Re: [PATCH] btf: emit linkage information in BTF_KIND_FUNC entries

[Indu Bhagat via Gcc-patches]

On 7/8/22 11:30 AM, Jose E. Marchesi via Gcc-patches wrote:


The kernel bpftool expects BTF_KIND_FUNC entries in BTF to include an
annotation reflecting the linkage of functions (static, global).  For
whatever reason they (ab)use the `vlen' field of the BTF_KIND_FUNC entry
instead of adding a variable-part to the record like it is done with
other entry kinds.


For BTF Variables, we have the linkage information in the output section 
as "btv_linkage".  To propagate that information from DWARF to BTF, we 
have the dvd_visibility in struct ctf_dvdef (in ctfc.h). Now that the 
linkage information is needed for the BTF_KIND_FUNC entries, what do you 
think about - adding something like dtd_visibility to ctf_dtdef.

Updating the BTF format documentation will be useful 
https://www.kernel.org/doc/Documentation/bpf/btf.rst. Let's see what can 
be done for that...

Also, adding some testcases with the current patch will be great.

I have created PR debug/106263 "BTF_KIND_FUNC type does not encode 
linkage" to track this.


This patch makes GCC to include this linkage info in BTF_KIND_FUNC
entries.

Tested in bpf-unknown-none target.

gcc/ChangeLog:

        * ctfc.h (struct ctf_itype): Add field ctti_linkage.
        * ctfc.cc (ctf_add_function): Set ctti_linkage.
        * dwarf2ctf.cc (gen_ctf_function_type): Pass a linkage for
        function types and subprograms.
        * btfout.cc (btf_asm_func_type): Emit linkage information for the
        function.
---
  gcc/btfout.cc    | 3 ++-
  gcc/ctfc.cc      | 3 ++-
  gcc/ctfc.h       | 3 ++-
  gcc/dwarf2ctf.cc | 4 +++-
  4 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index 31af50521da..417d87cf519 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -740,7 +740,8 @@ static void
  btf_asm_func_type (ctf_dtdef_ref dtd)
  {
    dw2_asm_output_data (4, dtd->dtd_data.ctti_name, "btt_name");
-  dw2_asm_output_data (4, BTF_TYPE_INFO (BTF_KIND_FUNC, 0, 0), "btt_info");
+  dw2_asm_output_data (4, BTF_TYPE_INFO (BTF_KIND_FUNC, 0,
+                                         dtd->dtd_data.ctti_linkage), 
"btt_info");
    dw2_asm_output_data (4, get_btf_id (dtd->dtd_data.ctti_type), "btt_type");
  }
  
diff --git a/gcc/ctfc.cc b/gcc/ctfc.cc
index f24e7bff948..ad7f8bb8e86 100644
--- a/gcc/ctfc.cc
+++ b/gcc/ctfc.cc
@@ -777,7 +777,7 @@ ctf_add_function_arg (ctf_container_ref ctfc, dw_die_ref 
func,
  ctf_id_t
  ctf_add_function (ctf_container_ref ctfc, uint32_t flag, const char * name,
                  const ctf_funcinfo_t * ctc, dw_die_ref die,
-                 bool from_global_func)
+                 bool from_global_func, int linkage)
  {
    ctf_dtdef_ref dtd;
    ctf_id_t type;
@@ -792,6 +792,7 @@ ctf_add_function (ctf_container_ref ctfc, uint32_t flag, 
const char * name,
  
    dtd->from_global_func = from_global_func;
    dtd->dtd_data.ctti_info = CTF_TYPE_INFO (CTF_K_FUNCTION, flag, vlen);
+  dtd->dtd_data.ctti_linkage = linkage;
    /* Caller must make sure CTF types for ctc->ctc_return are already added.  
*/
    dtd->dtd_data.ctti_type = (uint32_t) ctc->ctc_return;
    /* Caller must make sure CTF types for function arguments are already added
diff --git a/gcc/ctfc.h b/gcc/ctfc.h
index 001e544ef08..273997a2302 100644
--- a/gcc/ctfc.h
+++ b/gcc/ctfc.h
@@ -116,6 +116,7 @@ typedef struct GTY (()) ctf_itype
    } _u;
    uint32_t ctti_lsizehi;      /* High 32 bits of type size in bytes.  */
    uint32_t ctti_lsizelo;      /* Low 32 bits of type size in bytes.  */
+  uint32_t ctti_linkage;       /* Linkage info for function types.  */
  } ctf_itype_t;
  
  #define ctti_size _u._size
@@ -423,7 +424,7 @@ extern ctf_id_t ctf_add_forward (ctf_container_ref, 
uint32_t, const char *,
  extern ctf_id_t ctf_add_typedef (ctf_container_ref, uint32_t, const char *,
                                 ctf_id_t, dw_die_ref);
  extern ctf_id_t ctf_add_function (ctf_container_ref, uint32_t, const char *,
-                                 const ctf_funcinfo_t *, dw_die_ref, bool);
+                                 const ctf_funcinfo_t *, dw_die_ref, bool, 
int);
  extern ctf_id_t ctf_add_sou (ctf_container_ref, uint32_t, const char *,
                             uint32_t, size_t, dw_die_ref);
  
diff --git a/gcc/dwarf2ctf.cc b/gcc/dwarf2ctf.cc
index a6329ab6ee4..397100004c2 100644
--- a/gcc/dwarf2ctf.cc
+++ b/gcc/dwarf2ctf.cc
@@ -644,6 +644,7 @@ gen_ctf_function_type (ctf_container_ref ctfc, dw_die_ref 
function,
  
    ctf_funcinfo_t func_info;
    uint32_t num_args = 0;
+  int linkage = get_AT_flag (function, DW_AT_external);
  
    ctf_id_t return_type_id;
    ctf_id_t function_type_id;
@@ -687,7 +688,8 @@ gen_ctf_function_type (ctf_container_ref ctfc, dw_die_ref 
function,
                                       function_name,
                                       (const ctf_funcinfo_t *)&func_info,
                                       function,
-                                      from_global_func);
+                                      from_global_func,
+                                       linkage);
  
    /* Second pass on formals: generate the CTF types corresponding to
       them and add them as CTF function args.  */