Re: [PATCH] Fixed problem with BTF defining smaller enums.

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

Hi Cupertino,

A couple of comments inline.

On 11/13/23 14:37, Cupertino Miranda wrote:
> This patch fixes a BTF, which would become invalid when having
> smaller then 4 byte definitions of enums.
> For example, when using the __attribute__((mode(byte))) in the enum
> definition.

Please add a test for this case in the BTF testsuite.

> 
> Two problems were identified:
>  - it would incorrectly create an entry for enum64 when the size of the
>    enum was different then 4.
>  - it would allocate less then 4 bytes for the value entry in BTF, in
>    case the type was smaller.
> 
> BTF generated was validated against clang.
> 
> gcc/ChangeLog:
> 	* bpfout.cc (btf_calc_num_vbytes): Fixed logic for enum64.
> 	(btf_asm_enum_const): Corrected logic for enum64 and smaller
> 	than 4 bytes values.
> ---
>  gcc/btfout.cc | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
> index e07fed302c24..d2263ec6eec3 100644
> --- a/gcc/btfout.cc
> +++ b/gcc/btfout.cc
> @@ -299,7 +299,7 @@ btf_calc_num_vbytes (ctf_dtdef_ref dtd)
>        break;
>  
>      case BTF_KIND_ENUM:
> -      vlen_bytes += (dtd->dtd_data.ctti_size == 0x8)
> +      vlen_bytes += (dtd->dtd_data.ctti_size > 4)
>  			? vlen * sizeof (struct btf_enum64)
>  			: vlen * sizeof (struct btf_enum);
>        break;
> @@ -914,13 +914,13 @@ btf_asm_enum_const (unsigned int size, ctf_dmdef_t * dmd, unsigned int idx)
>  {
>    dw2_asm_output_data (4, dmd->dmd_name_offset, "ENUM_CONST '%s' idx=%u",
>  		       dmd->dmd_name, idx);
> -  if (size == 4)
> -    dw2_asm_output_data (size, dmd->dmd_value, "bte_value");
> -  else
> +  if (size > 4)
>      {
> -      dw2_asm_output_data (4, dmd->dmd_value & 0xffffffff, "bte_value_lo32");
> +      dw2_asm_output_data (4, dmd->dmd_value & 0xfffffffe, "bte_value_lo32");

I don't understand the mask change here.
Why clear the low bit of the enum value?

>        dw2_asm_output_data (4, (dmd->dmd_value >> 32) & 0xffffffff, "bte_value_hi32");
>      }
> +  else
> +    dw2_asm_output_data (size < 4 ? 4 : size, dmd->dmd_value, "bte_value");

In the else case isn't size guaranteed <= 4?
In which case, 'size < 4 ? 4 : size' always evaluates to 4.
So I would suggest to just write a literal 4 to keep it simple.


>  }
>  
>  /* Asm'out a function parameter description following a BTF_KIND_FUNC_PROTO.  */