On 8/31/26 4:01 AM, Jiayuan Chen wrote:
> btf_var_show() calls btf_type_id_resolve() unconditionally, which
> dereferences btf->resolved_ids. That is NULL for a base BTF - e.g. the
> vmlinux BTF that bpf_snprintf_btf() renders against - since base BTF is
> not resolved during parsing. btf_modifier_show() guards this with
> 'if (btf->resolved_ids)', but btf_var_show() does not.
>
> A BPF program that passes the type_id of a BTF_KIND_VAR from the vmlinux
> BTF to bpf_snprintf_btf() thus NULL-derefs:
>
> KASAN: probably user-memory-access in range [0x46638-0x4663f]
> RIP: 0010:btf_var_show (kernel/bpf/btf.c:2929)
> Call Trace:
> <TASK>
> btf_type_show (kernel/bpf/btf.c:8259)
> btf_type_snprintf_show (kernel/bpf/btf.c:8329)
> bpf_snprintf_btf (kernel/trace/bpf_trace.c:1047)
> bpf_prog_test_run_raw_tp (net/bpf/test_run.c:829)
> __sys_bpf (kernel/bpf/syscall.c:4804)
> do_syscall_64 (arch/x86/entry/syscall_64.c:84)
> entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
> </TASK>
>
> Resolve the var's type directly with btf_type_skip_modifiers() when
> resolved_ids is NULL, mirroring btf_modifier_show().
>
> Fixes: c4d0bfb45068 ("bpf: Add bpf_snprintf_btf helper")
> Signed-off-by: Jiayuan Chen <[email protected]>
> ---
> kernel/bpf/btf.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 6e267c4c4e1f..3cf35ee49c10 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -2926,7 +2926,15 @@ static void btf_var_show(const struct btf *btf, const
> struct btf_type *t,
> u32 type_id, void *data, u8 bits_offset,
> struct btf_show *show)
> {
> - t = btf_type_id_resolve(btf, &type_id);
> + /*
> + * btf_type_id_resolve() dereferences btf->resolved_ids, which is NULL
> + * for a base BTF (e.g. the vmlinux BTF that bpf_snprintf_btf() uses).
> + * Resolve the var's type directly in that case.
> + */
> + if (btf->resolved_ids)
> + t = btf_type_id_resolve(btf, &type_id);
> + else
> + t = btf_type_skip_modifiers(btf, t->type, &type_id);
Acked-by: Ihor Solodrai <[email protected]>
>
> btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
> }
