[Dwarf-Discuss] Segment selectors for the range list table.
Michael Eager
eager@eagercon.com
Thu Jul 16 16:30:06 GMT 2020
On 7/15/20 9:49 PM, David Blaikie wrote:
>
>
> On Wed, Jul 15, 2020 at 7:07 PM Michael Eager via Dwarf-Discuss
> <dwarf-discuss at lists.dwarfstd.org
> <mailto:dwarf-discuss at lists.dwarfstd.org>> wrote:
>
> 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.
>
>
> That seems at odds with the non-normative text of 2.12 "In some systems,
> addresses are specified as offsets within a given segment rather than as
> locations within a single flat address space."
That means that an x86 address could be represented as offset 0x1234 in
segment 0x4444, which would translate to 0x44440+0x1234=0x45674. Note
that x86 permits aliases, so that offset 0x0124 in segment 0x4555 is the
same address.
DWARF sometimes uses wording which is intended to generalize a concept.
Conceivably, another architecture could use the same DWARF attribute in
a similar way. That's why the non-normative text says "some systems"
rather than specifically referencing x86. But we do have that as the
only example listed in the table.
> And also would be confusing to me - if there is a contiguous linear
> address space, why would DWARF need to specify the use of a segment
> selector, and why do some references to addresses allow the inclusion of
> a segment selector and some don't? Why not just always use the
> non-segmented address description for DWARF?
So that a compiler can generate segment address values and offsets
independently. An x86 code generator may not know what segment it is
generating code in.
AFAIK, all addresses can be segmented addresses, except in the line
table where it isn't needed.
Perhaps we should have (long ago) required flat/linear addresses for x86
instead of segmented addresses.
> & I don't find any mention of this idea that some addresses are absolute
> and some are segment-relative in 2.12 - it does say that "If none of the
> entries in the chain of parents for this entry back to its containing
> compilation unit entry have DW_AT_segment attributes, then the entry is
> assumed to exist within a flat address space." - as though a flat (I
> assume this is synonymous with "linear"?) address space is distinct from
> the segmented address space being discussed otherwise?
Flat address space == linear address space.
From a certain perspective, x86 memory space is broken up into 65K
16-byte segments mapped onto a 256K linear address range.
>
> 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.
>
>
> Sorry, I don't quite follow the connections between all those statements.
Perhaps I didn't understand your comments about "reading the segment
selector" and "addrx encoding".
TL;DR:
DW_AT_segment was designed to describe x86 memory model addresses:
https://en.wikipedia.org/wiki/Intel_Memory_Model.
Possibly other architectures can use it, but I'm not familiar with any
that do.
> 2.17 says that if a DIE has a DW_AT_high_pc and DW_AT_segment, then the
> high_pc is relative to the specified segment. That's a bit redundant if
> high_pc uses FORM_addrx, because the address in the address pool can
> specify its own segment, but a producer could choose which way to go
> there. (presumably if the AT_segment is there, you should interpret the
> addrx high_pc relative to that segment - assuming debug_addr has no
> segment selector in it - or perhaps it should go the other way and
> ignore the local AT_segment and only rely on whatever segment is in
> debug_addr)
DW_FORM_addrx (and the .debug_addr section) were introduced in DWARF V5
to allow compression of DW_FORM_addr addresses. DW_AT_segment is
intended to describe an (x86) address in the form that the processor
uses. The first is one of many different compression schemes in DWARF,
the second is part of an architectural description.
> 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>
> > <mailto: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>
> >? ? ?<mailto: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>
> >? ? ?<mailto: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
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Michael Eager
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