Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump.
The existing 'file format' and 'architecture' parsing logic is brittle
and does not work with llvm-objcopy/llvm-objdump.
.BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag and
rename .BTF to BTF so that C code can reference the section via linker
synthesized __start_BTF and __stop_BTF. This fixes a small problem that
previous .BTF had the SHF_WRITE flag. Additionally, `objcopy -I binary`
synthesized symbols _binary__btf_vmlinux_bin_start and
_binary__btf_vmlinux_bin_start (not used elsewhere) are replaced with
more common __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.
Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
---
kernel/bpf/btf.c | 9 ++++-----
kernel/bpf/sysfs_btf.c | 11 +++++------
scripts/link-vmlinux.sh | 17 ++++++-----------
3 files changed, 15 insertions(+), 22 deletions(-)
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 50080add2ab9..6f397c4da05e 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
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)
diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
index 7ae5dddd1fe6..3b495773de5a 100644
--- a/kernel/bpf/sysfs_btf.c
+++ b/kernel/bpf/sysfs_btf.c
@@ -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);
}
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index ac569e197bfa..ae2048625f1e 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -133,17 +133,12 @@ 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}
+ # Extract .BTF, add SHF_ALLOC, rename to BTF so that we can reference
+ # it via linker synthesized __start_BTF and __stop_BTF. Change e_type
+ # to ET_REL so that it can be used to link final vmlinux.
+ ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
+ --rename-section .BTF=BTF ${1} ${2} 2>/dev/null && \
+ printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
}
# Create ${2} .o file with all symbols from the ${1} object file
--
2.25.1.481.gfbce0eb801-goog
