[PATCH,V3] gas: sframe: partially process DWARF unwind info in CFI_escape

[Jens Remus]

On 13.02.2025 02:14, Indu Bhagat wrote:
> CFI_escape is most commonly used to include DWARF expressions in the
> unwind information.  One may also use CFI_escape to add OS-specific CFI
> opcodes.  Up until now, SFrame generation process would skip generating
> SFrame FDE at the mere sight of a CFI_escape opcode.
> 
> Fine tune the handling of CFI_escape for SFrame generation by explicitly
> checking for few "harmless" (in context of SFrame generation)
> CFI_escape DWARF info:
>    - 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 on s390x.  A build
of Glibc with SFrame no longer skips FDEs because of .cfi_escape.

> Expose the current cfi_escape_data structure in dw2gencfi.c to the
> relevant header file to allow SFrame generation APIs to use it too.
> 
> Valid unwind info may be split across multiple .cfi_escape directives.
> Conversely, it is also allowed to simply put multiple DWARF expressions
> and/or operations in a single .cfi_escape directive.  Handling all of
> these cases correctly will need parsing/processing that is not deemed
> worth the effort in context of SFrame generation; We continue to skip
> generating SFrame FDE for these cases and warn the user.

Thank fou you for the clarification in the commit message!

> In future, SFrame stack trace may support non-SP/FP as base register
> (albeit in limited form).  Add an explicit check in
> sframe_xlate_do_escape_expr (to test against the current CFA register)
> to ensure the functionality continues to work.
> 
> Use differentiated warning text in sframe_xlate_do_val_offset to avoid
> confusion to the user as the same function is used for handling
> .cfi_val_offset and .cfi_escape DW_CFA_val_offset,...

Nice!

> Also, add a common test with DWARF reg 12 which is non SP / FP on x86_64
> and aarch64 (and s390x too).

Works on s390x.

> gas/
> 	* gas/dw2gencfi.c (struct cfi_escape_data): Move from ...
> 	* gas/dw2gencfi.h (struct cfi_escape_data): ... to.
> 	* gas/gen-sframe.c (sframe_xlate_do_val_offset): Include string
> 	for .cfi_escape conditionally.
> 	(sframe_xlate_do_escape_expr): New definition.
> 	(sframe_xlate_do_escape_val_offset): Likewise.
> 	(sframe_xlate_do_cfi_escape): Likewise.
> 	(sframe_do_cfi_insn): Handle CFI_escape explicitly.
> 
> gas/testsuite/
> 	* gas/cfi-sframe/cfi-sframe.exp: Add new tests.
> 	* gas/cfi-sframe/cfi-sframe-common-9.d: New test.
> 	* gas/cfi-sframe/cfi-sframe-common-9.s: New test.
> 	* gas/cfi-sframe/cfi-sframe-x86_64-empty-1.d: New test.
> 	* gas/cfi-sframe/cfi-sframe-x86_64-empty-1.s: New test.
> 	* gas/cfi-sframe/cfi-sframe-x86_64-empty-2.d: New test.
> 	* gas/cfi-sframe/cfi-sframe-x86_64-empty-2.s: New test.
> 
> ---

Jan had some good points concerning your patch introducing some cases
where SFrame FDE is silently skipped without a warning message.  It
would be great if there would be a message.

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

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