[Dwarf-Discuss] Segment selectors for the range list table.

[Michael Eager]

Segmented addresses have been in the DWARF specification since Version 2 
  and AFAIK have not been changed since that time.  DWARF V5 did not add 
any functionality to segmented addresses that was not present in DWARF 
V2/3.  At least, there was no intention to do so.  Segmented addresses 
are described in Section 2.12.

A segmented address maps into a linear address in a processor-specific 
fashion.  AFAIK, only the Intel 8086 and descendants have this 
functionality.  (It's a many to one mapping in the 8086 implementation, 
but that's a problem for a bygone era.)  There's a reference to i386 
memory models in Table 2.7.

DWARF assumes a linear address space.  A segmented address maps to a 
specific address in this linear address space.  The entries in 
DW_AT_ranges for subprograms with different segment addresses would 
usually be referenced by their address in the linear address space.  If 
DW_AT_ranges has a DW_AT_segment, this is an indication that the 
debugger is to perform the processor-specific computation to translate 
the segment-address pair to the linear address.

There is no need to do anything with segments in the line table, since 
the line table contains addresses in the linear address space.

There is some (perhaps considerable) confusion in terminology in the x86 
world, because the x86 has multiple segment registers which on other 
processors would be called base registers.  The values in these 
registers reference memory segments and are added to whatever offset is 
contained in the program to generate an address.  These segment 
registers, and the memory segments which they point to, are NOT the 
segments represented by DW_AT_segment.

Re "reading the segment selector" and "addrx encoding":  The addresses 
in DWARF DIEs are static, not dynamic.  There is no register+offset 
encoding, and processor registers are not read to determine where a 
subprogram is in memory.

On 7/15/20 4:31 PM, David Blaikie via Dwarf-Discuss wrote:
> Looking at how segment selectors work:
> 
> DW_AT_segment: Applies to a DIE subtree, including any ranges, high/low 
> pc, locations, labels, etc
> debug_range/loc (v4 and below): Doesn't seem to allow specifying segment 
> variation - inherits from the segment given on the nearest parent DIE 
> that refers to the entry
> debug_rnglist/loclist (v5): includes segment selector size in the 
> header, but doesn't seem to use it - segment selection via addresses in 
> the address pool (RLE/LLE_*x encodings) would allow fine-grained segment 
> selection, but direct address forms don't seem to allow segment 
> selection ("This operand is the
> 19 same size as used in DW_FORM_addr.")
> debug_addr: segment_size in header, then list of {segment selector, address}
> debug_aranges: segment_size in header says, then the list contains 
> triples of {segment selector, start address, length}
> debug_line: v5 encodes the address and segment selector size in the 
> header, but I'm not sure if/how it's used. The DW_LNE_set_address 
> operation says:
> "The DW_LNE_set_address opcode takes a single relocatable address as an 
> operand. The size of the operand is the size of an address on the target 
> machine. It sets the address register to the value given by the 
> relocatable address and sets the op_index register to 0." - doesn't 
> sound like it's reading the segment selector there.
> 
> So... I don't think DWARFv5 made anything worse - if anything it did 
> enable /a/ way to use fine grained segment selectors in range lists and 
> location lists that doesn't appear, to me, to have been provided before. 
> (it could be needed if you had some functions in some segment and some 
> functions in another segment (which could be represented at the 
> subprogram DIE level - DW_AT_segment 1 on one DW_TAG_subprogram, 
> DW_AT_segment 2 on another DW_TAG_subprogram - but how would you 
> represent the DW_AT_ranges for this CU (in DWARFv4, or in DWARFv5 
> without using addrx encodings)? I don't know how, because I think 
> debug_ranges could describe one range list entry as being from one 
> segment, and another range list entry as being in another segment - they 
> would all be in whatever segment was in DW_AT_segment on the CU)
> 
> does that make sense? Have I missed something about how you could use 
> segment selectors in a debug_loc, debug_ranges, or loclist/rnglist that 
> isn't using an addrx encoding?
> 
> On Wed, Jul 15, 2020 at 6:37 AM Robinson, Paul via Dwarf-Discuss 
> <dwarf-discuss at lists.dwarfstd.org 
> <mailto:dwarf-discuss at lists.dwarfstd.org>> wrote:
> 
> 
> 
>      > -----Original Message-----
>      > From: Dwarf-Discuss <dwarf-discuss-bounces at lists.dwarfstd.org
>     <mailto:dwarf-discuss-bounces at lists.dwarfstd.org>> On Behalf
>      > Of Xing GUO via Dwarf-Discuss
>      > Sent: Tuesday, July 14, 2020 10:39 PM
>      > To: dwarf-discuss at lists.dwarfstd.org
>     <mailto:dwarf-discuss at lists.dwarfstd.org>
>      > Subject: [Dwarf-Discuss] Segment selectors for the range list table.
>      >
>      > Hi there,
>      >
>      > The DWARFv5 spec mentioned that there might be segment selectors in
>      > the range list entries and when the segment_selector_size is 0, the
>      > segment selectors are omitted from the range list entries.
>     However, it
>      > didn't mention how the segment selector should be encoded when the
>      > segment_selector_size isn't 0. Can anyone help me figure it out?
>      > Thanks a lot!
> 
>     Hi Xing,
> 
>     The segment selectors in the range list would be encoded the same way
>     as they would be in the main .debug_info section.? Range lists and
>     location lists are essentially extensions to .debug_info, for cases
>     where the range or location cannot be represented by simple DW_AT_*
>     attribute values.
> 
>     The specifics of encoding the segment selector would be whatever is
>     appropriate to the target.? DWARF does not specify these details.
> 
>     Best Regards,
>     --paulr
> 
> 
>      >
>      > 7.28 (page 243)
>      > The segment size is given by the segment_selector_size field of the
>      > header, and the address size is given by the address_size field
>     of the
>      > header. If the segment_selector_size field in the header is zero, the
>      > segment selector is omitted from the range list entries.
>      >
>      > --
>      > Cheers,
>      > Xing



-- 
Michael Eager