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.
v6:
- drop llvm-objdump from the title. We don't run objdump now
- delete unused local variables: bin_arch, bin_format and bin_file
- mention in the comment that lld does not allow an ET_EXEC input
- rename BTF back to .BTF . The section name is assumed by bpftool
- add output section description to include/asm-generic/vmlinux.lds.h
- mention cb0cc635c7a9 ("powerpc: Include .BTF section")
v5:
- rebase on top of bpf-next/master
- rename .BTF to BTF
Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section")
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Reviewed-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Tested-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: clang-built-linux@xxxxxxxxxxxxxxxx
---
arch/powerpc/kernel/vmlinux.lds.S | 6 ------
include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++
kernel/bpf/btf.c | 9 ++++-----
kernel/bpf/sysfs_btf.c | 11 +++++------
scripts/link-vmlinux.sh | 24 ++++++++++--------------
5 files changed, 34 insertions(+), 31 deletions(-)
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index a32d478a7f41..b4c89a1acebb 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -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))
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e00f41aa8ec4..39da8d8b561d 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -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
*/
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..d09ab4afbda4 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -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
--
2.25.1.481.gfbce0eb801-goog
