[PATCH 1/2] sframe: Add SFrame v3 decode support for regular FDEs

[Jens Remus]

Hello Claudiu!

On 4/14/2026 2:23 PM, claudiu.zissulescu-ianculescu@oracle.com wrote:
> From: Claudiu Zissulescu <claudiu.zissulescu-ianculescu@oracle.com>
> 
> Add SFrame v3 format definitions and modify the backtrace routines to
> handle both v2 and v3 function descriptor layouts.
> 
> The sframe stack trace backtracer now detects the outer most frame
> returning without error. Also, the decoder reads v3 index/attribute
> records, performs explicit bounds checks for FRE access, and uses
> int64_t PC/start-address handling for version-agnostic lookup.

Nice work!

> @@ -279,7 +400,11 @@ sframe_fre_check_range_p (sframe_decoder_ctx *dctx, uint32_t func_idx,
>    /* For SFrame FDEs encoding information for repetitive pattern of insns,
>       masking with the rep_block_size is necessary to find the matching FRE.  */
>    if (mask_p)
> +    {
> +      if (rep_block_size == 0)
> +	return false;
>      pc_offset = pc_offset % rep_block_size;

Nit: Indentation appears to be wrong.

> +    }
>  
>    return (start_ip_offset <= pc_offset) && (end_ip_offset >= pc_offset);
>  }

> diff --git a/sysdeps/generic/sframe.c b/sysdeps/generic/sframe.c

> @@ -117,6 +117,10 @@ __stacktrace_sframe (void **ra_lst, int count, frame *frame)
>  	  return 0;
>  	}
>  
> +      /* A FRE with undefined RA indicates the outermost frame.  */
> +      if (SFRAME_V2_FRE_RA_UNDEFINED_P (frep->fre_info))
> +	return i;
> +

Thanks for taking care of the RA undefined indication for outermost
frames!

>        /* Get the CFA offset from the FRE.  If offset is unavailable,
>  	 sets err.  */
>        cfa_offset = __sframe_fre_get_cfa_offset (dctx, frep, &err);

> diff --git a/sysdeps/generic/sframe.h b/sysdeps/generic/sframe.h

> @@ -102,6 +107,7 @@ extern "C"
>  #define SFRAME_ABI_AARCH64_ENDIAN_BIG      1 /* AARCH64 big endian.  */
>  #define SFRAME_ABI_AARCH64_ENDIAN_LITTLE   2 /* AARCH64 little endian.  */
>  #define SFRAME_ABI_AMD64_ENDIAN_LITTLE     3 /* AMD64 little endian.  */
> +#define SFRAME_ABI_S390X_ENDIAN_BIG        4 /* s390x big endian.  */

Would it make sene to leave that out for my s390 64-bit support patch to
add it?

> @@ -307,6 +418,25 @@ typedef struct sframe_fre_info
>       fi
>       Note that in AAPCS64, a frame record, if created, will save both FP and
>       LR on stack.
> +
> +   s390x:
> +     offset1 (interpreted as CFA = BASE_REG + offset1)
> +     if RA is being tracked
> +       offset2 (interpreted as RA = CFA + offset2; an offset value of
> +	       SFRAME_FRE_RA_OFFSET_INVALID indicates a dummy padding RA offset
> +	       to represent FP without RA saved on stack)
> +       if FP is being tracked
> +	 offset3 (intrepreted as FP = CFA + offset3)
> +       fi
> +     else
> +      if FP is being tracked
> +	offset2 (intrepreted as FP = CFA + offset2)
> +      fi
> +    fi
> +    Note that in s390x, if a FP/RA is to be restored from a register, flex FDEs
> +    are used in SFrame V3.  In SFrame V2, default FDEs were used: the
> +    least-significant bit of the offset was set to indicate that the encoded
> +    value is a DWARF register number shifted to the left by 1.

Likewise?

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
jremus@de.ibm.com / jremus@linux.ibm.com

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