2021Q3
Date of Issue: 1st November 2021
This document is an alpha proposal for Morello extensions to DWARF for AArch64.
This document describes the use of the Morello extensions to the DWARF debug table format in the Application Binary Interface (ABI) for the Arm 64-bit architecture.
DWARF, DWARF 3.0, DWARF 5.0, use of DWARF format
Please check Application Binary Interface for the Arm® Architecture for the latest release of this document.
Please give feedback and report defects in this specification to the issue tracker page on GitHub.
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Contents
The following support level definitions are used by the Arm ABI specifications:
- Release
- Arm considers this specification to have enough implementations, which have received sufficient testing, to verify that it is correct. The details of these criteria are dependent on the scale and complexity of the change over previous versions: small, simple changes might only require one implementation, but more complex changes require multiple independent implementations, which have been rigorously tested for cross-compatibility. Arm anticipates that future changes to this specification will be limited to typographical corrections, clarifications and compatible extensions.
- Beta
- Arm considers this specification to be complete, but existing implementations do not meet the requirements for confidence in its release quality. Arm may need to make incompatible changes if issues emerge from its implementation.
- Alpha
- The content of this specification is a draft, and Arm considers the likelihood of future incompatible changes to be significant.
This document is a draft and all content is at the Alpha quality level.
If there is no entry in the change history table for a release, there are no changes to the content of the document for that release.
| Issue | Date | Change |
|---|---|---|
| 00alpha | September 2020 | Alpha release. |
| 2020Q4 | 21st December 2020 | Document released on Github. |
This document refers to, or is referred to by, the following documents:
| Ref | External reference or URL | Title |
|---|---|---|
| MDWARF64 | This document | DWARF supplement for Morello |
| AADWARF64 | DWARF for the Arm 64-bit Architecture (AArch64) | |
| AAPCS64 | Procedure Call Standard for the Arm 64-bit Architecture (AArch64) | |
| GDWARF | http://dwarfstd.org/Dwarf3Std.php | DWARF 3.0, the generic debug table format. |
| GDWARF5 | http://dwarfstd.org/Dwarf5Std.php | DWARF Debugging Information Format, Version 5 |
This ABI document uses the following terms and abbreviations:
- A64
- The instruction set that is available in AArch64 state.
- AAPCS64
- Procedure Call Standard for the Arm 64-bit Architecture (AArch64).
- AArch64
- The 64-bit general-purpose register width state of the Armv8 architecture.
- ABI
Application Binary Interface:
- The specifications to which an executable must conform in order to execute in a specific execution environment. For example, the Linux ABI for the Arm Architecture.
- A particular aspect of the specifications to which independently produced relocatable files must conform in order to be statically linkable and executable. For example, the C++ ABI for the Arm Architecture, ELF for the Arm Architecture, ...
- C64
- The instruction set available when the Morello extensions are used.
This specification only provides the Morello-specific extensions to the base DWARF specification for the Arm 64-bit Architecture (AArch64), and is expected to be used together with AADWARF64.
This specification adds DWARF register numbers and names for the capability registers.
| DWARF register number | AArch64 register name | Description |
|---|---|---|
| 198-228 | C0-C30 | Tagged 128+1-bit capability registers (Note 1, Note 2) |
| 229 | CSP | Tagged 128+1-bit capability stack pointer register (Note 1, Note 2) |
| 230 | PCC | Tagged 128+1-bit program counter capability register (Note 1, Note 2) |
| 231 | DDC | Tagged 128+1-bit default data capability register (Note 1, Note 2) |
| 232 | Reserved | |
| 233 | Reserved |
Notes
The architecture defines the following register overlaps:
- General registers (X registers) overlap with the capability registers (C registers). A given X register is mapped to the low 64 bits of the corresponding C register.
- Stack pointer (register SP) overlaps with the capability stack pointer (register CSP). Register SP is mapped to the low 64 bits of the CSP register.
- Program counter (register PC) overlaps with the program counter capability (register PCC). Register PC is mapped to the low 64 bits of the PCC register.
The DWARF call frame instructions do not explicitly specify the size of a register. This is implicit in the definition of the register. As a consequence, the overlapping registers have been allocated separate DWARF register number ranges which have their own definition for the size of these registers.
When searching the call frame information table for one of these registers a consumer must take into account the aliasing between them and their overlapping registers.
Capability registers are stored in memory including their tag bit. When debug information describes that a capability register is saved at some address, a consumer must use a proper load of a capability from memory to restore its value.
Capabilities are always stored in the little-endian byte order. This implies that if a given capability register C is stored in memory on a big-endian system, its corresponding X part is not stored in the natural byte order but as little-endian too.
The term Canonical Frame Address (CFA) is defined in GDWARF, §6.4, Call Frame Information. This ABI adopts the typical definition of CFA given there.
- The CFA is the value of the stack pointer at the call site in the previous frame.
A subroutine can define the CFA as either a 64-bit address or a 129-bit capability depending on whether the subroutine uses register SP as the stack pointer or register CSP, respectively.
The AAPCS64 document (AAPCS64, §5.2.2 Stack) describes what stack pointer is used by subroutines conforming to AAPCS64 and AAPCS64-cap:
- A subroutine conforming to AAPCS64 uses register SP as the stack pointer. The
CFA in the subroutine is then defined as a 64-bit address, typically as
SP+<offset>orX29+<offset>. - A subroutine conforming to AAPCS64-cap uses register CSP as the stack pointer.
The CFA in the subroutine is defined as a 129-bit capability, typically as
CSP+<offset>orC29+<offset>.
If a subroutine defines the CFA as a 64-bit address, then only the lowest 64 bits from the value stored in register CSP in the previous frame can be restored. The consumer should treat the remaining bits as undefined.
This specification adds one CIE augmentation character that might appear as part of a CIE augmentation string:
- The 'C' character indicates that the default unwind rules for this CIE should be initialized in accordance with the pure capability procedure call standard.
A subroutine can return to either a 64-bit address or a 129-bit capability depending on whether the subroutine expects the return address to be stored in register LR or CLR, respectively.
The AAPCS64 document (AAPCS64, §5.6 Function returns and the link register) describes where the return address for subroutines conforming to AAPCS64 and AAPCS64-cap is stored:
- A subroutine conforming to AAPCS64 expects the return address to be stored in
register LR. The
return_address_registerfield in the CIE is then set to a column for a 64-bit register, typically to the LR column directly. - A subroutine conforming to AAPCS64-cap expects the return address to be stored
in register CLR. The
return_address_registerfield in the CIE is set to a column for a 129-bit capability register, typically to the CLR column.
If a subroutine uses a 64-bit return address register then it must also guarantee that it does not modify the upper 65 bits of register PCC. The value of PCC in the previous frame can then be restored by a consumer by using the PCC from the current frame and replacing the lowest 64 bits by the calculated value of the return address register in the previous frame.
GDWARF, §5.3, Type Modifier Entries, describes attribute
DW_AT_address_class that denotes how objects having the given pointer or
reference type ought to be dereferenced. Section §2.12, Segmented Addresses then
describes that the set of permissible values is specific to each target
architecture.
This ABI uses the common address class value DW_ADDR_none and defines one
Morello-specific value DW_ADDR_capability.
| Address class name | Value |
|---|---|
DW_ADDR_none |
0x0 |
DW_ADDR_capability |
0x1 |
DW_ADDR_noneThe
DW_ADDR_nonevalue is defined in GDWARF, §2.12, Segmented Addresses and means that no address class has been specified.DW_ADDR_capabilityThe
DW_ADDR_capabilityvalue indicates that the type is an address capability and can be dereferenced as such.
GDWARF, §5.1, Base Type Entries, describes attribute DW_AT_encoding that
denotes how a base type is encoded and is to be interpreted.
This ABI uses two vendor-defined base-type encodings
DW_ATE_CHERI_signed_intcap and DW_ATE_CHERI_unsigned_intcap.
| Base type encoding name | Value |
|---|---|
DW_ATE_CHERI_signed_intcap |
0xa0 |
DW_ATE_CHERI_unsigned_intcap |
0xa1 |
DW_ATE_CHERI_signed_intcapThe
DW_ATE_CHERI_signed_intcapencoding describes a signed integer/capability type__intcap_t.DW_ATE_CHERI_unsigned_intcapThe
DW_ATE_CHERI_unsigned_intcapencoding describes an unsigned integer/capability type__uintcap_t.
A size attribute of an entry that describes a capability type should record the
untagged memory size of the data object. This typically means that such an entry
should have the DW_AT_byte_size attribute with value set to 16.
Given that the formal size of a capability is 16 bytes, consumers of DWARF will have to implement ways to read the additional tag bit when required. This is implementation specific.
When inspecting capabilities, a debugger might choose to display only the untagged, 16-byte value, and provide the possibility for the user to further query the tag only if interested. In such a case, the value and the tag can be read independently of each other. Alternatively, it is perfectly valid for an implementation to always read and display the full 129 bits of the capability.
The status of this appendix is informative.
The capability pointer type in example fragment Capability pointer: C++ source can be described in DWARF as shown in Capability pointer: DWARF description.
Capability pointer: C++ source
char * __capability cap;Capability pointer: DWARF description
1$: DW_TAG_base_type
DW_AT_name("char")
...
2$: DW_TAG_pointer_type
DW_AT_type(reference to 1$)
DW_AT_address_class(DW_ADDR_capability)
DW_AT_byte_size(16)
Capability reference and rvalue reference types can be represented in the same
way using DW_TAG_reference_type and DW_TAG_rvalue_reference_type,
respectively.
The __intcap_t and __uintcap_t types in example fragment
Intcap types: C++ source can be described in DWARF as illustrated in
Intcap types: DWARF description.
Intcap types: C++ source
__intcap_t intcap;
__uintcap_t uintcap;Intcap types: DWARF description
1$: DW_TAG_base_type
DW_AT_name("__intcap_t")
DW_AT_encoding(DW_ATE_CHERI_signed_intcap)
DW_AT_byte_size(16)
2$: DW_TAG_base_type
DW_AT_name("__uintcap_t")
DW_AT_encoding(DW_ATE_CHERI_unsigned_intcap)
DW_AT_byte_size(16)
Additional attributes needed to describe capability types can have recognizable impact on the size of debugging information in programs that extensively use capability pointers and references.
DWARF Version 5 introduced the new operand form DW_FORM_implicit_const
(GDWARF5, §7.5.3, Abbreviations Tables) which allows to extract common
attributes of capability types in section .debug_abbrev instead of repeating
them in .debug_info.
Two capability pointer types in example fragment Capability pointers: C++ source can be described in DWARF Version 5 as illustrated in Capability pointers: DWARF Version 5 description.
Capability pointers: C++ source
char * __capability ccap;
int * __capability icap;Capability pointers: DWARF Version 5 description
! *** Section .debug_abbrev content
a$h: 1
DW_TAG_compile_unit
...
0
2
DW_TAG_base_type
DW_CHILDREN_no
DW_AT_name DW_FORM_string
DW_AT_encoding DW_FORM_data1
DW_AT_byte_size DW_FORM_data1
0
3
DW_TAG_pointer_type
DW_CHILDREN_no
DW_AT_type DW_FORM_ref4
DW_AT_address_class DW_FORM_implicit_const
DW_ADDR_capability
DW_AT_byte_size DW_FORM_implicit_const
16
0
! *** Section .debug_info content
! Compilation unit header
<length> ; unit_length
5 ; version
DW_UT_compile ; unit_type
8 ; address_size
offset of a$h ; debug_abbrev_offset
! Entry for the compile unit
1
...
! Entry for "char"
i$1: 2
"char"
DW_ATE_unsigned_char
1
! Entry for "int"
i$2: 2
"int"
DW_ATE_signed
4
! Entry for "char * __capability"
3
offset of i$1
! Entry for "int * __capability"
3
offset of i$2
It is recommended for a toolchain vendor to introduce assembler syntax that allows the description of call frame information for subroutines that conform to AAPCS64-cap as easily as for the ones conforming to AAPCS64.
For instance, listing GNU assembler syntax for pure capability functions shows the extension that was introduced to the GNU assembler syntax:
GNU assembler syntax for pure capability functions
.cfi_startproc purecapUsing the purecap parameter causes .cfi_startproc to do the following:
- Initial CFI instructions consist of one instruction
DW_CFA_def_cfa CSP, 0. - Return address register is set to CLR.
- The 'C' character is included in the CIE augmentation string.