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

[Eugene Syromiatnikov <esyr@redhat.com>]

On Wed, May 18, 2022 at 09:55:05AM -0700, Yonghong Song wrote:
> 
> 
> 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?

As 32 bit arches are "unsupported" currently, the change would be more
a semantic one rather then practical;  I don't mind having it here (basically,
the tools/* part of [1]), though (assuming it is still possible to get it
in 5.18).

[1] https://lore.kernel.org/lkml/6ef675aeeea442fa8fc168cd1cb4e4e474f65a3f.1652772731.git.esyr@redhat.com/

> >
> >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)
>