Re: [PATCH v3] bpf: remove huge memory waste with string allocation.

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

Hi Cupertino,

On 4/18/24 13:58, Cupertino Miranda wrote:
> Hi David, everyone,
> 
> Following Davids last review I decided to properly detect error cases,
> as suggested.
> The error however should be reported earlier in compilation in
> pack_enum_valud function, where all the errors are reported.
> 
> Thanks for the quick and detailed reviews.
> 
> Regards,
> Cupertino

Thanks for taking the time on this.
This version is nice, just one little comment:

> 
> The BPF backend was allocating an unnecessarily large string when
> constructing CO-RE relocations for enum types.
> This patch further verifies if an enumerator is valid for CO-RE
> representability and returns an error in those cases.

The second sentence is a little awkward and seems to imply the error is
returned when the enumerator is valid :)
Perhaps "...verifies that an enumerator is valid for CO-RE, and returns
an error if it is not" or similar would be more clear?

Otherwise, OK.
Thanks!

> 
> gcc/ChangeLog:
> 	* config/bpf/core-builtins.cc (get_index_for_enum_value): Create
> 	function.
> 	(pack_enum_value): Check for enumerator and error out.
> 	(process_enum_value): Correct string allocation.
> ---
>  gcc/config/bpf/core-builtins.cc | 57 ++++++++++++++++++++++-----------
>  1 file changed, 38 insertions(+), 19 deletions(-)
> 
> diff --git a/gcc/config/bpf/core-builtins.cc b/gcc/config/bpf/core-builtins.cc
> index e03e986e2c1..829acea98f7 100644
> --- a/gcc/config/bpf/core-builtins.cc
> +++ b/gcc/config/bpf/core-builtins.cc
> @@ -795,6 +795,23 @@ process_field_expr (struct cr_builtins *data)
>  static GTY(()) hash_map<tree, tree> *bpf_enum_mappings;
>  tree enum_value_type = NULL_TREE;
>  
> +static int
> +get_index_for_enum_value (tree type, tree expr)
> +{
> +  gcc_assert (TREE_CODE (expr) == CONST_DECL
> +	      && TREE_CODE (type) == ENUMERAL_TYPE);
> +
> +  unsigned int index = 0;
> +  for (tree l = TYPE_VALUES (type); l; l = TREE_CHAIN (l))
> +    {
> +      gcc_assert (index < (1 << 16));
> +      if (TREE_VALUE (l) == expr)
> +	return index;
> +      index++;
> +    }
> +  return -1;
> +}
> +
>  /* Pack helper for the __builtin_preserve_enum_value.  */
>  
>  static struct cr_local
> @@ -846,6 +863,16 @@ pack_enum_value_fail:
>  	ret.reloc_data.default_value = integer_one_node;
>      }
>  
> +  if (ret.fail == false )
> +    {
> +      int index = get_index_for_enum_value (type, tmp);
> +      if (index == -1 || index >= (1 << 16))
> +	{
> +	  bpf_error ("enum value in CO-RE builtin cannot be represented");
> +	  ret.fail = true;
> +	}
> +    }
> +
>    ret.reloc_data.type = type;
>    ret.reloc_data.kind = kind;
>    return ret;
> @@ -864,25 +891,17 @@ process_enum_value (struct cr_builtins *data)
>  
>    struct cr_final ret = { NULL, type, data->kind };
>  
> -  if (TREE_CODE (expr) == CONST_DECL
> -     && TREE_CODE (type) == ENUMERAL_TYPE)
> -    {
> -      unsigned int index = 0;
> -      for (tree l = TYPE_VALUES (type); l; l = TREE_CHAIN (l))
> -	{
> -	  if (TREE_VALUE (l) == expr)
> -	    {
> -	      char *tmp = (char *) ggc_alloc_atomic ((index / 10) + 1);
> -	      sprintf (tmp, "%d", index);
> -	      ret.str = (const char *) tmp;
> -
> -	      break;
> -	    }
> -	  index++;
> -	}
> -    }
> -  else
> -    gcc_unreachable ();
> +  gcc_assert (TREE_CODE (expr) == CONST_DECL
> +	      && TREE_CODE (type) == ENUMERAL_TYPE);
> +
> +  int index = get_index_for_enum_value (type, expr);
> +  gcc_assert (index != -1 && index < (1 << 16));
> +
> +  /* Index can only be a value up to 2^16.  Should always fit
> +     in 6 chars.  */
> +  char tmp[6];
> +  sprintf (tmp, "%u", index);
> +  ret.str = CONST_CAST (char *, ggc_strdup(tmp));
>  
>    return ret;
>  }