Em Mon, Oct 10, 2022 at 09:06:46AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Fri, Oct 07, 2022 at 05:25:21PM -0700, Yonghong Song escreveu:
> > For function entry_ibpb, actually the kernel has a prorotype,
> > include/asm/nospec-branch.h:extern void entry_ibpb(void);
> > But unfortunately since the function is defined in asm code.
> > The actual type information is not encoded in dwarf so pahole
> > does not have enough info from vmlinux dwarf to get func
> > types. The compiler does not generate func types based on
> > declarations.
> Right
> > This may prevent people from tracing functions defined in
> > asm code but this should be extremely rare.
> > So I agree that BTF type id 0 is probably the best choice.
>
> Thanks for your comments, that is the way I'll do it.
Done, some prep patches then the patch below.
If possible, please ack :-)
- Arnaldo
commit 3abc72d9d56ec0cbfffc1794d2bf5d527d1e88ba
Author: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Date: Mon Oct 10 11:20:07 2022 -0300
btf_encoder: Encode DW_TAG_unspecified_type returning routines as void
Since we don´t have how to encode this info in BTF, and from what we
saw, at least in this case:
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 convert 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: 'entry.o' doesn't have '.data..percpu' section
Found 0 per-CPU variables!
Found 1 functions!
File entry.o:
[1] FUNC_PROTO (anon) return=0 args=(void)
[2] FUNC entry_ibpb type_id=1
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
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] FUNC_PROTO '(anon)' ret_type_id=0 vlen=0
[2] FUNC 'entry_ibpb' type_id=1 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.
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>,
Cc: Martin Liška <mliska@xxxxxxx>
Cc: Yonghong Song <yhs@xxxxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
diff --git a/btf_encoder.c b/btf_encoder.c
index fb2ca77e2e9bf144..a5fa04a84ee246ee 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -593,6 +593,19 @@ static int32_t btf_encoder__add_func_param(struct btf_encoder *encoder, const ch
}
}
+static int32_t btf_encoder__tag_type(struct btf_encoder *encoder, uint32_t type_id_off, uint32_t tag_type)
+{
+ if (tag_type == 0)
+ return 0;
+
+ if (encoder->cu->unspecified_type.tag && tag_type == encoder->cu->unspecified_type.type) {
+ // No provision for encoding this, turn it into void.
+ return 0;
+ }
+
+ return type_id_off + tag_type;
+}
+
static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct ftype *ftype, uint32_t type_id_off)
{
struct btf *btf = encoder->btf;
@@ -603,7 +616,7 @@ static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct f
/* add btf_type for func_proto */
nr_params = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);
- type_id = ftype->tag.type == 0 ? 0 : type_id_off + ftype->tag.type;
+ type_id = btf_encoder__tag_type(encoder, type_id_off, ftype->tag.type);
id = btf__add_func_proto(btf, type_id);
if (id > 0) {
@@ -966,6 +979,15 @@ 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:
+ /* Just don't encode this for now, converting anything with this type to void (0) instead.
+ *
+ * If we end up needing to encode this, one possible hack is to do as follows, as "const void".
+ *
+ * Returning zero means we skipped encoding a DWARF type.
+ */
+ // btf_encoder__add_ref_type(encoder, BTF_KIND_CONST, 0, NULL, false);
+ return 0;
default:
fprintf(stderr, "Unsupported DW_TAG_%s(0x%x): type: 0x%x\n",
dwarf_tag_name(tag->tag), tag->tag, ref_type_id);
@@ -1487,7 +1509,7 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co
{
uint32_t type_id_off = btf__type_cnt(encoder->btf) - 1;
struct llvm_annotation *annot;
- int btf_type_id, tag_type_id;
+ int btf_type_id, tag_type_id, skipped_types = 0;
uint32_t core_id;
struct function *fn;
struct tag *pos;
@@ -1510,8 +1532,13 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co
cu__for_each_type(cu, core_id, pos) {
btf_type_id = btf_encoder__encode_tag(encoder, pos, type_id_off, conf_load);
+ if (btf_type_id == 0) {
+ ++skipped_types;
+ continue;
+ }
+
if (btf_type_id < 0 ||
- tag__check_id_drift(pos, core_id, btf_type_id, type_id_off)) {
+ tag__check_id_drift(pos, core_id, btf_type_id + skipped_types, type_id_off)) {
err = -1;
goto out;
}
