Subject: [PATCH bpf-next v6] bpf: Support llvm-objcopy for vmlinux BTF

From: Fangrui Song <[email-masked]>
Date: Wed, 18 Mar 2020 15:27:46 -0700

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 <[email-masked]>
Reported-by: Nathan Chancellor <[email-masked]>
Reviewed-by: Stanislav Fomichev <[email-masked]>
Tested-by: Stanislav Fomichev <[email-masked]>
Cc: Alexei Starovoitov <[email-masked]>
Cc: Andrii Nakryiko <[email-masked]>
Cc: Daniel Borkmann <[email-masked]>
Cc: David S. Miller <[email-masked]>
Cc: Kees Cook <[email-masked]>
Cc: Michael Ellerman <[email-masked]>
Cc: Naveen N. Rao <[email-masked]>
Cc: Nick Desaulniers <[email-masked]>
Cc: [email-masked]
---
 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