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

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Aug 07, 2019 at 11:38:21AM -0700, 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).
> 
> 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.
> 
> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> ---
>  kernel/bpf/Makefile     |  3 +++
>  kernel/bpf/sysfs_btf.c  | 52 ++++++++++++++++++++++++++++++++++++++++
>  scripts/link-vmlinux.sh | 53 ++++++++++++++++++++++++++---------------
>  3 files changed, 89 insertions(+), 19 deletions(-)
>  create mode 100644 kernel/bpf/sysfs_btf.c
> 
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 29d781061cd5..e1d9adb212f9 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -22,3 +22,6 @@ obj-$(CONFIG_CGROUP_BPF) += cgroup.o
>  ifeq ($(CONFIG_INET),y)
>  obj-$(CONFIG_BPF_SYSCALL) += reuseport_array.o
>  endif
> +ifeq ($(CONFIG_SYSFS),y)
> +obj-$(CONFIG_DEBUG_INFO_BTF) += sysfs_btf.o
> +endif
> diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
> new file mode 100644
> index 000000000000..ac06ce1d62e8
> --- /dev/null
> +++ b/kernel/bpf/sysfs_btf.c
> @@ -0,0 +1,52 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Provide kernel BTF information for introspection and use by eBPF tools.
> + */
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/kobject.h>
> +#include <linux/init.h>
> +
> +/* See scripts/link-vmlinux.sh, gen_btf() func for details */
> +extern char __weak _binary__btf_kernel_bin_start[];
> +extern char __weak _binary__btf_kernel_bin_end[];
> +
> +static ssize_t
> +btf_kernel_read(struct file *file, struct kobject *kobj,
> +		struct bin_attribute *bin_attr,
> +		char *buf, loff_t off, size_t len)
> +{
> +	memcpy(buf, _binary__btf_kernel_bin_start + off, len);

hum, should you check the end of the btf data?
maybe use the memory_read_from_buffer function instead

jirka