Re: [PATCH v3 15/15] gas: Validate SFrame RA tracking and fixed RA offset

[Jens Remus <jremus@linux.ibm.com>]

Am 04.05.2024 um 02:22 schrieb Indu Bhagat:
> On 5/3/24 09:40, Jens Remus wrote:
>> Am 18.04.2024 um 22:38 schrieb Indu Bhagat:
>>> On 4/12/24 07:47, Jens Remus wrote:
>>>> If an architecture uses SFrame return-address (RA) tracking it must
>>>> specify the fixed RA offset as invalid. Otherwise, if an architecture
>>>> does not use RA tracking, it must specify a valid fixed RA offset.
>>>>
>>>> gas/
>>>>     * gen-sframe.c: Validate SFrame RA tracking and fixed
>>>>     RA offset.
>>>>
>>>> Signed-off-by: Jens Remus <jremus@linux.ibm.com>
>>>> ---
>>>>
>>>> Notes (jremus):
>>>>      Changes v2 -> v3:
>>>>      - New patch.
>>>>      This could be made dependent on ENABLE_CHECKING (configure option
>>>>      --enable-checking).
>>>>
>>>>   gas/gen-sframe.c | 12 ++++++++++++
>>>>   1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
>>>> index ca6565b0e45e..7e815f9603ef 100644
>>>> --- a/gas/gen-sframe.c
>>>> +++ b/gas/gen-sframe.c
>>>> @@ -1532,6 +1532,18 @@ output_sframe (segT sframe_seg)
>>>>     /* Setup the version specific access functions.  */
>>>>     sframe_set_version (SFRAME_VERSION_2);
>>>> +#ifdef SFRAME_FRE_RA_TRACKING
>>>> +  if (sframe_ra_tracking_p ())
>>>> +    /* With RA tracking the fixed RA offset must be invalid.  */
>>>> +    gas_assert (sframe_cfa_ra_offset () == 
>>>> SFRAME_CFA_FIXED_RA_INVALID);
>>>> +  else
>>>> +    /* Without RA tracking the fixed RA offset may not be invalid.  */
>>>> +    gas_assert (sframe_cfa_ra_offset () != 
>>>> SFRAME_CFA_FIXED_RA_INVALID);
>>>> +#else
>>>> +  /* Without RA tracking the fixed RA offset may not be invalid.  */
>>>> +  gas_assert (sframe_cfa_ra_offset () != SFRAME_CFA_FIXED_RA_INVALID);
>>>> +#endif
>>>> +
>>>
>>> I am not sure if the detailed checks are worth it here (simply 
>>> because of code patterns that follow).
>>
>> I agree, provided the checks are performed elsewhere as you suggest.
>>
>> My intention was to have checks that assist with getting SFrame 
>> support for another architecture implemented correctly, without having 
>> to chase subtle issues.
>>
>>>
>>> We use the sframe_cfa_ra_offset () function later and only in 
>>> output_sframe_internal () (shown below).  How about we simply put an 
>>> assert there (and get rid of the proposed thunk above):
>>>
>>> #ifdef sframe_ra_tracking_p
>>>    if (!sframe_ra_tracking_p ())
>>
>> See below.
>>
>>>      {
>>>        fixed_ra_offset = sframe_cfa_ra_offset ();
>>>        gas_assert (fixed_ra_offset != SFRAME_CFA_FIXED_RA_INVALID);
>>
>> That is clever and accounts for one potential implementation issue!
>>
>>>      }
>>> #endif
>>>    out_one (fixed_ra_offset);
>>>
>>> fixed_ra_offset is initialized to SFRAME_CFA_FIXED_RA_INVALID in 
>>> output_sframe_internal ().
>>
>> Above logic requires sframe_ra_tracking_p to be defined by an 
>> architecture that is not using RA tracking. Not defining 
>> sframe_ra_tracking_p would result in fixed_ra_offset being 
>> unexpectedly initialized to SFRAME_CFA_FIXED_RA_INVALID instead of 
>> being set to sframe_cfa_ra_offset().
>>
>> All checks but this do test SFRAME_FRE_RA_TRACKING first, which 
>> ensures both sframe_ra_tracking_p and SFRAME_CFA_RA_REG are defined, 
>> and then the predicate sframe_ra_tracking_p to determine whether RA 
>> tracking is used.
>> If SFRAME_FRE_RA_TRACKING is defined and sframe_ra_tracking_p returns 
>> true, then RA tracking is used.
>> Likewise, if SFRAME_FRE_RA_TRACKING is not defined or if 
>> sframe_ra_tracking_p returns false (evaluating lazily) RA tracking is 
>> not used.
>>
>> What about making the following change to make all RA tracking tests 
>> consistent depend on SFRAME_FRE_RA_TRACKING?
>>
>> #ifdef SFRAME_FRE_RA_TRACKING
>>    if (!sframe_ra_tracking_p ())
>> #endif
>>      {
>>        fixed_ra_offset = sframe_cfa_ra_offset ();
>>        /* Without RA tracking the fixed RA offset may not be invalid.  */
>>        gas_assert (fixed_ra_offset != SFRAME_CFA_FIXED_RA_INVALID);
>>      }
>>    out_one (fixed_ra_offset);
>>
> 
> Oops, that's my bad.  Guarding with SFRAME_FRE_RA_TRACKING is more 
> appropriate.

Included this in the previous patch in this series.

> 
> But, I think calling the sframe_cfa_ra_offset () out of 
> SFRAME_FRE_RA_TRACKING portrays an imprecise meaning.  Only backends 
> which opt in for SFrame define these vars/functions. (The cross build 
> will likely pass because of the way code is written, but I think you get 
> the idea).
> 
> I would do:
> 
> #ifdef SFRAME_FRE_RA_TRACKING
>     if (!sframe_ra_tracking_p ())
>       {
>         fixed_ra_offset = sframe_cfa_ra_offset ();
>         /* Without RA tracking the fixed RA offset may not be invalid.  */
>         gas_assert (fixed_ra_offset != SFRAME_CFA_FIXED_RA_INVALID);
>       }
> #endif
>    out_one (fixed_ra_offset);

Good catch! I did not consider that SFrame may not be implemented by an 
architecture at all.

> 
> 
>> What would still not be checked is the implementation error to define 
>> sframe_ra_tracking_p and have it return true without also defining 
>> SFRAME_CFA_RA_REG. This would be treated as if RA tracking was not used.
>>
>> Would it therefore make sense to add the following?
>>
>> #if defined (sframe_ra_tracking_p) && !defined (SFRAME_CFA_RA_REG)
>>    gas_assert (!sframe_ra_tracking_p ())
>> #endif
>>
>> Also when using RA tracking an architecture should implement 
>> sframe_cfa_ra_offset to return SFRAME_CFA_FIXED_RA_INVALID.
>>
>> Would it therefore make sense to add the following?
>>
>> #ifdef SFRAME_FRE_RA_TRACKING
>>    if (sframe_ra_tracking_p ())
>>      gas_assert (sframe_cfa_ra_offset () == SFRAME_CFA_FIXED_RA_INVALID);
>> #endif
>>
> 
> All these checks are around guarding against implementation errors, 
> opinions may vary. If you feel these add value, then it makes sense to 
> add them.
> 
> (That said, I am thinking the name sframe_cfa_ra_offset is confusing; 
> perhaps sframe_cfa_ra_fixed_offset () is better? I will think about it 
> and may be include this in my list of sframe-next patches.)

I'll leave it up to you then.

> 
>>>
>>>>     /* Process all fdes and create SFrame stack trace information.  */
>>>>     create_sframe_all ();
>>>
>>
>> Thanks and regards,
>> Jens
> 

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

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