[PATCH,V2 2/2] gas: sframe: partially process DWARF expressions in CFI_escape

[Jens Remus]

On 05.02.2025 00:10, Indu Bhagat wrote:

> Fine tune the handling of CFI_escape for SFrame generation by explicitly
> checking for some "harmless" (in context of SFrame generation)
> CFI_escape DWARF expressions:
>    - DW_CFA_expression affecting registers of no significance to SFrame
>      stack trace info
>    - DW_CFA_value_offset affecting registers of no significance to SFrame
>      stack trace info

I successfully tested the DW_CFA_value_offset handling with following
small fix on s390x.  A build of Glibc with SFrame no longer skips FDEs
because of .cfi_escape.

> diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c

> +/* Handle DW_CFA_val_offset in .cfi_escape.  */
> +
> +static int
> +sframe_xlate_do_escape_val_offset (struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
> +				   struct cfi_insn_data *cfi_insn)
> +{
> +  const struct cfi_escape_data *e = cfi_insn->u.esc;
> +  unsigned int reg = 0;
> +  offsetT offset = 0;
> +  int err = SFRAME_XLATE_OK;
> +  int i = 0;
> +
> +  if (!e || !e->next)
> +    return SFRAME_XLATE_ERR_INVAL;
> +
> +  /* Check for (DW_CFA_val_offset reg scaled_offset) sequence.  */
> +#define CFI_ESC_NUM_EXP 2
> +  offsetT items[CFI_ESC_NUM_EXP] = {0};
> +  while (e->next)
> +    {
> +      e = e->next;
> +      if (i >= CFI_ESC_NUM_EXP)
> +	return SFRAME_XLATE_ERR_NOTREPRESENTED;
> +      items[i] = e->exp.X_add_number;
> +      i++;
> +    }
> +  if (i <= CFI_ESC_NUM_EXP - 1)
> +    return SFRAME_XLATE_ERR_NOTREPRESENTED;
> +
> +  reg = items[0];
> +  offset = items[1];
> +#undef CFI_ESC_NUM_EXP
> +
> +  /* Invoke sframe_xlate_do_val_offset itself for checking.  */
> +  struct cfi_insn_data *temp = XCNEW (struct cfi_insn_data);
> +  temp->insn = DW_CFA_val_offset;
> +  temp->u.ri.reg = reg;
> +  temp->u.ri.offset = offset;

temp->u.ri.offset = offset * DWARF2_CIE_DATA_ALIGNMENT;

The scaling needs to be undone.  Looking at output_cfi_insn() handling
of DW_CFA_val_offset it makes sense to do it in this assignment instead
of when assigning offset = items[1] a few lines above.  But that is up
to you.

GAS cfi_add_CFA_val_offset() already has asserted that
DWARF2_CIE_DATA_ALIGNMENT is not zero.

> +
> +  err = sframe_xlate_do_val_offset (xlate_ctx, temp, true);
> +  XDELETE (temp);
> +
> +  return err;
> +}

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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