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

[Yonghong Song]

On 5/18/22 1:03 PM, Eugene Syromiatnikov wrote:
> 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@xxxxxxxxxx/

I think for patch [1], we only need libbpf and selftest change, no
kernel change is needed since we
explicitly does not support 32bit kernel in the
beginning of function bpf_kprobe_multi_link_attach():

        /* no support for 32bit archs yet */
        if (sizeof(u64) != sizeof(void *))
                return -EOPNOTSUPP;

and in kernel, address (pointer) size will be considered
long (64bit) which is exactly the libbpf change did that.

> > > Fixes: 0dcac272540613d4 ("bpf: Add multi kprobe link")
> > > Signed-off-by: Eugene Syromiatnikov <esyr@xxxxxxxxxx>
> > > ---
> > >   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)