Re: [PATCH] bpf: handle anonymous members in CO-RE reloc [PR106745]

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

On 8/29/22 12:57, Jose E. Marchesi wrote:
> 
> Hi David.
> 
>> The old method for computing a member index for a CO-RE relocation
>> relied on a name comparison, which could SEGV if the member in question
>> is itself part of an anonymous inner struct or union.
>>
>> This patch changes the index computation to not rely on a name, while
>> maintaining the ability to account for other sibling fields which may
>> not have a representation in BTF.
>>
>> Tested in bpf-unknown-none, no known regressions.
>> OK?
>>
>> Thanks.
>>
>> gcc/ChangeLog:
>>
>> 	PR target/106745
>> 	* config/bpf/coreout.cc (bpf_core_get_sou_member_index): Fix
>> 	computation of index for anonymous members.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	PR target/106745
>> 	* gcc.target/bpf/core-pr106745.c: New test.
>> ---
>>  gcc/config/bpf/coreout.cc                    | 19 +++++++++----
>>  gcc/testsuite/gcc.target/bpf/core-pr106745.c | 30 ++++++++++++++++++++
>>  2 files changed, 44 insertions(+), 5 deletions(-)
>>  create mode 100644 gcc/testsuite/gcc.target/bpf/core-pr106745.c
>>
>> diff --git a/gcc/config/bpf/coreout.cc b/gcc/config/bpf/coreout.cc
>> index cceaaa969cc..caad4380fa1 100644
>> --- a/gcc/config/bpf/coreout.cc
>> +++ b/gcc/config/bpf/coreout.cc
>> @@ -207,7 +207,6 @@ bpf_core_get_sou_member_index (ctf_container_ref ctfc, const tree node)
>>    if (TREE_CODE (node) == FIELD_DECL)
>>      {
>>        const tree container = DECL_CONTEXT (node);
>> -      const char * name = IDENTIFIER_POINTER (DECL_NAME (node));
>>  
>>        /* Lookup the CTF type info for the containing type.  */
>>        dw_die_ref die = lookup_type_die (container);
>> @@ -222,16 +221,26 @@ bpf_core_get_sou_member_index (ctf_container_ref ctfc, const tree node)
>>        if (kind != CTF_K_STRUCT && kind != CTF_K_UNION)
>>          return -1;
>>  
>> +      tree field = TYPE_FIELDS (container);
>>        int i = 0;
>>        ctf_dmdef_t * dmd;
>>        for (dmd = dtd->dtd_u.dtu_members;
>>             dmd != NULL; dmd = (ctf_dmdef_t *) ctf_dmd_list_next (dmd))
>>          {
>>            if (get_btf_id (dmd->dmd_type) > BTF_MAX_TYPE)
>> -            continue;
>> -          if (strcmp (dmd->dmd_name, name) == 0)
>> -            return i;
>> -          i++;
>> +	    {
>> +	      /* This field does not have a BTF representation.  */
>> +	      if (field == node)
>> +		return -1;
>> +	    }
>> +	  else
>> +	    {
>> +	      if (field == node)
>> +		return i;
>> +	      i++;
>> +	    }
>> +
>> +	  field = DECL_CHAIN (field);
>>          }
> 
> I find the logic of the new conditional a little difficult to follow.
> What about something like this instead:
> 
> for (dmd = dtd->dtd_u.dtu_members;
>      dmd != NULL; dmd = (ctf_dmdef_t *) ctf_dmd_list_next (dmd))
>   {
>     bool field_has_btf = get_btf_id (dmd->dmd_type) <= BTF_MAX_TYPE;
> 
>     if (field == node)
>       return field_has_btf ? i : -1;
> 
>     if (field_has_btf)
>       i++;
>     field = DECL_CHAIN (field);
>   }
> 
> WDYT?

Thanks, that is certainly easier to follow.

v2 coming shortly.


> 
>>      }
>>    return -1;
>> diff --git a/gcc/testsuite/gcc.target/bpf/core-pr106745.c b/gcc/testsuite/gcc.target/bpf/core-pr106745.c
>> new file mode 100644
>> index 00000000000..9d347006a69
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/bpf/core-pr106745.c
>> @@ -0,0 +1,30 @@
>> +/* { dg-do compile } */
>> +/* { dg-options "-O0 -gbtf -dA -mco-re" } */
>> +
>> +struct weird
>> +{
>> +  struct
>> +  {
>> +    int b;
>> +  };
>> +
>> +  char x;
>> +
>> +  union
>> +  {
>> +    int a;
>> +    int c;
>> +  };
>> +};
>> +
>> +
>> +int test (struct weird *arg) {
>> +  int *x = __builtin_preserve_access_index (&arg->b);
>> +  int *y = __builtin_preserve_access_index (&arg->c);
>> +
>> +  return *x + *y;
>> +}
>> +
>> +
>> +/* { dg-final { scan-assembler-times "ascii \"0:0:0.0\"\[\t \]+\[^\n\]*btf_aux_string" 1 } } */
>> +/* { dg-final { scan-assembler-times "ascii \"0:2:1.0\"\[\t \]+\[^\n\]*btf_aux_string" 1 } } */