Re: [PATCH v3 bpf-next 2/5] bpf: assign ID to vmlinux BTF and return extra info for BTF in GET_OBJ_INFO

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Nov 9, 2020 at 1:55 PM Martin KaFai Lau <kafai@fb.com> wrote:
>
> On Mon, Nov 09, 2020 at 01:00:21PM -0800, Andrii Nakryiko wrote:
> > Allocate ID for vmlinux BTF. This makes it visible when iterating over all BTF
> > objects in the system. To allow distinguishing vmlinux BTF (and later kernel
> > module BTF) from user-provided BTFs, expose extra kernel_btf flag, as well as
> > BTF name ("vmlinux" for vmlinux BTF, will equal to module's name for module
> > BTF).  We might want to later allow specifying BTF name for user-provided BTFs
> > as well, if that makes sense. But currently this is reserved only for
> > in-kernel BTFs.
> >
> > Having in-kernel BTFs exposed IDs will allow to extend BPF APIs that require
> > in-kernel BTF type with ability to specify BTF types from kernel modules, not
> > just vmlinux BTF. This will be implemented in a follow up patch set for
> > fentry/fexit/fmod_ret/lsm/etc.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> >  include/uapi/linux/bpf.h       |  3 +++
> >  kernel/bpf/btf.c               | 39 ++++++++++++++++++++++++++++++++--
> >  tools/include/uapi/linux/bpf.h |  3 +++
> >  3 files changed, 43 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 9879d6793e90..162999b12790 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -4466,6 +4466,9 @@ struct bpf_btf_info {
> >       __aligned_u64 btf;
> >       __u32 btf_size;
> >       __u32 id;
> > +     __aligned_u64 name;
> > +     __u32 name_len;
> > +     __u32 kernel_btf;
> >  } __attribute__((aligned(8)));
> >
> >  struct bpf_link_info {
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 894ee33f4c84..663c3fb4e614 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -215,6 +215,8 @@ struct btf {
> >       struct btf *base_btf;
> >       u32 start_id; /* first type ID in this BTF (0 for base BTF) */
> >       u32 start_str_off; /* first string offset (0 for base BTF) */
> > +     char name[MODULE_NAME_LEN];
> > +     bool kernel_btf;
> >  };
> >
> >  enum verifier_phase {
> > @@ -4430,6 +4432,8 @@ struct btf *btf_parse_vmlinux(void)
> >
> >       btf->data = __start_BTF;
> >       btf->data_size = __stop_BTF - __start_BTF;
> > +     btf->kernel_btf = true;
> > +     snprintf(btf->name, sizeof(btf->name), "vmlinux");
> >
> >       err = btf_parse_hdr(env);
> >       if (err)
> > @@ -4455,8 +4459,13 @@ struct btf *btf_parse_vmlinux(void)
> >
> >       bpf_struct_ops_init(btf, log);
> >
> > -     btf_verifier_env_free(env);
> >       refcount_set(&btf->refcnt, 1);
> > +
> > +     err = btf_alloc_id(btf);
> > +     if (err)
> > +             goto errout;
> > +
> > +     btf_verifier_env_free(env);
> >       return btf;
> >
> >  errout:
> > @@ -5554,7 +5563,8 @@ int btf_get_info_by_fd(const struct btf *btf,
> >       struct bpf_btf_info info;
> >       u32 info_copy, btf_copy;
> >       void __user *ubtf;
> > -     u32 uinfo_len;
> > +     char __user *uname;
> > +     u32 uinfo_len, uname_len, name_len;
> >
> >       uinfo = u64_to_user_ptr(attr->info.info);
> >       uinfo_len = attr->info.info_len;
> > @@ -5571,6 +5581,31 @@ int btf_get_info_by_fd(const struct btf *btf,
> >               return -EFAULT;
> >       info.btf_size = btf->data_size;
> >
> > +     info.kernel_btf = btf->kernel_btf;
> > +
> > +     uname = u64_to_user_ptr(info.name);
> > +     uname_len = info.name_len;
> > +     if (!uname ^ !uname_len)
> > +             return -EINVAL;
> > +
> > +     name_len = strlen(btf->name);
> > +     info.name_len = name_len;
> > +
> > +     if (uname) {
> > +             if (uname_len >= name_len + 1) {
> > +                     if (copy_to_user(uname, btf->name, name_len + 1))
> > +                             return -EFAULT;
> > +             } else {
> > +                     char zero = '\0';
> > +
> > +                     if (copy_to_user(uname, btf->name, uname_len - 1))
> > +                             return -EFAULT;
> > +                     if (put_user(zero, uname + uname_len - 1))
> > +                             return -EFAULT;
> > +                     return -ENOSPC;
> It should still do copy_to_user() even it will return -ENOSPC.
>

Good catch! I'll use "ret" variable, will set it to -ENOSPC here and
will let the code fall through.


> > +             }
> > +     }
> > +
> >       if (copy_to_user(uinfo, &info, info_copy) ||
> >           put_user(info_copy, &uattr->info.info_len))
> >               return -EFAULT;