Re: [RESEND][PATCH v3 bpf-next] btf: expose BTF info through sysfs

[Daniel Borkmann <daniel@iogearbox.net>]

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?

> 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@linuxfoundation.org>
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>

In any case, this is great progress, applied thanks!