On Tue, Aug 13, 2019 at 7:20 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 8/12/19 8:39 PM, Andrii Nakryiko wrote:
> > Make .BTF section allocated and expose its contents through sysfs.
> >
> > /sys/kernel/btf directory is created to contain all the BTFs present
> > inside kernel. Currently there is only kernel's main BTF, represented as
> > /sys/kernel/btf/kernel file. Once kernel modules' BTFs are supported,
> > each module will expose its BTF as /sys/kernel/btf/<module-name> file.
> >
> > Current approach relies on a few pieces coming together:
> > 1. pahole is used to take almost final vmlinux image (modulo .BTF and
> > kallsyms) and generate .BTF section by converting DWARF info into
> > BTF. This section is not allocated and not mapped to any segment,
> > though, so is not yet accessible from inside kernel at runtime.
> > 2. objcopy dumps .BTF contents into binary file and subsequently
> > convert binary file into linkable object file with automatically
> > generated symbols _binary__btf_kernel_bin_start and
> > _binary__btf_kernel_bin_end, pointing to start and end, respectively,
> > of BTF raw data.
> > 3. final vmlinux image is generated by linking this object file (and
> > kallsyms, if necessary). sysfs_btf.c then creates
> > /sys/kernel/btf/kernel file and exposes embedded BTF contents through
> > it. This allows, e.g., libbpf and bpftool access BTF info at
> > well-known location, without resorting to searching for vmlinux image
> > on disk (location of which is not standardized and vmlinux image
> > might not be even available in some scenarios, e.g., inside qemu
> > during testing).
>
> Small question: given modules will be covered later, would it not be more
> obvious to name it /sys/kernel/btf/vmlinux instead?
vmlinux totally makes sense, not sure why I didn't think about that initially...
I'll follow up with a rename.
>
> > Alternative approach using .incbin assembler directive to embed BTF
> > contents directly was attempted but didn't work, because sysfs_proc.o is
> > not re-compiled during link-vmlinux.sh stage. This is required, though,
> > to update embedded BTF data (initially empty data is embedded, then
> > pahole generates BTF info and we need to regenerate sysfs_btf.o with
> > updated contents, but it's too late at that point).
> >
> > If BTF couldn't be generated due to missing or too old pahole,
> > sysfs_btf.c handles that gracefully by detecting that
> > _binary__btf_kernel_bin_start (weak symbol) is 0 and not creating
> > /sys/kernel/btf at all.
> >
> > v2->v3:
> > - added Documentation/ABI/testing/sysfs-kernel-btf (Greg K-H);
> > - created proper kobject (btf_kobj) for btf directory (Greg K-H);
> > - undo v2 change of reusing vmlinux, as it causes extra kallsyms pass
> > due to initially missing __binary__btf_kernel_bin_{start/end} symbols;
> >
> > v1->v2:
> > - allow kallsyms stage to re-use vmlinux generated by gen_btf();
> >
> > Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
>
> In any case, this is great progress, applied thanks!
