netdev - [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux

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Message-Id: <20191127225759.39923-1-sdf@google.com>
Date:   Wed, 27 Nov 2019 14:57:59 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        Stanislav Fomichev <sdf@...gle.com>,
        Andrii Nakryiko <andriin@...com>
Subject: [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux

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).

Cc: Andrii Nakryiko <andriin@...com>
Fixes: da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF")
Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
---
 scripts/link-vmlinux.sh | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 2998ddb323e3..436379940356 100755
--- 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}
 }
-- 
2.24.0.432.g9d3f5f5b63-goog