Re: [PATCH bpf-next v1 1/8] bpf: Introduce pseudo_btf_id

[Yonghong Song <yhs@fb.com>]

On 8/19/20 3:40 PM, Hao Luo wrote:
> Pseudo_btf_id is a type of ld_imm insn that associates a btf_id to a
> ksym so that further dereferences on the ksym can use the BTF info
> to validate accesses. Internally, when seeing a pseudo_btf_id ld insn,
> the verifier reads the btf_id stored in the insn[0]'s imm field and
> marks the dst_reg as PTR_TO_BTF_ID. The btf_id points to a VAR_KIND,
> which is encoded in btf_vminux by pahole. If the VAR is not of a struct
> type, the dst reg will be marked as PTR_TO_MEM instead of PTR_TO_BTF_ID
> and the mem_size is resolved to the size of the VAR's type.
> 
>  From the VAR btf_id, the verifier can also read the address of the
> ksym's corresponding kernel var from kallsyms and use that to fill
> dst_reg.
> 
> Therefore, the proper functionality of pseudo_btf_id depends on (1)
> kallsyms and (2) the encoding of kernel global VARs in pahole, which
> should be available since pahole v1.18.

I tried your patch with latest pahole but it did not generate
expected BTF_TYPE_VARs. My pahole head is:
   f3d9054ba8ff btf_encoder: Teach pahole to store percpu variables in 
vmlinux BTF.

First I made the following changes to facilitate debugging:
diff --git a/btf_encoder.c b/btf_encoder.c
index 982f59d..f94c3a6 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -334,6 +334,9 @@ int cu__encode_btf(struct cu *cu, int verbose, bool 
force)
                 /* percpu variables are allocated in global space */
                 if (variable__scope(var) != VSCOPE_GLOBAL)
                         continue;
+               /* type 0 is void, probably an internal error */
+               if (var->ip.tag.type == 0)
+                       continue;
                 has_global_var = true;
                 head = &hash_addr[hashaddr__fn(var->ip.addr)];
                 hlist_add_head(&var->tool_hnode, head);
@@ -399,8 +402,8 @@ int cu__encode_btf(struct cu *cu, int verbose, bool 
force)
                 }

                 if (verbose)
-                       printf("symbol '%s' of address 0x%lx encoded\n",
-                              sym_name, addr);
+                       printf("symbol '%s' of address 0x%lx encoded, 
type %u\n",
+                              sym_name, addr, type);

                 /* add a BTF_KIND_VAR in btfe->types */
                 linkage = var->external ? BTF_VAR_GLOBAL_ALLOCATED : 
BTF_VAR_STATIC;
diff --git a/libbtf.c b/libbtf.c
index 7a01ded..3a0d8d7 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;

It would be good if you can add some of the above changes to
pahole for easier `pahole -JV` dump.

With the above change, I only got static per cpu variables.
For example,
    static DEFINE_PER_CPU(unsigned int , mirred_rec_level);
in net/sched/act_mirred.c.

[10] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[74536] VAR 'mirred_rec_level' type_id=10, linkage=static

The dwarf debug_info entry for `mirred_rec_level`:
0x0001d8d6:   DW_TAG_variable
                 DW_AT_name      ("mirred_rec_level")
                 DW_AT_decl_file 
("/data/users/yhs/work/net-next/net/sched/act_mirred.c")
                 DW_AT_decl_line (31)
                 DW_AT_decl_column       (0x08)
                 DW_AT_type      (0x00000063 "unsigned int")
                 DW_AT_location  (DW_OP_addr 0x0)
It is not a declaration and it contains type.

All global per cpu variables do not have BTF_KIND_VAR generated.
I did a brief investigation and found this mostly like to be a
pahole issue. For example, for global per cpu variable
bpf_prog_active,
   include/linux/bpf.h
         DECLARE_PER_CPU(int , bpf_prog_active);
   kernel/bpf/syscall.c
         DEFINE_PER_CPU(int , bpf_prog_active);
it is declared in the header include/linux/bpf.h and
defined in kernel/bpf/syscall.c.

In many cu's, you will see:
0x0003592a:   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      (0x0001fa7e "int")
                 DW_AT_external  (true)
                 DW_AT_declaration       (true)

In kernel/bpf/syscall.c, I see
the following dwarf entry for real definition:
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)

Note that for the second entry DW_AT_specification points to the 
declaration. I am not 100% sure whether pahole handle this properly or 
not. It generates a type id 0 (void) for bpf_prog_active variable.

Could you investigate this a little more?

I am using gcc 8.2.1. Using kernel default dwarf (dwarf 2) exposed
the above issue. Tries to use dwarf 4 and the problem still exists.


> 
> Signed-off-by: Hao Luo <haoluo@google.com>
> ---
>   include/linux/btf.h      | 15 +++++++++
>   include/uapi/linux/bpf.h | 38 ++++++++++++++++------
>   kernel/bpf/btf.c         | 15 ---------
>   kernel/bpf/verifier.c    | 68 ++++++++++++++++++++++++++++++++++++++++
>   4 files changed, 112 insertions(+), 24 deletions(-)
> 
[...]