Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Sedat Dilek
2021-02-09T17:09:06Z

On Tue, Feb 9, 2021 at 5:35 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Tue, Feb 09, 2021 at 05:13:38PM +0100, Jiri Olsa wrote:
> > On Tue, Feb 09, 2021 at 04:09:36PM +0100, Jiri Olsa wrote:
> >
> > SNIP
> >
> > > > > > > DW_AT_prototyped (true)
> > > > > > > DW_AT_type (0x01cfdfe4 "long int")
> > > > > > > DW_AT_external (true)
> > > > > > >
> > > > > >
> > > > > > Ok, the problem appears to be not in DWARF, but in mcount_loc data.
> > > > > > vfs_truncate's address is not recorded as ftrace-attachable, and thus
> > > > > > pahole ignores it. I don't know why this happens and it's quite
> > > > > > strange, given vfs_truncate is just a normal global function.
> > > >
> > > > right, I can't see it in mcount adresses.. but it begins with instructions
> > > > that appears to be nops, which would suggest it's traceable
> > > >
> > > > ffff80001031f430 <vfs_truncate>:
> > > > ffff80001031f430: 5f 24 03 d5 hint #34
> > > > ffff80001031f434: 1f 20 03 d5 nop
> > > > ffff80001031f438: 1f 20 03 d5 nop
> > > > ffff80001031f43c: 3f 23 03 d5 hint #25
> > > >
> > > > > >
> > > > > > I'd like to understand this issue before we try to fix it, but there
> > > > > > is at least one improvement we can make: pahole should check ftrace
> > > > > > addresses only for static functions, not the global ones (global ones
> > > > > > should be always attachable, unless they are special, e.g., notrace
> > > > > > and stuff). We can easily check that by looking at the corresponding
> > > > > > symbol. But I'd like to verify that vfs_truncate is ftrace-attachable
> > >
> > > I'm still trying to build the kernel.. however ;-)
> >
> > I finally reproduced.. however arm's not using mcount_loc
> > but some other special section.. so it's new mess for me
> >
> > however I tried the same build without LLVM=1 and it passed,
> > so my guess is that clang is doing something unexpected
> >
> > jirka
> >
>
> Additionally, if I remove the btfid generation section in
> scripts/link-vmlinux.sh to bypass that and get a working Image.gz,
> vfs_truncate is in the list of available functions:
>

I did the same trick by commenting in that section to provide a BROKEN
vmlinux file for linux-bpf folks.

[ BTFIDS-vmlinux.diff ]
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index eef40fa9485d..40f1b6aae553 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -330,7 +330,7 @@ vmlinux_link vmlinux "${kallsymso}" ${btf_vmlinux_bin_o}
# fill in BTF IDs
if [ -n "${CONFIG_DEBUG_INFO_BTF}" -a -n "${CONFIG_BPF}" ]; then
info BTFIDS vmlinux
- ${RESOLVE_BTFIDS} vmlinux
+ ##${RESOLVE_BTFIDS} vmlinux
fi

if [ -n "${CONFIG_BUILDTIME_TABLE_SORT}" ]; then
- EOF -

We should ask linux-kbuild/Masahiro to have an option to OVERRIDE:
When scripts/link-vmlinux.sh fails all generated files like vmlinux get removed.
For further debugging they should be kept.

My €0.02.

- Sedat -

> / # grep vfs_truncate /sys/kernel/debug/tracing/available_filter_functions
> vfs_truncate
>
> / # cat /proc/version
> Linux version 5.11.0-rc7-dirty (nathan@ubuntu-m3-large-x86) (ClangBuiltLinux clang version 13.0.0 (https://github.com/llvm/llvm-project 8f130f108fedfcf6cb80ef594560a87341028a37), LLD 13.0.0 (https://github.com/llvm/llvm-project 8f130f108fedfcf6cb80ef594560a87341028a37)) #1 SMP PREEMPT Tue Feb 9 09:25:36 MST 2021
>
> / # zcat /proc/config.gz | grep "DEBUG_INFO_BTF\|FTRACE\|BPF"
> # CONFIG_CGROUP_BPF is not set
> CONFIG_BPF=y
> # CONFIG_BPF_LSM is not set
> CONFIG_BPF_SYSCALL=y
> CONFIG_ARCH_WANT_DEFAULT_BPF_JIT=y
> # CONFIG_BPF_JIT_ALWAYS_ON is not set
> CONFIG_BPF_JIT_DEFAULT_ON=y
> # CONFIG_BPF_PRELOAD is not set
> # CONFIG_NETFILTER_XT_MATCH_BPF is not set
> # CONFIG_BPFILTER is not set
> # CONFIG_NET_CLS_BPF is not set
> # CONFIG_NET_ACT_BPF is not set
> CONFIG_BPF_JIT=y
> CONFIG_HAVE_EBPF_JIT=y
> # CONFIG_PSTORE_FTRACE is not set
> CONFIG_DEBUG_INFO_BTF=y
> CONFIG_DEBUG_INFO_BTF_MODULES=y
> CONFIG_HAVE_DYNAMIC_FTRACE=y
> CONFIG_HAVE_DYNAMIC_FTRACE_WITH_REGS=y
> CONFIG_HAVE_FTRACE_MCOUNT_RECORD=y
> CONFIG_FTRACE=y
> CONFIG_DYNAMIC_FTRACE=y
> CONFIG_DYNAMIC_FTRACE_WITH_REGS=y
> # CONFIG_FTRACE_SYSCALLS is not set
> CONFIG_BPF_EVENTS=y
> CONFIG_FTRACE_MCOUNT_RECORD=y
> # CONFIG_FTRACE_RECORD_RECURSION is not set
> # CONFIG_FTRACE_STARTUP_TEST is not set
> # CONFIG_TEST_BPF is not set
>
> Cheers,
> Nathan
>
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