Commit 90ceddcb authored by Fangrui Song's avatar Fangrui Song Committed by Daniel Borkmann
Browse files

bpf: Support llvm-objcopy for vmlinux BTF 



Simplify gen_btf logic to make it work with llvm-objcopy. The existing
'file format' and 'architecture' parsing logic is brittle and does not
work with llvm-objcopy/llvm-objdump.

'file format' output of llvm-objdump>=11 will match GNU objdump, but
'architecture' (bfdarch) may not.

.BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag
because it is part of vmlinux image used for introspection. C code
can reference the section via linker script defined __start_BTF and
__stop_BTF. This fixes a small problem that previous .BTF had the
SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data).

Additionally, `objcopy -I binary` synthesized symbols
_binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not
used elsewhere) are replaced with more commonplace __start_BTF and
__stop_BTF.

Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns
"empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"

We use a dd command to change the e_type field in the ELF header from
ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o.  Accepting
ET_EXEC as an input file is an extremely rare GNU ld feature that lld
does not intend to support, because this is error-prone.

The output section description .BTF in include/asm-generic/vmlinux.lds.h
avoids potential subtle orphan section placement issues and suppresses
--orphan-handling=warn warnings.

Fixes: df786c9b ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Fixes: cb0cc635 ("powerpc: Include .BTF section")
Reported-by: default avatarNathan Chancellor <natechancellor@gmail.com>
Signed-off-by: default avatarFangrui Song <maskray@google.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Tested-by: default avatarStanislav Fomichev <sdf@google.com>
Tested-by: default avatarAndrii Nakryiko <andriin@fb.com>
Reviewed-by: default avatarStanislav Fomichev <sdf@google.com>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com
parent 483d7a30

arch/powerpc/kernel/vmlinux.lds.S

+0 −6
Original line number Diff line number Diff line
@@ -303,12 +303,6 @@ SECTIONS 
		*(.branch_lt)

	}



#ifdef CONFIG_DEBUG_INFO_BTF

	.BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {

		*(.BTF)

	}

#endif



	.opd : AT(ADDR(.opd) - LOAD_OFFSET) {

		__start_opd = .;

		KEEP(*(.opd))

include/asm-generic/vmlinux.lds.h

+15 −0
Original line number Diff line number Diff line
@@ -535,6 +535,7 @@ 
									\

	RO_EXCEPTION_TABLE						\

	NOTES								\

	BTF								\

									\

	. = ALIGN((align));						\

	__end_rodata = .;
@@ -621,6 +622,20 @@ 
		__stop___ex_table = .;					\

	}



/*

 * .BTF

 */

#ifdef CONFIG_DEBUG_INFO_BTF

#define BTF								\

	.BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {				\

		__start_BTF = .;					\

		*(.BTF)							\

		__stop_BTF = .;						\

	}

#else

#define BTF

#endif



/*

 * Init task

 */

kernel/bpf/btf.c

+4 −5
Original line number Diff line number Diff line
@@ -3477,8 +3477,8 @@ errout: 
	return ERR_PTR(err);

}



extern char __weak _binary__btf_vmlinux_bin_start[];

extern char __weak _binary__btf_vmlinux_bin_end[];

extern char __weak __start_BTF[];

extern char __weak __stop_BTF[];

extern struct btf *btf_vmlinux;



#define BPF_MAP_TYPE(_id, _ops)
@@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) 
	}

	env->btf = btf;



	btf->data = _binary__btf_vmlinux_bin_start;

	btf->data_size = _binary__btf_vmlinux_bin_end -

		_binary__btf_vmlinux_bin_start;

	btf->data = __start_BTF;

	btf->data_size = __stop_BTF - __start_BTF;



	err = btf_parse_hdr(env);

	if (err)

kernel/bpf/sysfs_btf.c

+5 −6
Original line number Diff line number Diff line
@@ -9,15 +9,15 @@ 
#include <linux/sysfs.h>



/* See scripts/link-vmlinux.sh, gen_btf() func for details */

extern char __weak _binary__btf_vmlinux_bin_start[];

extern char __weak _binary__btf_vmlinux_bin_end[];

extern char __weak __start_BTF[];

extern char __weak __stop_BTF[];



static ssize_t

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_vmlinux_bin_start + off, len);

	memcpy(buf, __start_BTF + off, len);

	return len;

}
@@ -30,15 +30,14 @@ static struct kobject *btf_kobj; 


static int __init btf_vmlinux_init(void)

{

	if (!_binary__btf_vmlinux_bin_start)

	if (!__start_BTF)

		return 0;



	btf_kobj = kobject_create_and_add("btf", kernel_kobj);

	if (!btf_kobj)

		return -ENOMEM;



	bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end -

				    _binary__btf_vmlinux_bin_start;

	bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;



	return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);

}

scripts/link-vmlinux.sh

+10 −14
Original line number Diff line number Diff line
@@ -113,9 +113,6 @@ vmlinux_link() 
gen_btf()

{

	local pahole_ver

	local bin_arch

	local bin_format

	local bin_file



	if ! [ -x "$(command -v ${PAHOLE})" ]; then

		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -133,17 +130,16 @@ gen_btf() 
	info "BTF" ${2}

	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}



	# dump .BTF section into raw binary file to link with final vmlinux

	bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \

		cut -d, -f1 | cut -d' ' -f2)

	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \

		awk '{print $4}')

	bin_file=.btf.vmlinux.bin

	${OBJCOPY} --change-section-address .BTF=0 \

		--set-section-flags .BTF=alloc -O binary \

		--only-section=.BTF ${1} $bin_file

	${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \

		--rename-section .data=.BTF $bin_file ${2}

	# Create ${2} which contains just .BTF section but no symbols. Add

	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all

	# deletes all symbols including __start_BTF and __stop_BTF, which will

	# be redefined in the linker script. Add 2>/dev/null to suppress GNU

	# objcopy warnings: "empty loadable segment detected at ..."

	${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \

		--strip-all ${1} ${2} 2>/dev/null

	# Change e_type to ET_REL so that it can be used to link final vmlinux.

	# Unlike GNU ld, lld does not allow an ET_EXEC input.

	printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none

}



# Create ${2} .o file with all symbols from the ${1} object file

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