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bpf: Support llvm-objcopy for vmlinux BTF

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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: df786c9b94 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Fixes: cb0cc635c7 ("powerpc: Include .BTF section")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Stanislav Fomichev <sdf@google.com>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Andrii 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
This commit is contained in:
Fangrui Song
2020-03-18 15:27:46 -07:00
committed by Daniel Borkmann
parent 483d7a30f5
commit 90ceddcb49
5 changed files with 34 additions and 31 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
kernel/bpf/btf.c
View File

@@ -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)

11
kernel/bpf/sysfs_btf.c
View File

@@ -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);
}