On Tue, Feb 9, 2021 at 7:09 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > On Tue, Feb 09, 2021 at 01:36:41PM +0100, Jiri Olsa wrote: > > On Tue, Feb 09, 2021 at 12:49:04AM -0700, Nathan Chancellor wrote: > > > On Mon, Feb 08, 2021 at 10:56:36PM -0800, Andrii Nakryiko wrote: > > > > On Mon, Feb 8, 2021 at 10:13 PM Andrii Nakryiko > > > > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > > > > > > > On Mon, Feb 8, 2021 at 10:09 PM Andrii Nakryiko > > > > > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > > > > > > > > > On Mon, Feb 8, 2021 at 9:23 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > On Mon, Feb 08, 2021 at 08:45:43PM -0800, Andrii Nakryiko wrote: > > > > > > > > On Mon, Feb 8, 2021 at 7:44 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > Hi all, > > > > > > > > > > > > > > > > > > Recently, an issue with CONFIG_DEBUG_INFO_BTF was reported for arm64: > > > > > > > > > https://groups.google.com/g/clang-built-linux/c/de_mNh23FOc/m/E7cu5BwbBAAJ > > > > > > > > > > > > > > > > > > $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \ > > > > > > > > > LLVM=1 O=build/aarch64 defconfig > > > > > > > > > > > > > > > > > > $ scripts/config \ > > > > > > > > > --file build/aarch64/.config \ > > > > > > > > > -e BPF_SYSCALL \ > > > > > > > > > -e DEBUG_INFO_BTF \ > > > > > > > > > -e FTRACE \ > > > > > > > > > -e FUNCTION_TRACER > > > > > > > > > > > > > > > > > > $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \ > > > > > > > > > LLVM=1 O=build/aarch64 olddefconfig all > > > > > > > > > ... > > > > > > > > > FAILED unresolved symbol vfs_truncate > > > > > > > > > ... > > > > > > > > > > > > > > > > > > My bisect landed on commit 6e22ab9da793 ("bpf: Add d_path helper") > > > > > > > > > although that seems obvious given that is what introduced > > > > > > > > > BTF_ID(func, vfs_truncate). > > > > > > > > > > > > > > > > > > I am using the latest pahole v1.20 and LLVM is at > > > > > > > > > https://github.com/llvm/llvm-project/commit/14da287e18846ea86e45b421dc47f78ecc5aa7cb > > > > > > > > > although I can reproduce back to LLVM 10.0.1, which is the earliest > > > > > > > > > version that the kernel supports. I am very unfamiliar with BPF so I > > > > > > > > > have no idea what is going wrong here. Is this a known issue? > > > > > > > > > > > > > > > > > > > > > > > > > I'll skip the reproduction games this time and will just request the > > > > > > > > vmlinux image. Please upload somewhere so that we can look at DWARF > > > > > > > > and see what's going on. Thanks. > > > > > > > > > > > > > > > > > > > > > > Sure thing, let me know if this works. I uploaded in two places to make > > > > > > > it easier to grab: > > > > > > > > > > > > > > zstd compressed: > > > > > > > https://github.com/nathanchance/bug-files/blob/3b2873751e29311e084ae2c71604a1963f5e1a48/btf-aarch64/vmlinux.zst > > > > > > > > > > > > > > > > > > > Thanks. I clearly see at least one instance of seemingly well-formed > > > > > > vfs_truncate DWARF declaration. Also there is a proper ELF symbol for > > > > > > it. Which means it should have been generated in BTF, but it doesn't > > > > > > appear to be, so it does seem like a pahole bug. I (or someone else > > > > > > before me) will continue tomorrow. > > > > > > > > > > > > $ llvm-dwarfdump vmlinux > > > > > > ... > > > > > > > > > > > > 0x00052e6f: DW_TAG_subprogram > > > > > > DW_AT_name ("vfs_truncate") > > > > > > DW_AT_decl_file > > > > > > ("/home/nathan/cbl/src/linux/include/linux/fs.h") > > > > > > DW_AT_decl_line (2520) > > > > > > DW_AT_prototyped (true) > > > > > > DW_AT_type (0x000452cb "long int") > > > > > > DW_AT_declaration (true) > > > > > > DW_AT_external (true) > > > > > > > > > > > > 0x00052e7b: DW_TAG_formal_parameter > > > > > > DW_AT_type (0x00045fc6 "const path*") > > > > > > > > > > > > 0x00052e80: DW_TAG_formal_parameter > > > > > > DW_AT_type (0x00045213 "long long int") > > > > > > > > > > > > ... > > > > > > > > > > > > > > > > ... and here's the *only* other one (not marked as declaration, but I > > > > > thought we already handle that, Jiri?): > > > > > > > > > > 0x01d0da35: DW_TAG_subprogram > > > > > DW_AT_low_pc (0xffff80001031f430) > > > > > DW_AT_high_pc (0xffff80001031f598) > > > > > DW_AT_frame_base (DW_OP_reg29) > > > > > DW_AT_GNU_all_call_sites (true) > > > > > DW_AT_name ("vfs_truncate") > > > > > DW_AT_decl_file ("/home/nathan/cbl/src/linux/fs/open.c") > > > > > DW_AT_decl_line (69) > > > > > 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 ;-) > > patch below adds the ftrace check only for static functions > and lets the externa go through.. but as you said, in this > case we'll need to figure out the 'notrace' and other checks > ftrace is doing > > jirka > > > --- > diff --git a/btf_encoder.c b/btf_encoder.c > index b124ec20a689..4d147406cfa5 100644 > --- a/btf_encoder.c > +++ b/btf_encoder.c > @@ -734,7 +734,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force, > continue; > if (!has_arg_names(cu, &fn->proto)) > continue; > - if (functions_cnt) { > + if (!fn->external && functions_cnt) { I wouldn't trust DWARF, honestly. Wouldn't checking GLOBAL vs LOCAL FUNC ELF symbol be more reliable? > struct elf_function *func; > const char *name; > > @@ -746,9 +746,6 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force, > if (!func || func->generated) > continue; > func->generated = true; > - } else { > - if (!fn->external) > - continue; > } > > btf_fnproto_id = btf_elf__add_func_proto(btfe, cu, &fn->proto, type_id_off); >
