Commit 341dfcf8 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Daniel Borkmann
Browse files

btf: expose BTF info through sysfs 



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.

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: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent a664a834

Documentation/ABI/testing/sysfs-kernel-btf

0 → 100644
+17 −0
Original line number Diff line number Diff line 
What:		/sys/kernel/btf

Date:		Aug 2019

KernelVersion:	5.5

Contact:	bpf@vger.kernel.org

Description:

		Contains BTF type information and related data for kernel and

		kernel modules.



What:		/sys/kernel/btf/kernel

Date:		Aug 2019

KernelVersion:	5.5

Contact:	bpf@vger.kernel.org

Description:

		Read-only binary attribute exposing kernel's own BTF type

		information with description of all internal kernel types. See

		Documentation/bpf/btf.rst for detailed description of format

		itself.

kernel/bpf/Makefile

+3 −0
Original line number Diff line number Diff line
@@ -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

kernel/bpf/sysfs_btf.c

0 → 100644
+51 −0
Original line number Diff line number Diff line 
// 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>

#include <linux/sysfs.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 bin_attr_btf_kernel __ro_after_init = {

	.attr = { .name = "kernel", .mode = 0444, },

	.read = btf_kernel_read,

};



static struct kobject *btf_kobj;



static int __init btf_kernel_init(void)

{

	int err;



	if (!_binary__btf_kernel_bin_start)

		return 0;



	btf_kobj = kobject_create_and_add("btf", kernel_kobj);

	if (IS_ERR(btf_kobj)) {

		err = PTR_ERR(btf_kobj);

		btf_kobj = NULL;

		return err;

	}



	bin_attr_btf_kernel.size = _binary__btf_kernel_bin_end -

				   _binary__btf_kernel_bin_start;



	return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_kernel);

}



subsys_initcall(btf_kernel_init);

scripts/link-vmlinux.sh

+33 −19
Original line number Diff line number Diff line
@@ -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}

}



# 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_vmlinux.btf .btf.kernel.bin.o ; then

		btf_kernel_bin_o=.btf.kernel.bin.o

	fi

fi



kallsymso=""

kallsyms_vmlinux=""

if [ -n "${CONFIG_KALLSYMS}" ]; then
@@ -246,11 +265,11 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then 
	kallsyms_vmlinux=.tmp_vmlinux2



	# step 1

	vmlinux_link "" .tmp_vmlinux1

	vmlinux_link .tmp_vmlinux1 ${btf_kernel_bin_o}

	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 +280,13 @@ 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

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