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Subject[PATCH 5.4 008/102] bpf: Force .BTF section start to zero when dumping from vmlinux
Date
From: Stanislav Fomichev <sdf@google.com>

commit df786c9b947639aedbc7bb44b5dae2a7824af360 upstream.

While trying to figure out why fentry_fexit selftest doesn't pass for me
(old pahole, broken BTF), I found out that my latest patch can break vmlinux
.BTF generation. objcopy preserves section start when doing --only-section,
so there is a chance (depending on where pahole inserts .BTF section) to
have leading empty zeroes. Let's explicitly force section offset to zero.

Before:

$ objcopy --set-section-flags .BTF=alloc -O binary \
--only-section=.BTF vmlinux .btf.vmlinux.bin
$ xxd .btf.vmlinux.bin | head -n1
00000000: 0000 0000 0000 0000 0000 0000 0000 0000 ................

After:

$ objcopy --change-section-address .BTF=0 \
--set-section-flags .BTF=alloc -O binary \
--only-section=.BTF vmlinux .btf.vmlinux.bin
$ xxd .btf.vmlinux.bin | head -n1
00000000: 9feb 0100 1800 0000 0000 0000 80e1 1c00 ................
^BTF magic

As part of this change, I'm also dropping '2>/dev/null' from objcopy
invocation to be able to catch possible other issues (objcopy doesn't
produce any warnings for me anymore, it did before with --dump-section).

Fixes: da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191127225759.39923-1-sdf@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
scripts/link-vmlinux.sh | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -127,8 +127,9 @@ gen_btf()
cut -d, -f1 | cut -d' ' -f2)
bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
awk '{print $4}')
- ${OBJCOPY} --set-section-flags .BTF=alloc -O binary \
- --only-section=.BTF ${1} .btf.vmlinux.bin 2>/dev/null
+ ${OBJCOPY} --change-section-address .BTF=0 \
+ --set-section-flags .BTF=alloc -O binary \
+ --only-section=.BTF ${1} .btf.vmlinux.bin
${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
--rename-section .data=.BTF .btf.vmlinux.bin ${2}
}

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