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@xxxxxxxxx>
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!
