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

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
...@@ -303,12 +303,6 @@ SECTIONS ...@@ -303,12 +303,6 @@ SECTIONS
*(.branch_lt) *(.branch_lt)
} }
#ifdef CONFIG_DEBUG_INFO_BTF
.BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {
*(.BTF)
}
#endif
.opd : AT(ADDR(.opd) - LOAD_OFFSET) { .opd : AT(ADDR(.opd) - LOAD_OFFSET) {
__start_opd = .; __start_opd = .;
KEEP(*(.opd)) KEEP(*(.opd))
......
...@@ -535,6 +535,7 @@ ...@@ -535,6 +535,7 @@
\ \
RO_EXCEPTION_TABLE \ RO_EXCEPTION_TABLE \
NOTES \ NOTES \
BTF \
\ \
. = ALIGN((align)); \ . = ALIGN((align)); \
__end_rodata = .; __end_rodata = .;
...@@ -621,6 +622,20 @@ ...@@ -621,6 +622,20 @@
__stop___ex_table = .; \ __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 * Init task
*/ */
......
...@@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size, ...@@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
return ERR_PTR(err); return ERR_PTR(err);
} }
extern char __weak _binary__btf_vmlinux_bin_start[]; extern char __weak __start_BTF[];
extern char __weak _binary__btf_vmlinux_bin_end[]; extern char __weak __stop_BTF[];
extern struct btf *btf_vmlinux; extern struct btf *btf_vmlinux;
#define BPF_MAP_TYPE(_id, _ops) #define BPF_MAP_TYPE(_id, _ops)
...@@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) ...@@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void)
} }
env->btf = btf; env->btf = btf;
btf->data = _binary__btf_vmlinux_bin_start; btf->data = __start_BTF;
btf->data_size = _binary__btf_vmlinux_bin_end - btf->data_size = __stop_BTF - __start_BTF;
_binary__btf_vmlinux_bin_start;
err = btf_parse_hdr(env); err = btf_parse_hdr(env);
if (err) if (err)
......
...@@ -9,15 +9,15 @@ ...@@ -9,15 +9,15 @@
#include <linux/sysfs.h> #include <linux/sysfs.h>
/* See scripts/link-vmlinux.sh, gen_btf() func for details */ /* See scripts/link-vmlinux.sh, gen_btf() func for details */
extern char __weak _binary__btf_vmlinux_bin_start[]; extern char __weak __start_BTF[];
extern char __weak _binary__btf_vmlinux_bin_end[]; extern char __weak __stop_BTF[];
static ssize_t static ssize_t
btf_vmlinux_read(struct file *file, struct kobject *kobj, btf_vmlinux_read(struct file *file, struct kobject *kobj,
struct bin_attribute *bin_attr, struct bin_attribute *bin_attr,
char *buf, loff_t off, size_t len) 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; return len;
} }
...@@ -30,15 +30,14 @@ static struct kobject *btf_kobj; ...@@ -30,15 +30,14 @@ static struct kobject *btf_kobj;
static int __init btf_vmlinux_init(void) static int __init btf_vmlinux_init(void)
{ {
if (!_binary__btf_vmlinux_bin_start) if (!__start_BTF)
return 0; return 0;
btf_kobj = kobject_create_and_add("btf", kernel_kobj); btf_kobj = kobject_create_and_add("btf", kernel_kobj);
if (!btf_kobj) if (!btf_kobj)
return -ENOMEM; return -ENOMEM;
bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end - bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
_binary__btf_vmlinux_bin_start;
return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux); return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
} }
......
...@@ -113,9 +113,6 @@ vmlinux_link() ...@@ -113,9 +113,6 @@ vmlinux_link()
gen_btf() gen_btf()
{ {
local pahole_ver local pahole_ver
local bin_arch
local bin_format
local bin_file
if ! [ -x "$(command -v ${PAHOLE})" ]; then if ! [ -x "$(command -v ${PAHOLE})" ]; then
echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available" echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
...@@ -133,17 +130,16 @@ gen_btf() ...@@ -133,17 +130,16 @@ gen_btf()
info "BTF" ${2} info "BTF" ${2}
LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1} LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
# dump .BTF section into raw binary file to link with final vmlinux # Create ${2} which contains just .BTF section but no symbols. Add
bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \ # SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
cut -d, -f1 | cut -d' ' -f2) # deletes all symbols including __start_BTF and __stop_BTF, which will
bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \ # be redefined in the linker script. Add 2>/dev/null to suppress GNU
awk '{print $4}') # objcopy warnings: "empty loadable segment detected at ..."
bin_file=.btf.vmlinux.bin ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
${OBJCOPY} --change-section-address .BTF=0 \ --strip-all ${1} ${2} 2>/dev/null
--set-section-flags .BTF=alloc -O binary \ # Change e_type to ET_REL so that it can be used to link final vmlinux.
--only-section=.BTF ${1} $bin_file # Unlike GNU ld, lld does not allow an ET_EXEC input.
${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \ printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
--rename-section .data=.BTF $bin_file ${2}
} }
# Create ${2} .o file with all symbols from the ${1} object file # Create ${2} .o file with all symbols from the ${1} object file
......
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