* [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux
@ 2019-11-27 22:57 Stanislav Fomichev
2019-11-28 4:37 ` John Fastabend
0 siblings, 1 reply; 3+ messages in thread
From: Stanislav Fomichev @ 2019-11-27 22:57 UTC (permalink / raw)
To: netdev, bpf; +Cc: davem, ast, daniel, Stanislav Fomichev, Andrii Nakryiko
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
^ permalink raw reply related [flat|nested] 3+ messages in thread* RE: [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux
2019-11-27 22:57 [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux Stanislav Fomichev
@ 2019-11-28 4:37 ` John Fastabend
2019-11-29 15:16 ` Daniel Borkmann
0 siblings, 1 reply; 3+ messages in thread
From: John Fastabend @ 2019-11-28 4:37 UTC (permalink / raw)
To: Stanislav Fomichev, netdev, bpf
Cc: davem, ast, daniel, Stanislav Fomichev, Andrii Nakryiko
Stanislav Fomichev wrote:
> 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).
Agree dropping /dev/null seems like a good choice. Otherwise seems reasonable
to me.
Acked-by: John Fastabend <john.fastabend@gmail.com>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux
2019-11-28 4:37 ` John Fastabend
@ 2019-11-29 15:16 ` Daniel Borkmann
0 siblings, 0 replies; 3+ messages in thread
From: Daniel Borkmann @ 2019-11-29 15:16 UTC (permalink / raw)
To: John Fastabend, Stanislav Fomichev, netdev, bpf
Cc: davem, ast, Andrii Nakryiko
On 11/28/19 5:37 AM, John Fastabend wrote:
> Stanislav Fomichev wrote:
>> 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).
>
> Agree dropping /dev/null seems like a good choice. Otherwise seems reasonable
> to me.
>
> Acked-by: John Fastabend <john.fastabend@gmail.com>
Could reproduce as well:
root@apoc:~/bpf# xxd .btf.vmlinux.bin.old | head -n1 (original)
00000000: 9feb 0100 1800 0000 0000 0000 5088 2000 ............P. .
root@apoc:~/bpf# ls -l .btf.vmlinux.bin.old
-rw-r--r-- 1 root root 3439882 Nov 29 15:59 .btf.vmlinux.bin.old
root@apoc:~/bpf# xxd .btf.vmlinux.bin.new-buggy | head -n1 ('bpf: Support pre-2.25-binutils objcopy for vmlinux BTF')
00000000: 0000 0000 0000 0000 0000 0000 0000 0000 ................
root@apoc:~/bpf# ls -l .btf.vmlinux.bin.new-buggy
-rwxr-xr-x 1 root root 45705482 Nov 29 16:01 .btf.vmlinux.bin.new-buggy
root@apoc:~/bpf# xxd .btf.vmlinux.bin.new-fixed | head -n1 ('bpf: Force .BTF section start to zero when dumping from vmlinux')
00000000: 9feb 0100 1800 0000 0000 0000 5088 2000 ............P. .
root@apoc:~/bpf# ls -l .btf.vmlinux.bin.new-fixed
-rwxr-xr-x 1 root root 3439882 Nov 29 16:02 .btf.vmlinux.bin.new-fixed
root@apoc:~/bpf# diff .btf.vmlinux.bin.old .btf.vmlinux.bin.new-buggy
Binary files .btf.vmlinux.bin.old and .btf.vmlinux.bin.new-buggy differ
root@apoc:~/bpf# diff .btf.vmlinux.bin.old .btf.vmlinux.bin.new-fixed
root@apoc:~/bpf#
Applied, thanks!
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2019-11-29 15:38 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2019-11-27 22:57 [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux Stanislav Fomichev
2019-11-28 4:37 ` John Fastabend
2019-11-29 15:16 ` Daniel Borkmann
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