[PATCH 09/12] objdump/readelf: adjust for SFRAME_VERSION_2

[Indu Bhagat]

On 6/28/23 16:04, Hans-Peter Nilsson wrote:
> On Tue, 27 Jun 2023, Indu Bhagat via Binutils wrote:
>> Make adjustments to account for a new format version and its name.
>> Also shuffle some code to make it more readable and use the available
>> APIs rather than direct access.
>>
>> objdump/readelf will show the following message to the user if .sframe
>> section in SFRAME_VERSION_1 format is seen:
>>
>>   "No further information can be displayed.  SFrame version not
>>   supported."
>>
>> In other words, like the rest of the binutils, only the current SFrame
>> format version, i.e., SFRAME_VERSION_2 is supported by the textual dump
>> facilities.
> 
> A decision that IMHO is counter to binutils philosophy (AFAIU),
> unless version 1 is actually generated by nobody nowhere.
> 
> If there are object files out there generated with version 1,
> support should be kept, at the very least for inspection tools.
> 

SFrame Version 1 format definition had issues related to unaligned 
accesses. In most cases of when there are version 1 .sframe sections out 
there, those applications should ideally be rebuilt to switch to using 
SFRAME_VERSION_2.  When other assemblers and linkers begin to add 
support for SFrame format, SFRAME_VERSION_2 should be used as it is the 
latest - there is little incentive to implement SFRAME_VERSION_1 there.

The trouble with supporting both SFRAME_VERSION_1 and SFRAME_VERSION_2 
in readelf/objdump is that the abstraction sframe_func_desc_entry used 
in libsframe is the same as the binary representation defined in 
include/sframe.h. This means that to support two versions of SFrame FDE, 
libsframe will need to revisit some of its APIs and will be a bit of 
rejig. I tried to do some of this, but it looked very clunky to me.

The choice to keep libsframe's internal representation for SFrame FDE to 
be the same as the on-disk representation has the advantage that an 
unwinder does not need to do any copy operations at run time.  It could 
simply read the SFrame FDE from memory.

Thanks
Indu