On Tue, Feb 09, 2021 at 10:41:44PM +0100, Jiri Olsa wrote: > On Tue, Feb 09, 2021 at 09:50:48PM +0100, Jiri Olsa wrote: > > On Tue, Feb 09, 2021 at 12:09:31PM -0800, Nick Desaulniers wrote: > > > On Tue, Feb 9, 2021 at 11:06 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > > > > > On Tue, Feb 09, 2021 at 05:13:42PM +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 > > > > > > > > so ftrace data actualy has vfs_truncate address but with extra 4 bytes: > > > > > > > > ffff80001031f434 > > > > > > > > real vfs_truncate address: > > > > > > > > ffff80001031f430 g F .text 0000000000000168 vfs_truncate > > > > > > > > vfs_truncate disasm: > > > > > > > > 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 > > > > > > > > thats why we don't match it in pahole.. I checked few other functions > > > > and some have the same problem and some match the function boundary > > > > > > > > those that match don't have that first hint instrucion, like: > > > > > > > > ffff800010321e40 <do_faccessat>: > > > > ffff800010321e40: 1f 20 03 d5 nop > > > > ffff800010321e44: 1f 20 03 d5 nop > > > > ffff800010321e48: 3f 23 03 d5 hint #25 > > > > > > > > any hints about hint instructions? ;-) > > > > > > aarch64 makes *some* newer instructions reuse the "hint" ie "nop" > > > encoding space to make software backwards compatible on older hardware > > > that doesn't support such instructions. Is this BTI, perhaps? (The > > > function is perhaps the destination of an indirect call?) > > > > I see, I think we can't take ftrace addresses as start of the function > > because there could be extra instruction(s) related to the call before > > it like here > > > > we need to check ftrace address be within the function/symbol, > > not exact start > > the build with gcc passed only because mcount data are all zeros > and pahole falls back to 'not-ftrace' mode > > $ llvm-objdump -t build/aarch64-gcc/vmlinux | grep mcount > ffff800011eb4840 g .init.data 0000000000000000 __stop_mcount_loc > ffff800011e47d58 g .init.data 0000000000000000 __start_mcount_loc > > $ llvm-objdump -s build/aarch64-gcc/vmlinux > ffff800011e47d50 00000000 00000000 00000000 00000000 ................ > ffff800011e47d60 00000000 00000000 00000000 00000000 ................ > ffff800011e47d70 00000000 00000000 00000000 00000000 ................ > ffff800011e47d80 00000000 00000000 00000000 00000000 ................ > ffff800011e47d90 00000000 00000000 00000000 00000000 ................ > ... > > we should check on why it's zero > > Nathan, any chance you could run kernel built with gcc and check on ftrace? Sure, with GCC 10.2.0: / # cat /proc/version Linux version 5.11.0-rc7 (nathan@ubuntu-m3-large-x86) (aarch64-linux-gcc (GCC) 10.2.0, GNU ld (GNU Binutils) 2.35) #1 SMP PREEMPT Tue Feb 9 16:04:19 MST 2021 / # grep vfs_truncate /sys/kernel/debug/tracing/available_filter_functions vfs_truncate / # 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 > the build with clang fails because the ftrace data are there, > but pahole takes them as 'start' of the function, which is wrong > > $ llvm-objdump -t build/aarch64/vmlinux | grep mcount > ffff800011d27d10 g .init.data 0000000000000000 __start_mcount_loc > ffff800011d90038 g .init.data 0000000000000000 __stop_mcount_loc > > $ llvm-objdump -s build/aarch64-gcc/vmlinux > ffff800011d27d10 cc330110 0080ffff 1c340110 0080ffff .3.......4...... > ffff800011d27d20 6c340110 0080ffff 1004c111 0080ffff l4.............. > ffff800011d27d30 3804c111 0080ffff 6004c111 0080ffff 8.......`....... > ffff800011d27d40 8804c111 0080ffff 0405c111 0080ffff ................ > ffff800011d27d50 3805c111 0080ffff 7c05c111 0080ffff 8.......|....... > ... > > I think if we fix pahole to take check the ftrace address is > within the processed function, we should be fine.. I'll try to > send something soon > > jirka >
