Merge branch 'bpf-libbpf-read-sysfs-btf'

Andrii Nakryiko says:

====================
Now that kernel's BTF is exposed through sysfs at well-known location, attempt
to load it first as a target BTF for the purpose of BPF CO-RE relocations.

Patch #1 is a follow-up patch to rename /sys/kernel/btf/kernel into
/sys/kernel/btf/vmlinux.

Patch #2 adds ability to load raw BTF contents from sysfs and expands the list
of locations libbpf attempts to load vmlinux BTF from.
====================
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Documentation/ABI/testing/sysfs-kernel-btf

@@ -6,7 +6,7 @@ Description:
 		Contains BTF type information and related data for kernel and
 		kernel modules.
 
-What:		/sys/kernel/btf/kernel
+What:		/sys/kernel/btf/vmlinux
 Date:		Aug 2019
 KernelVersion:	5.5
 Contact:	bpf@vger.kernel.org

kernel/bpf/sysfs_btf.c

@@ -9,30 +9,30 @@
 #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[];
+extern char __weak _binary__btf_vmlinux_bin_start[];
+extern char __weak _binary__btf_vmlinux_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)
+btf_vmlinux_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);
+	memcpy(buf, _binary__btf_vmlinux_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 bin_attribute bin_attr_btf_vmlinux __ro_after_init = {
+	.attr = { .name = "vmlinux", .mode = 0444, },
+	.read = btf_vmlinux_read,
 };
 
 static struct kobject *btf_kobj;
 
-static int __init btf_kernel_init(void)
+static int __init btf_vmlinux_init(void)
 {
 	int err;
 
-	if (!_binary__btf_kernel_bin_start)
+	if (!_binary__btf_vmlinux_bin_start)
 		return 0;
 
 	btf_kobj = kobject_create_and_add("btf", kernel_kobj);
@@ -42,10 +42,10 @@ static int __init btf_kernel_init(void)
 		return err;
 	}
 
-	bin_attr_btf_kernel.size = _binary__btf_kernel_bin_end -
-				   _binary__btf_kernel_bin_start;
+	bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end -
+				    _binary__btf_vmlinux_bin_start;
 
-	return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_kernel);
+	return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
 }
 
-subsys_initcall(btf_kernel_init);
+subsys_initcall(btf_vmlinux_init);

scripts/link-vmlinux.sh

@@ -117,9 +117,9 @@ gen_btf()
 	# 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} --dump-section .BTF=.btf.vmlinux.bin ${1} 2>/dev/null
 	${OBJCOPY} -I binary -O ${CONFIG_OUTPUT_FORMAT} -B ${bin_arch} \
-		--rename-section .data=.BTF .btf.kernel.bin ${2}
+		--rename-section .data=.BTF .btf.vmlinux.bin ${2}
 }
 
 # Create ${2} .o file with all symbols from the ${1} object file
@@ -227,10 +227,10 @@ ${MAKE} -f "${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=""
+btf_vmlinux_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
+	if gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
+		btf_vmlinux_bin_o=.btf.vmlinux.bin.o
 	fi
 fi
 
@@ -265,11 +265,11 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 	kallsyms_vmlinux=.tmp_vmlinux2
 
 	# step 1
-	vmlinux_link .tmp_vmlinux1 ${btf_kernel_bin_o}
+	vmlinux_link .tmp_vmlinux1 ${btf_vmlinux_bin_o}
 	kallsyms .tmp_vmlinux1 .tmp_kallsyms1.o
 
 	# step 2
-	vmlinux_link .tmp_vmlinux2 .tmp_kallsyms1.o ${btf_kernel_bin_o}
+	vmlinux_link .tmp_vmlinux2 .tmp_kallsyms1.o ${btf_vmlinux_bin_o}
 	kallsyms .tmp_vmlinux2 .tmp_kallsyms2.o
 
 	# step 3
@@ -280,13 +280,13 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 		kallsymso=.tmp_kallsyms3.o
 		kallsyms_vmlinux=.tmp_vmlinux3
 
-		vmlinux_link .tmp_vmlinux3 .tmp_kallsyms2.o ${btf_kernel_bin_o}
+		vmlinux_link .tmp_vmlinux3 .tmp_kallsyms2.o ${btf_vmlinux_bin_o}
 		kallsyms .tmp_vmlinux3 .tmp_kallsyms3.o
 	fi
 fi
 
 info LD vmlinux
-vmlinux_link vmlinux "${kallsymso}" "${btf_kernel_bin_o}"
+vmlinux_link vmlinux "${kallsymso}" "${btf_vmlinux_bin_o}"
 
 if [ -n "${CONFIG_BUILDTIME_EXTABLE_SORT}" ]; then
 	info SORTEX vmlinux

tools/lib/bpf/libbpf.c

@@ -2807,15 +2807,61 @@ static int bpf_core_reloc_insn(struct bpf_program *prog, int insn_off,
 	return 0;
 }
 
+static struct btf *btf_load_raw(const char *path)
+{
+	struct btf *btf;
+	size_t read_cnt;
+	struct stat st;
+	void *data;
+	FILE *f;
+
+	if (stat(path, &st))
+		return ERR_PTR(-errno);
+
+	data = malloc(st.st_size);
+	if (!data)
+		return ERR_PTR(-ENOMEM);
+
+	f = fopen(path, "rb");
+	if (!f) {
+		btf = ERR_PTR(-errno);
+		goto cleanup;
+	}
+
+	read_cnt = fread(data, 1, st.st_size, f);
+	fclose(f);
+	if (read_cnt < st.st_size) {
+		btf = ERR_PTR(-EBADF);
+		goto cleanup;
+	}
+
+	btf = btf__new(data, read_cnt);
+
+cleanup:
+	free(data);
+	return btf;
+}
+
 /*
  * Probe few well-known locations for vmlinux kernel image and try to load BTF
  * data out of it to use for target BTF.
  */
 static struct btf *bpf_core_find_kernel_btf(void)
 {
-	const char *locations[] = {
-		"/lib/modules/%1$s/vmlinux-%1$s",
-		"/usr/lib/modules/%1$s/kernel/vmlinux",
+	struct {
+		const char *path_fmt;
+		bool raw_btf;
+	} locations[] = {
+		/* try canonical vmlinux BTF through sysfs first */
+		{ "/sys/kernel/btf/vmlinux", true /* raw BTF */ },
+		/* fall back to trying to find vmlinux ELF on disk otherwise */
+		{ "/boot/vmlinux-%1$s" },
+		{ "/lib/modules/%1$s/vmlinux-%1$s" },
+		{ "/lib/modules/%1$s/build/vmlinux" },
+		{ "/usr/lib/modules/%1$s/kernel/vmlinux" },
+		{ "/usr/lib/debug/boot/vmlinux-%1$s" },
+		{ "/usr/lib/debug/boot/vmlinux-%1$s.debug" },
+		{ "/usr/lib/debug/lib/modules/%1$s/vmlinux" },
 	};
 	char path[PATH_MAX + 1];
 	struct utsname buf;
@@ -2825,14 +2871,18 @@ static struct btf *bpf_core_find_kernel_btf(void)
 	uname(&buf);
 
 	for (i = 0; i < ARRAY_SIZE(locations); i++) {
-		snprintf(path, PATH_MAX, locations[i], buf.release);
+		snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
 
 		if (access(path, R_OK))
 			continue;
 
-		btf = btf__parse_elf(path, NULL);
-		pr_debug("kernel BTF load from '%s': %ld\n",
-			 path, PTR_ERR(btf));
+		if (locations[i].raw_btf)
+			btf = btf_load_raw(path);
+		else
+			btf = btf__parse_elf(path, NULL);
+
+		pr_debug("loading kernel BTF '%s': %ld\n",
+			 path, IS_ERR(btf) ? PTR_ERR(btf) : 0);
 		if (IS_ERR(btf))
 			continue;