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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
diff --git a/Documentation/ABI/testing/sysfs-kernel-btf b/Documentation/ABI/testing/sysfs-kernel-btf
new file mode 100644
index 0000000..5390f80
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-btf
@@ -0,0 +1,17 @@
+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.
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index 29d7810..e1d9adb 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -22,3 +22,6 @@
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 0000000..092e63b
--- /dev/null
+++ b/kernel/bpf/sysfs_btf.c
@@ -0,0 +1,51 @@
+// 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);
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index a7124f8..cb93832 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -56,8 +56,8 @@
}
# 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 @@
--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 @@
-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 @@
}
# 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 @@
# 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 @@
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 @@
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 @@
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
