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

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Date:   Tue, 13 Aug 2019 11:08:14 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [RESEND][PATCH v3 bpf-next] btf: expose BTF info through sysfs

On Tue, Aug 13, 2019 at 7:20 AM Daniel Borkmann <daniel@...earbox.net> 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@...uxfoundation.org>
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
>
> In any case, this is great progress, applied thanks!