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

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

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?

>      }
>    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 } } */