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

[David Faust <david.faust@oracle.com>]

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.

> 
>> +	  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,
>