Re: [PATCH 3/3] btf: correct generation for extern funcs [PR106773]

[Indu Bhagat <indu.bhagat@oracle.com>]

On 12/12/22 12:31, David Faust wrote:
> 
> 
> On 12/8/22 23:36, Indu Bhagat wrote:
>> On 12/7/22 12:57, David Faust wrote:
>>> The eBPF loader expects to find entries for functions declared as extern
>>> in the corresponding BTF_KIND_DATASEC record, but we were not generating
>>> these entries.
>>>
>>> This patch adds support for the 'extern' linkage of function types in
>>> BTF, and creates entries for for them BTF_KIND_DATASEC records as needed.
>>>
>>> 	PR target/106773
>>>
>>> gcc/
>>>
>>> 	* btfout.cc (get_section_name): New function.
>>> 	(btf_collect_datasec): Use it here. Process functions, marking them
>>> 	'extern' and generating DATASEC entries for them as appropriate. Move
>>> 	creation of BTF_KIND_FUNC records to here...
>>> 	(btf_dtd_emit_preprocess_cb): ... from here.
>>>
>>> gcc/testsuite/
>>>
>>> 	* gcc.dg/debug/btf/btf-datasec-2.c: New test.
>>> 	* gcc.dg/debug/btf/btf-function-6.c: New test.
>>>
>>> include/
>>>
>>> 	* btf.h (struct btf_var_secinfo): Update comments with notes about
>>> 	extern functions.
>>> ---
>>>    gcc/btfout.cc                                 | 129 ++++++++++++------
>>>    .../gcc.dg/debug/btf/btf-datasec-2.c          |  28 ++++
>>>    .../gcc.dg/debug/btf/btf-function-6.c         |  19 +++
>>>    include/btf.h                                 |   9 +-
>>>    4 files changed, 139 insertions(+), 46 deletions(-)
>>>    create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-datasec-2.c
>>>    create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-function-6.c
>>>
>>> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
>>> index 05f3a3f9b6e..d7ead377ec5 100644
>>> --- a/gcc/btfout.cc
>>> +++ b/gcc/btfout.cc
>>> @@ -294,7 +294,35 @@ btf_datasec_push_entry (ctf_container_ref ctfc, const char *secname,
>>>      ds.entries.safe_push (info);
>>>    
>>>      datasecs.safe_push (ds);
>>> -  num_types_created++;
>>> +}
>>> +
>>> +
>>> +/* Return the section name, as of interest to btf_collect_datasec, for the
>>> +   given symtab node. Note that this deliberately returns NULL for objects
>>> +   which do not go in a section btf_collect_datasec cares about.  */
>>
>> "Dot, space, space, new sentence."
>>
>>> +static const char *
>>> +get_section_name (symtab_node *node)
>>> +{
>>> +  const char *section_name = node->get_section ();
>>> +
>>> +  if (section_name == NULL)
>>> +    {
>>> +      switch (categorize_decl_for_section (node->decl, 0))
>>> +	{
>>> +	case SECCAT_BSS:
>>> +	  section_name = ".bss";
>>> +	  break;
>>> +	case SECCAT_DATA:
>>> +	  section_name = ".data";
>>> +	  break;
>>> +	case SECCAT_RODATA:
>>> +	  section_name = ".rodata";
>>> +	  break;
>>> +	default:;
>>> +	}
>>> +    }
>>> +
>>> +  return section_name;
>>>    }
>>>    
>>>    /* Construct all BTF_KIND_DATASEC records for CTFC. One such record is created
>>> @@ -305,7 +333,60 @@ btf_datasec_push_entry (ctf_container_ref ctfc, const char *secname,
>>>    static void
>>>    btf_collect_datasec (ctf_container_ref ctfc)
>>>    {
>>> -  /* See cgraph.h struct symtab_node, which varpool_node extends.  */
>>> +  cgraph_node *func;
>>> +  FOR_EACH_FUNCTION (func)
>>> +    {
>>> +      dw_die_ref die = lookup_decl_die (func->decl);
>>> +      if (die == NULL)
>>> +	continue;
>>> +
>>> +      ctf_dtdef_ref dtd = ctf_dtd_lookup (ctfc, die);
>>> +      if (dtd == NULL)
>>> +	continue;
>>> +
>>> +      /* Functions actually get two types: a BTF_KIND_FUNC_PROTO, and
>>> +	 also a BTF_KIND_FUNC. But the CTF container only allocates one
>>> +	 type per function, which matches closely with BTF_KIND_FUNC_PROTO.
>>> +	 For each such function, also allocate a BTF_KIND_FUNC entry.
>>> +	 These will be output later.  */
>>
>> "Dot, space, space, new sentence."
>>
>>> +      ctf_dtdef_ref func_dtd = ggc_cleared_alloc<ctf_dtdef_t> ();
>>> +      func_dtd->dtd_data = dtd->dtd_data;
>>> +      func_dtd->dtd_data.ctti_type = dtd->dtd_type;
>>> +      func_dtd->linkage = dtd->linkage;
>>> +      func_dtd->dtd_type = num_types_added + num_types_created;
>>> +
>>> +      /* Only the BTF_KIND_FUNC type actually references the name. The
>>> +	 BTF_KIND_FUNC_PROTO is always anonymous.  */
>>> +      dtd->dtd_data.ctti_name = 0;
>>> +
>>> +      vec_safe_push (funcs, func_dtd);
>>> +      num_types_created++;
>>> +
>>> +      /* Mark any 'extern' funcs and add DATASEC entries for them.  */
>>> +      if (DECL_EXTERNAL (func->decl))
>>> +	{
>>> +	  func_dtd->linkage = BTF_LINKAGE_EXTERN;
>>> +
>>
>> What is the expected BTF when both decl and definition are present:
>>
>> extern int extfunc(int x);
>> int extfunc (int x) {
>>     int y = foo ();
>>     return y;
>> }
> 
> Using clang implementation as the reference, a single FUNC record
> for "extfunc" with "global" linkage:
> 
> $ cat extfuncdef.c
> extern int extfunc (int x);
> int extfunc (int x) {
>    int y = foo ();
>    return y;
> }
> 
> $ clang -target bpf -c -g extfuncdef.c -o extfuncdef.o
> 
> $ /usr/sbin/bpftool btf dump file extfuncdef.o
> [1] FUNC_PROTO '(anon)' ret_type_id=2 vlen=1
> 	'(anon)' type_id=2
> [2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> [3] FUNC 'extfunc' type_id=1 linkage=global
> 
> With this patch we do the same in GCC.
> 

OK. Thanks for confirming.

>>
>>> +	  const char *section_name = get_section_name (func);
>>> +	  /* Note: get_section_name () returns NULL for functions in text
>>> +	     section. This is intentional, since we do not want to generate
>>> +	     DATASEC entries for them.  */
>>
>> "Dot, space, space, new sentence."
>>
>>> +	  if (section_name == NULL)
>>> +	    continue;
>>> +
>>> +	  struct btf_var_secinfo info;
>>> +
>>> +	  /* +1 for the sentinel type not in the types map.  */
>>> +	  info.type = func_dtd->dtd_type + 1;
>>> +
>>> +	  /* Both zero at compile time.  */
>>> +	  info.size = 0;
>>> +	  info.offset = 0;
>>> +
>>> +	  btf_datasec_push_entry (ctfc, section_name, info);
>>> +	}
>>> +    }
>>> +
>>>      varpool_node *node;
>>>      FOR_EACH_VARIABLE (node)
>>>        {
>>> @@ -317,28 +398,13 @@ btf_collect_datasec (ctf_container_ref ctfc)
>>>          if (dvd == NULL)
>>>    	continue;
>>>    
>>> -      const char *section_name = node->get_section ();
>>>          /* Mark extern variables.  */
>>>          if (DECL_EXTERNAL (node->decl))
>>>    	dvd->dvd_visibility = BTF_LINKAGE_EXTERN;
>>>    
>>> +      const char *section_name = get_section_name (node);
>>>          if (section_name == NULL)
>>> -	{
>>> -	  switch (categorize_decl_for_section (node->decl, 0))
>>> -	    {
>>> -	    case SECCAT_BSS:
>>> -	      section_name = ".bss";
>>> -	      break;
>>> -	    case SECCAT_DATA:
>>> -	      section_name = ".data";
>>> -	      break;
>>> -	    case SECCAT_RODATA:
>>> -	      section_name = ".rodata";
>>> -	      break;
>>> -	    default:
>>> -	      continue;
>>> -	    }
>>> -	}
>>> +	continue;
>>>    
>>>          struct btf_var_secinfo info;
>>>    
>>> @@ -363,6 +429,8 @@ btf_collect_datasec (ctf_container_ref ctfc)
>>>    
>>>          btf_datasec_push_entry (ctfc, section_name, info);
>>>        }
>>> +
>>> +  num_types_created += datasecs.length ();
>>>    }
>>>    
>>>    /* Return true if the type ID is that of a type which will not be emitted (for
>>> @@ -461,29 +529,6 @@ btf_dtd_emit_preprocess_cb (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
>>>      if (!btf_emit_id_p (dtd->dtd_type))
>>>        return;
>>>    
>>> -  uint32_t btf_kind
>>> -    = get_btf_kind (CTF_V2_INFO_KIND (dtd->dtd_data.ctti_info));
>>> -
>>> -  if (btf_kind == BTF_KIND_FUNC_PROTO)
>>> -    {
>>> -      /* Functions actually get two types: a BTF_KIND_FUNC_PROTO, and
>>> -	 also a BTF_KIND_FUNC. But the CTF container only allocates one
>>> -	 type per function, which matches closely with BTF_KIND_FUNC_PROTO.
>>> -	 For each such function, also allocate a BTF_KIND_FUNC entry.
>>> -	 These will be output later.  */
>>> -      ctf_dtdef_ref func_dtd = ggc_cleared_alloc<ctf_dtdef_t> ();
>>> -      func_dtd->dtd_data = dtd->dtd_data;
>>> -      func_dtd->dtd_data.ctti_type = dtd->dtd_type;
>>> -      func_dtd->linkage = dtd->linkage;
>>> -
>>> -      vec_safe_push (funcs, func_dtd);
>>> -      num_types_created++;
>>> -
>>> -      /* Only the BTF_KIND_FUNC type actually references the name. The
>>> -	 BTF_KIND_FUNC_PROTO is always anonymous.  */
>>> -      dtd->dtd_data.ctti_name = 0;
>>> -    }
>>> -
>>>      ctfc->ctfc_num_vlen_bytes += btf_calc_num_vbytes (dtd);
>>>    }
>>>    
>>> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-datasec-2.c b/gcc/testsuite/gcc.dg/debug/btf/btf-datasec-2.c
>>> new file mode 100644
>>> index 00000000000..f4b298cf019
>>> --- /dev/null
>>> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-datasec-2.c
>>> @@ -0,0 +1,28 @@
>>> +/* Test BTF generation of DATASEC records for extern functions.
>>> +
>>> +   Only functions declared extern should have entries in DATASEC records.  */
>>> +
>>> +/* { dg-do compile } */
>>> +/* { dg-options "-O0 -gbtf -dA" } */
>>> +
>>> +/* Expect one DATASEC with vlen=1 (.foo_sec) and one with vlen=2 (.bar_sec) */
>>> +/* { dg-final { scan-assembler-times "0xf000002\[\t \]+\[^\n\]*btt_info" 1 } } */
>>> +/* { dg-final { scan-assembler-times "0xf000001\[\t \]+\[^\n\]*btt_info" 1 } } */
>>> +
>>> +/* Function entries should have offset and size of 0 at compile time.  */
>>> +/* { dg-final { scan-assembler-times "0\[\t \]+\[^\n\]*bts_offset" 3 } } */
>>> +/* { dg-final { scan-assembler-times "0\[\t \]+\[^\n\]*bts_size" 3 } } */
>>> +
>>> +extern int foo (int a) __attribute__((section(".foo_sec")));
>>> +
>>> +
>>> +extern int bar (int b) __attribute__((section(".bar_sec")));
>>> +extern void chacha (void) __attribute__((section(".bar_sec")));
>>> +
>>> +__attribute__((section(".foo_sec")))
>>> +void baz (int *x)
>>> +{
>>> +  chacha ();
>>> +
>>> +  *x = foo (bar (*x));
>>> +}
>>> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-function-6.c b/gcc/testsuite/gcc.dg/debug/btf/btf-function-6.c
>>> new file mode 100644
>>> index 00000000000..48a946ab14b
>>> --- /dev/null
>>> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-function-6.c
>>> @@ -0,0 +1,19 @@
>>> +/* Test BTF extern linkage for functions.
>>> +
>>> +   We expect to see one BTF_KIND_FUNC type with global linkage (foo), and
>>> +   one BTF_KIND_FUNC type with extern linkage (extfunc).  */
>>> +
>>> +/* { dg-do compile } */
>>> +/* { dg-options "-O0 -gbtf -dA" } */
>>> +
>>> +/* { dg-final { scan-assembler-times "btt_info: kind=12, kflag=0, linkage=2" 1 } } */
>>> +/* { dg-final { scan-assembler-times "btt_info: kind=12, kflag=0, linkage=1" 1 } } */
>>> +
>>> +extern int extfunc(int a, int b);
>>> +
>>> +int foo (int x) {
>>> +
>>> +  int y = extfunc (x, x+1);
>>> +
>>> +  return y;
>>> +}
>>> diff --git a/include/btf.h b/include/btf.h
>>> index 9a757ce5bc9..f41ea94b75f 100644
>>> --- a/include/btf.h
>>> +++ b/include/btf.h
>>> @@ -186,12 +186,13 @@ struct btf_var
>>>    };
>>>    
>>>    /* BTF_KIND_DATASEC is followed by VLEN struct btf_var_secinfo entries,
>>> -   which describe all BTF_KIND_VAR types contained in the section.  */
>>> +   which describe all BTF_KIND_VAR or extern BTF_KIND_FUNC types contained
>>> +   in the section.  */
>>>    struct btf_var_secinfo
>>>    {
>>> -  uint32_t type;	/* Type of variable.  */
>>> -  uint32_t offset;	/* In-section offset of variable (in bytes).  */
>>> -  uint32_t size;	/* Size (in bytes) of variable.  */
>>> +  uint32_t type;	/* Type of BTF_KIND_VAR or BTF_KIND_FUNC item.  */
>>> +  uint32_t offset;	/* In-section offset (in bytes) of item.  */
>>> +  uint32_t size;	/* Size (in bytes) of item.  */
>>>    };
>>>    
>>>    /* BTF_KIND_ENUM64 is followed by VLEN struct btf_enum64 entries,
>>