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From: Stanislav Fomichev <sdf@google.com>
To: netdev@vger.kernel.org, bpf@vger.kernel.org
Cc: davem@davemloft.net, ast@kernel.org, daniel@iogearbox.net,
	Stanislav Fomichev <sdf@google.com>,
	Andrii Nakryiko <andriin@fb.com>
Subject: [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux
Date: Wed, 27 Nov 2019 14:57:59 -0800	[thread overview]
Message-ID: <20191127225759.39923-1-sdf@google.com> (raw)

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@fb.com>
Fixes: da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF")
Signed-off-by: Stanislav Fomichev <sdf@google.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


             reply	other threads:[~2019-11-27 22:58 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-27 22:57 Stanislav Fomichev [this message]
2019-11-28  4:37 ` [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux John Fastabend
2019-11-29 15:16   ` Daniel Borkmann

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