On 8/7/19 5:32 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).
>
> 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.
>
> v1->v2:
> - allow kallsyms stage to re-use vmlinux generated by gen_btf();
>
> Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> ---
> kernel/bpf/Makefile | 3 +++
> kernel/bpf/sysfs_btf.c | 52 ++++++++++++++++++++++++++++++++++++++
> scripts/link-vmlinux.sh | 55 +++++++++++++++++++++++++++--------------
> 3 files changed, 91 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);
> + return len;
> +}
> +
> +static struct bin_attribute btf_kernel_attr __ro_after_init = {
> + .attr = {
> + .name = "kernel",
> + .mode = 0444,
> + },
> + .read = btf_kernel_read,
> +};
> +
> +static struct bin_attribute *btf_attrs[] __ro_after_init = {
> + &btf_kernel_attr,
> + NULL,
> +};
> +
> +static struct attribute_group btf_group_attr __ro_after_init = {
> + .name = "btf",
> + .bin_attrs = btf_attrs,
> +};
> +
> +static int __init btf_kernel_init(void)
> +{
> + if (!_binary__btf_kernel_bin_start)
> + return 0;
> +
> + btf_kernel_attr.size = _binary__btf_kernel_bin_end -
> + _binary__btf_kernel_bin_start;
> +
> + return sysfs_create_group(kernel_kobj, &btf_group_attr);
> +}
> +
> +subsys_initcall(btf_kernel_init);
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index a7124f895b24..e05abe19b11f 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -56,8 +56,8 @@ modpost_link()
> }
>
> # Link of vmlinux
> -# ${1} - optional extra .o files
> -# ${2} - output file
> +# ${1} - output file
> +# ${@:2} - optional extra .o files
> vmlinux_link()
> {
> local lds="${objtree}/${KBUILD_LDS}"
> @@ -70,9 +70,9 @@ vmlinux_link()
> --start-group \
> ${KBUILD_VMLINUX_LIBS} \
> --end-group \
> - ${1}"
> + ${@:2}"
>
> - ${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux} -o ${2} \
> + ${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux} -o ${1} \
> -T ${lds} ${objects}
> else
> objects="-Wl,--whole-archive \
> @@ -81,9 +81,9 @@ vmlinux_link()
> -Wl,--start-group \
> ${KBUILD_VMLINUX_LIBS} \
> -Wl,--end-group \
> - ${1}"
> + ${@:2}"
>
> - ${CC} ${CFLAGS_vmlinux} -o ${2} \
> + ${CC} ${CFLAGS_vmlinux} -o ${1} \
> -Wl,-T,${lds} \
> ${objects} \
> -lutil -lrt -lpthread
> @@ -92,23 +92,34 @@ vmlinux_link()
> }
>
> # generate .BTF typeinfo from DWARF debuginfo
> +# ${1} - vmlinux image
> +# ${2} - file to dump raw BTF data into
> gen_btf()
> {
> - local pahole_ver;
> + local pahole_ver
> + local bin_arch
>
> if ! [ -x "$(command -v ${PAHOLE})" ]; then
> info "BTF" "${1}: pahole (${PAHOLE}) is not available"
> - return 0
> + return 1
> fi
>
> pahole_ver=$(${PAHOLE} --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/')
> if [ "${pahole_ver}" -lt "113" ]; then
> info "BTF" "${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.13"
> - return 0
> + return 1
> fi
>
> - info "BTF" ${1}
> + info "BTF" ${2}
> + vmlinux_link ${1}
> LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> +
> + # dump .BTF section into raw binary file to link with final vmlinux
> + bin_arch=$(${OBJDUMP} -f ${1} | grep architecture | \
> + cut -d, -f1 | cut -d' ' -f2)
> + ${OBJCOPY} --dump-section .BTF=.btf.kernel.bin ${1} 2>/dev/null
> + ${OBJCOPY} -I binary -O ${CONFIG_OUTPUT_FORMAT} -B ${bin_arch} \
> + --rename-section .data=.BTF .btf.kernel.bin ${2}
Currently, the binary size on my config is about 2.6MB. Do you think
we could or need to compress it to make it smaller? I tried gzip
and the compressed size is 0.9MB.
> }
>
> # Create ${2} .o file with all symbols from the ${1} object file
> @@ -153,6 +164,7 @@ sortextable()
> # Delete output files in case of error
> cleanup()
> {
> + rm -f .btf.*
> rm -f .tmp_System.map
> rm -f .tmp_kallsyms*
> rm -f .tmp_vmlinux*
> @@ -215,6 +227,13 @@ ${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o
> info MODINFO modules.builtin.modinfo
> ${OBJCOPY} -j .modinfo -O binary vmlinux.o modules.builtin.modinfo
>
> +btf_kernel_bin_o=""
> +if [ -n "${CONFIG_DEBUG_INFO_BTF}" ]; then
> + if gen_btf .tmp_vmlinux1 .btf.kernel.bin.o ; then
> + btf_kernel_bin_o=.btf.kernel.bin.o
> + fi
> +fi
> +
> kallsymso=""
> kallsyms_vmlinux=""
> if [ -n "${CONFIG_KALLSYMS}" ]; then
> @@ -246,11 +265,14 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
> kallsyms_vmlinux=.tmp_vmlinux2
>
> # step 1
> - vmlinux_link "" .tmp_vmlinux1
> + # skip building .tmp_vmlinux1 if gen_btf() already did that
> + if [ -z "${btf_kernel_bin_o}" ]; then
> + vmlinux_link .tmp_vmlinux1
> + fi
> kallsyms .tmp_vmlinux1 .tmp_kallsyms1.o
>
> # step 2
> - vmlinux_link .tmp_kallsyms1.o .tmp_vmlinux2
> + vmlinux_link .tmp_vmlinux2 .tmp_kallsyms1.o ${btf_kernel_bin_o}
> kallsyms .tmp_vmlinux2 .tmp_kallsyms2.o
>
> # step 3
> @@ -261,18 +283,13 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
> kallsymso=.tmp_kallsyms3.o
> kallsyms_vmlinux=.tmp_vmlinux3
>
> - vmlinux_link .tmp_kallsyms2.o .tmp_vmlinux3
> -
> + vmlinux_link .tmp_vmlinux3 .tmp_kallsyms2.o ${btf_kernel_bin_o}
> kallsyms .tmp_vmlinux3 .tmp_kallsyms3.o
> fi
> fi
>
> info LD vmlinux
> -vmlinux_link "${kallsymso}" vmlinux
> -
> -if [ -n "${CONFIG_DEBUG_INFO_BTF}" ]; then
> - gen_btf vmlinux
> -fi
> +vmlinux_link vmlinux "${kallsymso}" "${btf_kernel_bin_o}"
>
> if [ -n "${CONFIG_BUILDTIME_EXTABLE_SORT}" ]; then
> info SORTEX vmlinux
>
