Re: [PATCH bpf v3 2/2] bpf_trace: bail out from bpf_kprobe_multi_link_attach when in compat

[Yonghong Song <yhs@fb.com>]

On 5/18/22 5:22 AM, Eugene Syromiatnikov wrote:
> Since bpf_kprobe_multi_link_attach doesn't support 32-bit kernels
> for whatever reason, having it enabled for compat processes on 64-bit
> kernels makes even less sense due to discrepances in the type sizes
> that it does not handle.

If I understand correctly, the reason is due to
in libbpf we have
struct bpf_link_create_opts {
         size_t sz; /* size of this struct for forward/backward 
compatibility */
         __u32 flags;
         union bpf_iter_link_info *iter_info;
         __u32 iter_info_len;
         __u32 target_btf_id;
         union {
                 struct {
                         __u64 bpf_cookie;
                 } perf_event;
                 struct {
                         __u32 flags;
                         __u32 cnt;
                         const char **syms;
                         const unsigned long *addrs;
                         const __u64 *cookies;
                 } kprobe_multi;
         };
         size_t :0;
};

Note that we have `const unsigned long *addrs;`

If we have 32-bit user space application and 64bit kernel,
and we will have userspace 32-bit pointers and kernel as
64bit pointers and current kernel doesn't handle 32-bit
user pointer properly.

Consider this may involve libbpf uapi change, maybe
we should change "const unsigned long *addrs;" to
"const __u64 *addrs;" considering we haven't freeze
libbpf UAPI yet.

Otherwise, we stick to current code with this patch,
it will make it difficult to support 32-bit app with
64-bit kernel for kprobe_multi in the future due to
uapi issues.

WDYT?

> 
> Fixes: 0dcac272540613d4 ("bpf: Add multi kprobe link")
> Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
> ---
>   kernel/trace/bpf_trace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 212faa4..2f83489 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2412,7 +2412,7 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>   	int err;
>   
>   	/* no support for 32bit archs yet */
> -	if (sizeof(u64) != sizeof(void *))
> +	if (sizeof(u64) != sizeof(void *) || in_compat_syscall())
>   		return -EOPNOTSUPP;
>   
>   	if (prog->expected_attach_type != BPF_TRACE_KPROBE_MULTI)