hi,
we're hitting a problem on s390 with BTF data alignment.
When running simple test, we're getting this message from
verifier and console:
bpf_common.c:91: BROK: Failed verification: in-kernel BTF is malformed
[ 41.545572] BPF:Total section length too long
AFAICS it happens when .BTF section data size is not an even number ;-)
DISCLAIMER I'm quite ignorant of s390x arch details, so most likely I'm
totally wrong and perhaps missing something important and there's simple
explanation.. but here's what got me here:
... so BTF data is placed in .BTF section via linker script:
.BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { \
__start_BTF = .; \
*(.BTF) \
__stop_BTF = .; \
}
and the .BTF data size in btf_parse_vmlinux is computed as:
btf->data_size = __stop_BTF - __start_BTF;
this computation is compiled as:
00000000002aeb20 <btf_parse_vmlinux>:
...
2aeb8a: larl %r1,cda3ac <__start_BTF+0x2084a8> # loads r1 with end
2aeb90: larl %r2,ad1f04 <__start_BTF> # loads r2 with start
2aeb96: sgr %r1,%r2 # substract r1 - r2
having following values for start/stop_BTF symbols:
# nm ./vmlinux | grep __start_BTF
0000000000ad1f04 R __start_BTF
# nm ./vmlinux | grep __stop_BTF
0000000000cda3ad R __stop_BTF
-> the BTF data size is 0x2084a9
but as you can see the instruction that loads the 'end' symbol:
larl %r1,cda3ac <__start_BTF+0x2084a8>
is loading '__start_BTF + 0x2084a8', which is '__stop_BTF - 1'
>From spec it seems that larl instruction's argument must be even
number ([1] page 7-214):
2. For LOAD RELATIVE LONG, the second oper-and must be aligned
on an integral boundary cor-responding to the operand’s size.
I also found an older bug complaining about this issue [2]:
...
larl instruction can only load even values - instructions on s390 are 2-byte
aligned and the instruction encodes offset to the target in 2-byte units.
...
The GNU BFD linker for s390 doesn't bother to check if relocations fit or are
properly aligned.
...
I tried to fix that aligning the end to even number, but then
btf_check_sec_info logic needs to be adjusted as well, and
probably other places as well.. so I decided to share this
first.. because it all seems wrong ;-)
thoughts? thanks,
jirka
[1] http://publibfi.boulder.ibm.com/epubs/pdf/dz9zr008.pdf
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=18960
