Em Mon, Aug 24, 2020 at 05:45:23PM -0700, Hao Luo escreveu:
> It is found on gcc 8.2 that global percpu variables generate the
> following dwarf entry in the cu where the variable is defined[1].
>
> Take the global variable "bpf_prog_active" defined in
> kernel/bpf/syscall.c as an example. The debug info for syscall.c
> has two dwarf entries for "bpf_prog_active".
>
> > readelf -wi kernel/bpf/syscall.o
>
> 0x00013534: DW_TAG_variable
> DW_AT_name ("bpf_prog_active")
> DW_AT_decl_file
> ("/data/users/yhs/work/net-next/include/linux/bpf.h")
> DW_AT_decl_line (1074)
> DW_AT_decl_column (0x01)
> DW_AT_type (0x000000d6 "int")
> DW_AT_external (true)
> DW_AT_declaration (true)
>
> 0x00021a25: DW_TAG_variable
> DW_AT_specification (0x00013534 "bpf_prog_active")
> DW_AT_decl_file
> ("/data/users/yhs/work/net-next/kernel/bpf/syscall.c")
> DW_AT_decl_line (43)
> DW_AT_location (DW_OP_addr 0x0)
Interesting, here I get, with binutils' readelf:
[root@quaco perf]# readelf -wi ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep bpf_prog_active
<f6a1> DW_AT_name : (indirect string, offset: 0xb70d): bpf_prog_active
[root@quaco perf]#
Just one, as:
[root@quaco perf]# readelf -wi ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep bpf_prog_active -B1 -A8
<1><f6a0>: Abbrev Number: 103 (DW_TAG_variable)
<f6a1> DW_AT_name : (indirect string, offset: 0xb70d): bpf_prog_active
<f6a5> DW_AT_decl_file : 11
<f6a6> DW_AT_decl_line : 1008
<f6a8> DW_AT_decl_column : 1
<f6a9> DW_AT_type : <0xcf>
<f6ad> DW_AT_external : 1
<f6ad> DW_AT_declaration : 1
<1><f6ad>: Abbrev Number: 103 (DW_TAG_variable)
<f6ae> DW_AT_name : (indirect string, offset: 0x3a5d): bpf_stats_enabled_mutex
[root@quaco perf]#
I get what you have when I use elfutils' readelf:
[root@quaco perf]# eu-readelf -winfo ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep bpf_prog_active
name (strp) "bpf_prog_active"
[ 0] addr .data..percpu+0 <bpf_prog_active>
[root@quaco perf]#
[root@quaco perf]# eu-readelf -winfo ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep -B1 -A8 \"bpf_prog_active\"
[ f6a0] variable abbrev: 103
name (strp) "bpf_prog_active"
decl_file (data1) bpf.h (11)
decl_line (data2) 1008
decl_column (data1) 1
type (ref4) [ cf]
external (flag_present) yes
declaration (flag_present) yes
[ f6ad] variable abbrev: 103
name (strp) "bpf_stats_enabled_mutex"
[root@quaco perf]#
And:
[root@quaco perf]# eu-readelf -winfo ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep -B5 \<bpf_prog_active\>
[ 1bdf5] variable abbrev: 212
specification (ref4) [ f6a0]
decl_file (data1) syscall.c (1)
decl_line (data1) 43
location (exprloc)
[ 0] addr .data..percpu+0 <bpf_prog_active>
[root@quaco perf]#
> Note that second DW_TAG_variable entry contains specification that
> points to the first entry.
So you are not considering the first when encoding since it is just a
DW_AT_declaration, considers the second, as it should be, and then needs
to go see its DW_AT_specification, right?
Sounds correct, applying, will test further and then push out,
Thanks,
- Arnaldo
> This causes problem for btf_encoder when
> encoding global variables. The tag generated for the second entry
> doesn't have the type and scope info. Therefore the BTF VARs encoded
> using this tag has incorrect type_id and scope.
>
> As fix, when creating variable, examine the dwarf entry. If it has
> a DW_AT_specification, store the referred struct variable in a 'spec'
> field. When encoding VARs, check this 'spec', if it's non-empty, follow
> the pointer to use the referred var.
>
> [1] https://www.mail-archive.com/netdev@xxxxxxxxxxxxxxx/msg348144.html
>
> Tested: Tested using gcc 4.9 and gcc 8.2. The types and scopes of global
> vars are now generated correctly.
>
> [21] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> [21102] VAR 'bpf_prog_active' type_id=21, linkage=global-alloc
>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
> btf_encoder.c | 16 +++++++++++++---
> dwarf_loader.c | 25 ++++++++++++++++++++++++-
> dwarves.h | 1 +
> libbtf.c | 4 +++-
> 4 files changed, 41 insertions(+), 5 deletions(-)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 982f59d..29b2095 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -329,10 +329,10 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force)
> struct hlist_head *head;
>
> var = tag__variable(pos);
> - if (var->declaration)
> + if (var->declaration && !var->spec)
> continue;
> /* percpu variables are allocated in global space */
> - if (variable__scope(var) != VSCOPE_GLOBAL)
> + if (variable__scope(var) != VSCOPE_GLOBAL && !var->spec)
> continue;
> has_global_var = true;
> head = &hash_addr[hashaddr__fn(var->ip.addr)];
> @@ -376,6 +376,8 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force)
> var = hashaddr__find_variable(hash_addr, addr);
> if (var == NULL)
> continue;
> + if (var->spec)
> + var = var->spec;
>
> sym_name = elf_sym__name(&sym, btfe->symtab);
> if (!btf_name_valid(sym_name)) {
> @@ -387,7 +389,15 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force)
> break;
> }
> name = strings__add(strings, sym_name);
> - type = var->ip.tag.type + type_id_off;
> + if (var->ip.tag.type == 0) {
> + dump_invalid_symbol("Found symbol of void type when encoding btf",
> + sym_name, cu->name, verbose, force);
> + if (force)
> + continue;
> + err = -1;
> + break;
> + }
> + type = type_id_off + var->ip.tag.type;
> size = elf_sym__size(&sym);
> if (!size) {
> dump_invalid_symbol("Found symbol of zero size when encoding btf",
> diff --git a/dwarf_loader.c b/dwarf_loader.c
> index b8d7b35..46f86cb 100644
> --- a/dwarf_loader.c
> +++ b/dwarf_loader.c
> @@ -576,7 +576,15 @@ const char *variable__scope_str(const struct variable *var)
>
> static struct variable *variable__new(Dwarf_Die *die, struct cu *cu)
> {
> - struct variable *var = tag__alloc(cu, sizeof(*var));
> + struct variable *var;
> + bool has_specification;
> +
> + has_specification = dwarf_hasattr(die, DW_AT_specification);
> + if (has_specification) {
> + var = tag__alloc_with_spec(cu, sizeof(*var));
> + } else {
> + var = tag__alloc(cu, sizeof(*var));
> + }
>
> if (var != NULL) {
> tag__init(&var->ip.tag, cu, die);
> @@ -589,6 +597,10 @@ static struct variable *variable__new(Dwarf_Die *die, struct cu *cu)
> var->ip.addr = 0;
> if (!var->declaration && cu->has_addr_info)
> var->scope = dwarf__location(die, &var->ip.addr, &var->location);
> + if (has_specification) {
> + dwarf_tag__set_spec(var->ip.tag.priv,
> + attr_type(die, DW_AT_specification));
> + }
> }
>
> return var;
> @@ -2008,6 +2020,17 @@ static int tag__recode_dwarf_type(struct tag *tag, struct cu *cu)
> if (dtype != NULL)
> goto out;
> goto find_type;
> + case DW_TAG_variable: {
> + struct variable *var = tag__variable(tag);
> + dwarf_off_ref specification = dwarf_tag__spec(dtag);
> +
> + if (specification.off) {
> + dtype = dwarf_cu__find_tag_by_ref(cu->priv, &specification);
> + if (dtype)
> + var->spec = tag__variable(dtype->tag);
> + }
> + }
> +
> }
>
> if (dtag->type.off == 0) {
> diff --git a/dwarves.h b/dwarves.h
> index 771a02c..bbd5478 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -679,6 +679,7 @@ struct variable {
> enum vscope scope;
> struct location location;
> struct hlist_node tool_hnode;
> + struct variable *spec;
> };
>
> static inline struct variable *tag__variable(const struct tag *tag)
> diff --git a/libbtf.c b/libbtf.c
> index 7a01ded..ac06b79 100644
> --- a/libbtf.c
> +++ b/libbtf.c
> @@ -304,6 +304,8 @@ static const char * const btf_kind_str[NR_BTF_KINDS] = {
> [BTF_KIND_RESTRICT] = "RESTRICT",
> [BTF_KIND_FUNC] = "FUNC",
> [BTF_KIND_FUNC_PROTO] = "FUNC_PROTO",
> + [BTF_KIND_VAR] = "VAR",
> + [BTF_KIND_DATASEC] = "DATASEC",
> };
>
> static const char *btf_elf__name_in_gobuf(const struct btf_elf *btfe, uint32_t offset)
> @@ -671,7 +673,7 @@ int32_t btf_elf__add_var_type(struct btf_elf *btfe, uint32_t type, uint32_t name
> return -1;
> }
>
> - btf_elf__log_type(btfe, &t.type, false, false, "type=%u name=%s",
> + btf_elf__log_type(btfe, &t.type, false, false, "type=%u name=%s\n",
> t.type.type, btf_elf__name_in_gobuf(btfe, t.type.name_off));
>
> return btfe->type_index;
> --
> 2.28.0.297.g1956fa8f8d-goog
>
--
- Arnaldo
