On Wed, Nov 10, 2021 at 9:04 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 11/10/21 8:40 AM, Alexei Starovoitov wrote:
> > On Tue, Nov 9, 2021 at 10:26 PM Yonghong Song <yhs@xxxxxx> wrote:
> >>
> >>
> >>
> >> On 11/9/21 9:28 PM, Alexei Starovoitov wrote:
> >>> On Tue, Nov 09, 2021 at 09:19:40PM -0800, Yonghong Song wrote:
> >>>> LLVM patches ([1] for clang, [2] and [3] for BPF backend)
> >>>> added support for btf_type_tag attributes. This patch
> >>>> added support for the kernel.
> >>>>
> >>>> The main motivation for btf_type_tag is to bring kernel
> >>>> annotations __user, __rcu etc. to btf. With such information
> >>>> available in btf, bpf verifier can detect mis-usages
> >>>> and reject the program. For example, for __user tagged pointer,
> >>>> developers can then use proper helper like bpf_probe_read_kernel()
> >>>> etc. to read the data.
> >>>
> >>> +#define __tag1 __attribute__((btf_type_tag("tag1")))
> >>> +#define __tag2 __attribute__((btf_type_tag("tag2")))
> >>> +
> >>> +struct btf_type_tag_test {
> >>> + int __tag1 * __tag1 __tag2 *p;
> >>> +} g;
> >>>
> >>> Can we build the kernel with the latest clang and get __user in BTF ?
> >>
> >> Not yet. The following are the steps:
> >> 1. land this patch set in the kernel
> >> 2. sync to libbpf repo.
> >> 3. pahole sync with libbpf repo, and pahole convert btf_type_tag
> >> in llvm to BTF
> >> 4. another kernel patch to define __user as
> >> __attribute__((btf_type_tag("user")))
> >> and then we will get __user in vmlinux BTF.
> >
> > Makes sense. I was wondering whether clang can handle
> > the whole kernel source code with
> > #define __user __attribute__((btf_type_tag("user")))
> > Steps 1,2,3 are necessary to make use of it,
> > but step 4 can be tried out already?
>
> Yes, you try clang -> vmlinux dwarf part of step 4 with
> the following kernel hack:
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 05ceb2e92b0e..30e199c30a53 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -32,7 +32,7 @@ static inline void __chk_io_ptr(const volatile void
> __iomem *ptr) { }
> # ifdef STRUCTLEAK_PLUGIN
> # define __user __attribute__((user))
> # else
> -# define __user
> +# define __user __attribute__((btf_type_tag("user")))
> # endif
> # define __iomem
> # define __percpu
I've tried the latest LLVM with the above diff and it seems to work!
$ llvm-dwarfdump kernel/bpf/built-in.a |grep -3 btf_type_tag|head
0x00003ace: DW_TAG_pointer_type
0x00003acf: DW_TAG_LLVM_annotation
DW_AT_name ("btf_type_tag")
DW_AT_const_value ("user")
Nice!
Didn't notice any warnings. Great.
