accessing unions
passed by value.
Think it through and update the selftest to make sure it tests
the support end-to-end and covers the bug in this patch.
pw-bot: cr
