On Wed, Jul 13, 2022 at 11:42 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 7/13/22 11:00 PM, Andrii Nakryiko wrote:
> > On Wed, Jul 13, 2022 at 3:37 PM Indu Bhagat <indu.bhagat@xxxxxxxxxx> wrote:
> >>
> >> The vlen bits in the BTF type of kind BTF_KIND_FUNC are used to convey the
> >> linkage information for functions.
> >>
> >> Signed-off-by: Indu Bhagat <indu.bhagat@xxxxxxxxxx>
> >> ---
> >> Documentation/bpf/btf.rst | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
> >> index f49aeef62d0c..b3a9d5ac882c 100644
> >> --- a/Documentation/bpf/btf.rst
> >> +++ b/Documentation/bpf/btf.rst
> >> @@ -369,7 +369,7 @@ No additional type data follow ``btf_type``.
> >> * ``name_off``: offset to a valid C identifier
> >> * ``info.kind_flag``: 0
> >> * ``info.kind``: BTF_KIND_FUNC
> >> - * ``info.vlen``: 0
> >> + * ``info.vlen``: linkage information (static=0, global=1)
> >
> > there is also extern=2, but I think we should just refer to enum
> > btf_func_linkage, defined in UAPI (include/uapi/linux/btf.h) ?
>
> Currently kernel rejects extern=2. In kernel btf.c, we have
>
> if (btf_type_vlen(t) > BTF_FUNC_GLOBAL) {
> btf_verifier_log_type(env, t, "Invalid func linkage");
> return -EINVAL;
> }
>
> and extern=2 will cause btf loading failure.
>
> The BTF_FUNC_EXTERN is generated when you call an extern *global*
> function. I suspect that during static linking, all these
> extern globals should become true global/static functions and
> not extern func's any more so kernel is okay.
>
> So looks like it is worthwhile to mention that BTF_KIND_FUNC
> supports all three modes as specified in enum btf_func_linkage.
> But only static/global is supported in the kernel.
>
Exactly. This is specification of BTF format (for clang and gcc to
agree upon, for example), while kernel BTF verifier puts extra
conditions on top of BTF format itself (as required by C and kernel
semantics). So it makes sense to document all possibilities and extra
kernel checks.
> >
> >> * ``type``: a BTF_KIND_FUNC_PROTO type
> >>
> >> No additional type data follow ``btf_type``.
> >> --
> >> 2.31.1
> >>
