Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

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

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                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

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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:

/ # 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