Em Thu, Oct 06, 2022 at 02:23:16PM -0300, Arnaldo Carvalho de Melo escreveu: > Em Thu, Oct 06, 2022 at 09:04:58AM -0700, Andrii Nakryiko escreveu: > > On Thu, Oct 6, 2022 at 7:00 AM Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx> wrote: > > > Em Thu, Oct 06, 2022 at 10:43:22AM -0300, Arnaldo Carvalho de Melo escreveu: > > > > 15e20ce2324a:~/git/linux # readelf -wi ./arch/x86/entry/entry.o > > > > Contents of the .debug_info section: > > > > > Compilation Unit @ offset 0: > > > > Length: 0x35 (32-bit) > > > > Version: 5 > > > > Unit Type: DW_UT_compile (1) > > > > Abbrev Offset: 0 > > > > Pointer Size: 8 > > > > <0><c>: Abbrev Number: 1 (DW_TAG_compile_unit) > > > > <d> DW_AT_stmt_list : 0 > > > > <11> DW_AT_low_pc : 0 > > > > <19> DW_AT_high_pc : 19 > > > > <1a> DW_AT_name : (indirect string, offset: 0): arch/x86/entry/entry.S > > > > <1e> DW_AT_comp_dir : (indirect string, offset: 0x17): /root/git/linux > > > > <22> DW_AT_producer : (indirect string, offset: 0x27): GNU AS 2.39.50 > > > > <26> DW_AT_language : 32769 (MIPS assembler) > > > > <1><28>: Abbrev Number: 2 (DW_TAG_subprogram) > > > > <29> DW_AT_name : (indirect string, offset: 0x36): entry_ibpb > > > > <2d> DW_AT_external : 1 > > > > <2d> DW_AT_type : <0x37> > > > > <2e> DW_AT_low_pc : 0 > > > > <36> DW_AT_high_pc : 19 > > > > <1><37>: Abbrev Number: 3 (DW_TAG_unspecified_type) > > > > <1><38>: Abbrev Number: 0 > > > > > 15e20ce2324a:~/git/linux # > > > > > Which pahole -J barfs on: > > > > > 15e20ce2324a:~/git/linux # pahole -J ./arch/x86/entry/entry.o > > > > [1] UNKNOWN (anon) Unexpected kind for reference > > > > 15e20ce2324a:~/git/linux # > > > > > But if we ask it to exclude asm CUs (<26> DW_AT_language : 32769 > > > > (MIPS assembler)) then it ignores it, so this is a workaround. > > > > > 15e20ce2324a:~/git/linux # pahole --lang_exclude asm -V -J ./arch/x86/entry/entry.o > > > > 15e20ce2324a:~/git/linux # > > > > > Now I'm looking at how to get the BTF encoder grokking this. > > > > This is what I came up with, Andrii, Yonghong, wdyt? > > > Does `const void` make sense? Why not just keeping it as "void"? > > "const void" might confuse tooling and BTF verifier in kernel, but I > > haven't checked. Just trying to understand why we need extra "const". > > I was trying to have an entry for that DW_TAG_unspecified_type type > instead of plain changing the type for entry_ibpb (a DW_TAG_subprogram) > straight to 0 (void). I think this is the best way forward at the moment, i.e. to just change the type for such DW_TAG_subprogram to zero, aka void, losing this information by default. I'll have a --btf_encode_unspecified_type as this const void trick as an opt-in, who knows if this ends up being useful :-) - Arnaldo > I.e. we wouldn't lose that info that the return type for that "function" > (it was encoded as a DW_TAG_subprogram) isn't defined. Using straight > void would state that it doesn't return anything. > > I couldn't find a way to use KCFLAGS to ask Kbuild to pass that -B flag > to gcc so that I could rebuild the whole kernel with it to check if the > build goes thru and then try booting with such BTF info. > > - Arnaldo > > > > As an additional info clang 14 (haven't tested with other versions) > > > encodes such ASM Labels as DW_TAG_label and this thus isn't an issue > > > there. > > > > > > - Arnaldo > > > > > > commit 15ec614672da043008df31aa6ee85ebc5105d4fd > > > Author: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> > > > Date: Tue Oct 4 18:22:53 2022 -0300 > > > > > > btf_encoder: Encode DW_TAG_unspecified_type as BTF_KIND_CONST > > > > > > This first appeared for assembler files in the Linux kernel with recent > > > GNU compilers, we don't have anything in BTF, AFAIK, to properly > > > represent that, so, for now, lets go with BTF_KIND_CONST. > > > > > > Testing it: > > > > > > Built binutils from git://sourceware.org/git/binutils-gdb.git, then used > > > gcc's -B option to point to the directory with the new as, that is built > > > as as-new, so make a symlink, ending up with: > > > > > > 15e20ce2324a:~/git/linux # readelf -wi ./arch/x86/entry/entry.o > > > Contents of the .debug_info section: > > > > > > Compilation Unit @ offset 0: > > > Length: 0x35 (32-bit) > > > Version: 5 > > > Unit Type: DW_UT_compile (1) > > > Abbrev Offset: 0 > > > Pointer Size: 8 > > > <0><c>: Abbrev Number: 1 (DW_TAG_compile_unit) > > > <d> DW_AT_stmt_list : 0 > > > <11> DW_AT_low_pc : 0 > > > <19> DW_AT_high_pc : 19 > > > <1a> DW_AT_name : (indirect string, offset: 0): arch/x86/entry/entry.S > > > <1e> DW_AT_comp_dir : (indirect string, offset: 0x17): /root/git/linux > > > <22> DW_AT_producer : (indirect string, offset: 0x27): GNU AS 2.39.50 > > > <26> DW_AT_language : 32769 (MIPS assembler) > > > <1><28>: Abbrev Number: 2 (DW_TAG_subprogram) > > > <29> DW_AT_name : (indirect string, offset: 0x36): entry_ibpb > > > <2d> DW_AT_external : 1 > > > <2d> DW_AT_type : <0x37> > > > <2e> DW_AT_low_pc : 0 > > > <36> DW_AT_high_pc : 19 > > > <1><37>: Abbrev Number: 3 (DW_TAG_unspecified_type) > > > <1><38>: Abbrev Number: 0 > > > > > > So we have that asm label encoded by GNU AS 2.39.50 as a > > > DW_TAG_subprogram that has as its DW_AT_type the DW_TAG_unspecified_type > > > 0x37 that we encode as a BTF_KIND_CONST pointing to 0 (void): > > > > > > 15e20ce2324a:~/git/linux # pahole -J ./arch/x86/entry/entry.o > > > 15e20ce2324a:~/git/linux # pahole -JV ./arch/x86/entry/entry.o > > > btf_encoder__new: './arch/x86/entry/entry.o' doesn't have '.data..percpu' section > > > Found 0 per-CPU variables! > > > Found 1 functions! > > > File ./arch/x86/entry/entry.o: > > > [1] CONST (anon) type_id=0 > > > [2] FUNC_PROTO (anon) return=1 args=(void) > > > [3] FUNC entry_ibpb type_id=2 > > > 15e20ce2324a:~/git/linux # pfunct -F btf ./arch/x86/entry/entry.o > > > entry_ibpb > > > 15e20ce2324a:~/git/linux # pfunct --proto -F btf ./arch/x86/entry/entry.o > > > const void entry_ibpb(void); > > > 15e20ce2324a:~/git/linux # > > > > > > 15e20ce2324a:~/git/linux # tools/bpf/bpftool/bpftool btf dump file ./arch/x86/entry/entry.o format raw > > > [1] CONST '(anon)' type_id=0 > > > [2] FUNC_PROTO '(anon)' ret_type_id=1 vlen=0 > > > [3] FUNC 'entry_ibpb' type_id=2 linkage=static > > > 15e20ce2324a:~/git/linux # > > > > > > I think this is what can be done to avoid having to skip ASM DWARF when > > > gets widely used, i.e. binutils gets updated. > > > > > > Reported-by: Martin Liška <mliska@xxxxxxx> > > > Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> > > > Cc: Yonghong Song <yhs@xxxxxx> > > > Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> > > > > > > diff --git a/btf_encoder.c b/btf_encoder.c > > > index 7ad3f29ef153d8d6..7e50ba711ccc14d6 100644 > > > --- a/btf_encoder.c > > > +++ b/btf_encoder.c > > > @@ -962,6 +962,8 @@ static int btf_encoder__encode_tag(struct btf_encoder *encoder, struct tag *tag, > > > return btf_encoder__add_enum_type(encoder, tag, conf_load); > > > case DW_TAG_subroutine_type: > > > return btf_encoder__add_func_proto(encoder, tag__ftype(tag), type_id_off); > > > + case DW_TAG_unspecified_type: > > > + return btf_encoder__add_ref_type(encoder, BTF_KIND_CONST, 0, NULL, false); > > > default: > > > fprintf(stderr, "Unsupported DW_TAG_%s(0x%x): type: 0x%x\n", > > > dwarf_tag_name(tag->tag), tag->tag, ref_type_id); > > -- > > - Arnaldo -- - Arnaldo
