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

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

Am 06.05.2024 um 13:41 schrieb Jens Remus:
> 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.

This breaks x86-64, as gas/config/tc-i386.h does not define SFRAME_CFA_RA_REG.

Either this specific check must stay, possibly with an additional comment:

   /* Offset for the return address from CFA is fixed for some ABIs
      (e.g., AMD64), output a SFRAME_CFA_FIXED_RA_INVALID otherwise.
      NOTE: sframe_ra_tracking_p may be defined without SFRAME_CFA_RA_REG
            (e.g., AMD64), so that SFRAME_FRE_RA_TRACKING won't be defined.  */
#ifdef sframe_ra_tracking_p
   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);

or gas/config/tc-i386.h needs to define SFRAME_CFA_RA_REG, for instance as follows:

#define SFRAME_CFA_RA_REG DWARF2_DEFAULT_RETURN_COLUMN

Since an architecture may not have a RA register I wonder whether keeping the existing logic as is would be better. What is your opinion?

> 
>>
>> 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

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

IBM

IBM Deutschland Research & Development GmbH; Vorsitzender des Aufsichtsrats: Wolfgang Wendt; Geschäftsführung: David Faller; Sitz der Gesellschaft: Böblingen; Registergericht: Amtsgericht Stuttgart, HRB 243294
IBM Data Privacy Statement: https://www.ibm.com/privacy/