2023Q1
Date of Issue: 06th April 2023
This document is an alpha proposal for Morello extensions to ELF for AArch64.
This document describes the use of the Morello extensions to the ELF binary file format in the Application Binary Interface (ABI) for the Arm 64-bit architecture.
ELF, AArch64 ELF, Morello, C64, ...
Please check Application Binary Interface for the Arm® Architecture for the latest release of this document.
Please report defects in this specification to the issue tracker page on GitHub.
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Contents
- 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. The relocation codes in Relocation in particular are expected to change.
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 | 1st October 2020 | Alpha release. |
| 2020Q4 | 21st December 2020 | Document released on Github. |
| 2021Q3 | 1st November 2021 | ELF markers to identify Morello purecap binaries. |
| 2022Q3 | 20th October 2022 | Describe the purecap TLS implementation. |
This document refers to, or is referred to by, the following documents.
| Ref | External reference or URL | Title |
|---|---|---|
| MORELLO_AAELF64 | This document | Morello extensions to ELF for the Arm 64-bit Architecture (AArch64). |
| AAELF64 | IHI 0056 | ELF for the Arm 64-bit Architecture (AArch64). |
| MORELLO_ARM | DDI0606 | Arm® Architecture Reference Manual Supplement Morello for A-profile Architecture. |
| TLSDESC | http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf | TLS Descriptors for Arm. Original proposal document. |
| CHERI_ELF | https://github.com/CTSRD-CHERI/cheri-elf-gabi/blob/main/gabi.md | CHERI ELF gABI Extensions |
| TLS | https://akkadia.org/drepper/tls.pdf | ELF Handling For Thread-Local Storage |
The ABI for the Morello extensions to the Arm 64-bit Architecture uses the following terms and abbreviations.
- C64
- The instruction set available when the Morello extensions are used.
- A64
- The instruction set available when in AArch64 state.
Other terms may be defined when first used.
This specification only provides the Morello-specific extensions to the base ELF specification for the Arm 64-bit Architecture (AArch64), and is expected to be used along with AAELF64.
Object Files is structured to correspond to the chapter with the same name in AAELF64:
The ELF header provides a number of fields that assist in interpretation of the file. Most of these are specified in the base standard. The following fields have Morello-specific meanings.
e_flags- The processor-specific flags are shown in the following table.
| Value | Description |
EF_AARCH64_CHERI_PURECAP
(0x00010000) |
The ELF file uses an ABI where all pointers are implemented using capabilities (Pure-capability ABI). |
A Morello toolchain can emit ELF Note sections in accordance to [CHERI_ELF].
All code symbols exported from an object file (symbols with binding
STB_GLOBAL) shall have type STT_FUNC. All extern data objects shall have
type STT_OBJECT. No STB_GLOBAL data symbol shall have type STT_FUNC.
The type of an undefined symbol shall be STT_NOTYPE or the type of its
expected definition.
The type of any other symbol defined in an executable section can be
STT_NOTYPE. A linker is only required to provide long-branch and PLT support
for symbols of type STT_FUNC. A linker is also only required to provide
interworking support for A64 and C64 symbols of type STT_FUNC (interworking
for untyped symbols must be encoded directly in the object file)
A symbol that names a C or assembly language entity should have the name of that
entity. For example, a C function called calculate generates a symbol called
calculate (not _calculate).
Symbol names are case sensitive and are matched exactly by linkers.
Any symbol with binding STB_LOCAL may be removed from an object and replaced
with an offset from another symbol in the same section under the following
conditions:
- The original symbol and replacement symbol are not of type
STT_FUNC, or both symbols are of typeSTT_FUNCand describe code of the same instruction set state (either both A64 or both C64). - The symbol is not described by the debug information.
- The symbol is not a mapping symbol (Mapping symbols).
- The resulting object, or image, is not required to preserve accurate symbol information to permit de-compilation or other post-linking optimization techniques.
- If the symbol labels an object in a section with the
SHF_MERGEflag set, the relocation using symbol may be changed to use the section symbol only if the initial addend of the relocation is zero.
No tool is required to perform the above transformations; an object consumer must be prepared to do this itself if it might find the additional symbols confusing.
A section of an ELF file can contain a mixture of A64 code, C64 code and data. There are inline transitions between code and data at literal pool boundaries.
Linkers, file decoders and other tools need to map binaries correctly. To
support this, a number of symbols, termed mapping symbols, appear in the symbol
table to label the start of each sequence of bytes of the appropriate class. All
mapping symbols have type STT_NOTYPE and binding STB_LOCAL. The
st_size field is unused and must be zero.
The mapping symbols are defined in Mapping symbols table. It is an error for a relocation to reference a mapping symbol. Two forms of mapping symbol are supported:
- A short form that uses a dollar character and a single letter denoting the class. This form can be used when an object producer creates mapping symbols automatically. Its use minimizes string table size.
- A longer form in which the short form is extended with a period, followed by any sequence of characters that are legal for a symbol. This form can be used when assembler files have to be annotated manually and the assembler does not support multiple definitions of symbols.
Mapping symbols defined in a section (relocatable view) or segment (executable view) define a sequence of half-open intervals that cover the address range of the section or segment. Each interval starts at the address defined by the mapping symbol, and continues up to, but not including, the address defined by the next (in address order) mapping symbol or the end of the section or segment. A section that contains instructions must have a mapping symbol defined at the beginning of the section. If a section contains only data, no mapping symbol is required. A platform ABI should specify whether or not mapping symbols are present in the executable view; they will never be present in a stripped executable file.
| Name | Description |
|---|---|
|
Start of a sequence of A64 instructions. |
|
Start of a sequence of C64 instructions. |
|
Start of a sequence of data items (for example, a literal pool). |
In addition to the normal rules for symbol values, the following rules shall also
apply to symbols of type STT_FUNC and STT_GNU_IFUNC:
- If the symbol addresses an A64 instruction, its value is the address of the instruction (in a relocatable object, the offset of the instruction from the start of the section containing it).
- If the symbol addresses a C64 instruction, its value is the address of the instruction with bit 0 set (in a relocatable object, the section offset with bit 0 set).
Note
This allows a linker to distinguish A64 and C64 code symbols without having to refer to the map. An A64 symbol will always have an even value, while a C64 symbol will always have an odd value. However, a linker should strip the discriminating bit from the value before using it for relocation.
Morello uses the private relocation code space for vendor experiments [0xE000, 0xF000) specified in AAELF64.
Static Morello relocation codes begin at 0xE000(57344); dynamic ones at 0xE800(59392). Relocation codes starting at 0xEA00(59904) are reserved for private Morello experiments.
The following nomenclature is used in the descriptions of relocation operations:
S(when used on its own) is the address of the symbol.Ais the addend for the relocation.Pis the address of the place being relocated (derived fromr_offset).Cis 1 if the target symbolShas typeSTT_FUNCand the symbol addresses a C64 instruction; it is 0 otherwise.Xis the result of a relocation operation, before any masking or bit-selection operation is appliedPage(expr)is the page address of the expression expr, defined as (expr & ~0xFFF). This applies even if the machine page size supported by the platform has a different value.GOTis the address of the Global Offset Table, the table of code and data addresses to be resolved at dynamic link time. TheGOTand each entry in it must be aligned to the pointer-size.GDAT(S+A)represents a pointer-sized entry in theGOTfor addressS+A. The entry will be relocated at run time with relocationR_MORELLO_GLOB_DAT(S+A).G(expr)is the address of the GOT entry for the expression expr.GTLSDESC(S+A)represents a consecutive pair of pointer-sized entries in the GOT which contain atlsdescstructure describing the thread-local variable located at offsetAfrom thread-local symbolS. The first entry holds a pointer to the variable's TLS descriptor resolver function and the second entry holds a platform-specific offset or pointer. The pair of pointer-sized entries will be relocated withR_MORELLO_TLSDESC(S+A).TPREL(S)resolves to a pair of two 64-bit values. The first value contains the offset in the static TLS block of the thread-local symbolS. The second value contains the size of the symbolSGTPREL(S)represents an entry in the GOT containing a pair of two 64-bit values. The first value contains the offset in the static TLS block of the symbolS. The second value contains the size of the symbolS.TLSDESC(S+A)resolves to a contiguous pair of pointer-sized values, as created by GTLSDESC(S+A).CAP_INITgenerates a capability with all required information. When used on its own represents the operations needs to be done for handlingR_MORELLO_CAPINIT.CAP_SIZEis the size of the underlying memory region that the capability can reference. This may not directly map to the symbol size.SIZE(S)is the symbol size of the symbolS.CAP_PERMis the permission of the capability. This may not directly map to the type of the symbol.[msb:lsb]is a bit-mask operation representing the selection of bits in a value. The bits selected range fromlsbup tomsbinclusive. For example, ‘bits [3:0]’ represents the bits under the mask 0x0000000F. When range checking is applied to a value, it is applied before the masking operation is performed.
pointer-size
The pointer-size is 64 bits for the A64 ABI and 128 bits for the pure capability (C64) ABI.
Warning
The ELF64 Code of the relocations are subject to change.
| ELF64 Code | Name | Operation | Comment |
|---|---|---|---|
| 57348 | R_MORELLO_LD_PREL_LO17 |
S+A -
(P&~0xF) |
Set a load-literal immediate value to bits [20:4] of X.
Check that -220 <= X < 220.
Check that X & 15 = 0. |
| 57349 | R_MORELLO_ADR_PREL_PG_HI20 |
Page(S+A)
- Page(P) |
Set an ADRP immediate value to bits [31:12] of the X. Check that -231 <= X < 231. |
| 57350 | R_MORELLO_ADR_PREL_PG_HI20_NC |
Page(S+A)
- Page(P) |
Set an ADRP immediate value to bits [31:12] of the X. No overflow check. Although overflow must not be checked, a linker should check that the value of X is aligned to a multiple of the datum size. |
| ELF64 Code | Name | Operation | Comment |
|---|---|---|---|
| 57344 | R_MORELLO_TSTBR14 |
((S+A)|C)-P |
Set the immediate field of a TBZ/TBNZ instruction to bits [15:2] of X. Check that -215 <= X < 215. See Call and Jump relocations. |
| 57345 | R_MORELLO_CONDBR19 |
((S+A)|C)-P |
Set the immediate field of a conditional branch instruction to bits [20:2] of X. Check that -227 <= X < 227. See Call and Jump relocations. |
| 57346 | R_MORELLO_JUMP26 |
((S+A)|C)-P |
Set a B immediate field to bits [27:2] of X. Check that -227 <= X < 227. See Call and Jump relocations. |
| 57347 | R_MORELLO_CALL26 |
((S+A)|C)-P |
Set a BL immediate field to bits [27:2] of X. Check that -227 <= X < 227. See Call and Jump relocations. |
| ELF64 Code | Name | Operation | Comment |
|---|---|---|---|
| 57353 | R_MORELLO_MOVW_SIZE_G0 | SIZE | Set a MOV[KZ] immediate field to bits [15:0] of X; check that 0 <= X < 216 |
| 57354 | R_MORELLO_MOVW_SIZE_G0_NC | SIZE | Set a MOV[KZ] immediate field to bits [15:0] of X. No overflow check |
| 57355 | R_MORELLO_MOVW_SIZE_G1 | SIZE | Set a MOV[KZ] immediate field to bits [31:16] of X; check that 0 <= X < 232 |
| 57356 | R_MORELLO_MOVW_SIZE_G1_NC | SIZE | Set a MOV[KZ] immediate field to bits [31:16] of X. No overflow check |
| 57357 | R_MORELLO_MOVW_SIZE_G2 | SIZE | Set a MOV[KZ] immediate field to bits [47:32] of X; check that 0 <= X < 248 |
| 57358 | R_MORELLO_MOVW_SIZE_G2_NC | SIZE | Set a MOV[KZ] immediate field to bits [47:32] of X. No overflow check |
| 57359 | R_MORELLO_MOVW_SIZE_G3 | SIZE | Set a MOV[KZ] immediate field to bits [63:48] of X (no overflow check needed) |
Note
The group relocations to create a 16-, 32-, 48-, or 64-bit symbol size inline do not accept an addend.
There is one relocation code (R_MORELLO_CALL26) for function call (BL)
instructions and one (R_MORELLO_JUMP26) for jump (B) instructions.
A linker may use a veneer (a sequence of instructions) to implement a relocated branch if the relocation is either
R_MORELLO_CALL26 or R_MORELLO_JUMP26 and:
- The target symbol has type
STT_FUNC. - Or, the target symbol and relocated place are in separate sections input to the linker.
- Or, the target symbol is undefined (external to the link unit).
In all other cases a linker shall diagnose an error if a relocation cannot be
effected without a veneer. A linker generated veneer may corrupt register
c16 and the condition flags, but must preserve all other registers. Linker
veneers may be needed for a number of reasons, including, but not limited to:
- Interworking: The branch source and target symbol are in different execution states(A64/C64).
- Range Extension: The branch source and target symbol are in C64 execution state and the target is outside the addressable span of the branch instruction (+/- 128MB).
- The target address will not be known until run time, or the target address might be pre-empted.
Long branches with 64-bit range are not supported yet for range extensions or for interworking. Interworking between ABIs are not supported yet.
| ELF64 Code | Name | Operation | Comment |
|---|---|---|---|
| 57351 | R_MORELLO_ADR_GOT_PAGE |
Page(G(GDAT(S+A)))
- Page(P) |
Set the immediate value of an ADRP to bits [31:12] of X. Check that -231 <= X < 231. |
| 57352 | R_MORELLO_LD128_GOT_LO12_NC |
G(GDAT(S+A)) |
Set the LD/ST immediate field to bits [11:4] of X. No overflow check. Check that X&15 = 0. Also see Static linking with Morello. |
Morello only defines the relocations needed to implement the descriptor based thread-local storage (TLS) models in a SysV-type environment. The details of TLS descriptors are beyond the scope of this specification; a general introduction can be found in [TLSDESC]. Also, only the relocations needed to implement the General Dynamic (GD) access model and the Local Executable (LE) access models are defined.
Relocations needed to define the traditional TLS models are undefined.
| ELF64 Code | Name | Operation | Comment |
|---|---|---|---|
| 57600 | R_MORELLO_TLSDESC_ADR_PAGE20 |
|
Set the immediate value of an ADRP to bits [31:12] of X. Check that -231 <= X < 231. |
| 57601 | R_MORELLO_TLSDESC_LD128_LO12 |
G(GTLSDESC(S+A)) |
Set the LD/ST immediate field to bits [11:4] of X. No overflow check. Check that X&15 = 0. |
| 57602 | R_MORELLO_TLSDESC_CALL |
None | For relaxation only. Must be used to identify a BLR
instruction which performs an indirect call to the TLS
descriptor function for S + A. |
| 57603 | R_MORELLO_TLSIE_ADR_GOTTPREL_PAGE20 |
Page(G(GTPREL(S)))
- Page(P) |
Set the immediate value of an ADRP to bits [31:12] of X. Check that -231 <= X < 231. |
| 57604 | R_MORELLO_TLSIE_ADD_LO12 |
G(GTPREL(S)) |
Set the ADD immediate field to bits [11:0] of X. No overflow check. |
| ELF64 Code | Name | Operation | Comment |
|---|---|---|---|
| 59392 | R_MORELLO_CAPINIT |
CAP_INIT(S, A, CAP_SIZE, CAP_PERM) |
See note below. |
| 59393 | R_MORELLO_GLOB_DAT |
CAP_INIT(S, A, CAP_SIZE, CAP_PERM) |
See note below. |
| 59394 | R_MORELLO_JUMP_SLOT |
CAP_INIT(S, A, CAP_SIZE, CAP_PERM) |
See note below. |
| 59395 | R_MORELLO_RELATIVE |
CAP_INIT(S, A, CAP_SIZE, CAP_PERM) |
See note below. |
| 59396 | R_MORELLO_IRELATIVE |
CAP_INIT(S, A, CAP_SIZE, CAP_PERM) |
See note below. |
| 59397 | R_MORELLO_TLSDESC |
TLSDESC(S+A) |
Identifies a TLS descriptor to be filled. |
| 59398 | R_MORELLO_TPREL128 |
TPREL(S) |
See note below. |
Note
R_MORELLO_CAPINIT instructs the runtime or dynamic loader to create a 16-byte
capability at r_offset. r_offset must be 16-byte aligned. An object
producer may communicate a hint about the size of the capability to the static
linker in the 16-byte fragment identified by r_offset. The fragment has
the following format:
| 64-bits empty | 64-bits size |
R_MORELLO_GLOB_DAT instructs the runtime or dynamic loader to create a 16-byte
capability in the GOT entry identified by r_offset. The capability holds the
address of a data symbol which must be resolved at load time when dynamic
linking.
R_MORELLO_JUMP_SLOT instructs the dynamic loader to create a 16-byte capability
in the GOT entry identified by r_offset. The capability holds the address of a
function symbol which must be resolved at load time.
R_MORELLO_RELATIVE represents an optimization of R_MORELLO_GLOB_DAT. It can be
used when the symbol resolves to the current shared object or executable. S
must be the Null symbol (Index 0). The address and permissions must be written
to the fragment. See Dynamic linking with Morello for details.
R_MORELLO_IRELATIVE is used by the linker when transforming IFUNC s. The
rest are the same as R_MORELLO_RELATIVE
R_MORELLO_TLSDESC : identifies a TLS descriptor to be filled by the dynamic loader.
If the size of S is known by the static linker the 256-bit fragment will contain the size
of the symbol in the last 64 bits of the fragment. Otherwise the fragment will contain
all zeroes. The fragment has the following format:
| 192-bits empty | 64-bits size |
R_MORELLO_TPREL128 : instructs the dynamic loader to create a pair of two
64-bit integers, the first integer containing the offset of S in the TLS
block and the second integer containing the size of the symbol S. The first
64-bit integer (the offset) has the same fragment encoding as R_AARCH64_TLS_TPREL.
If the size of S is known by the static linker the second 64-bit integer in the
fragment will contain the size of the symbol. The fragment has the following format:
| 64-bits offset | 64-bits size |
A capability has more associated information than a conventional pointer. It has extra information. For example: base, offset, size and permissions.
Capabilities cannot be statically initialised. Global capability initialization
when static linking is performed by the runtime at program startup. The
communication between the static linker and runtime is implementation defined.
This document describes an implementation based on a table of capability
descriptions created at static link time, where each capability-generating
relocation results in one entry in the table. When static linking,
all capability descriptions will be explicitly grouped into a single table of
capability descriptions where each table entry is a struct capdesc (listed
below).
In the current LLVM based Morello toolchain, the runtime iterates through each
capdesc entry creating a capability in the location pointed to by
cap_location, with the specified base, offset, size and permissions given by the
entry. To aid in the finding of the capability descriptions table, the linker
emits two symbols to denote the start and end of the table:
__cap_relocs_start and __cap_relocs_end respectively. The capability
descriptions table is placed inside the __cap_relocs section.
struct capdesc
{
void*__capability cap_location;
void* base;
uint64_t offset;
uint64_t size;
uint64_t permissions;
};The permission bits of a capability constructed for a capdesc entry is the
inverse of the permissions[17:0] field in the capdesc entry.
Additionally, the MSB (bit 64) of the capdesc permissions field is set
for Executable symbols to indicate that the PCC is to be used to construct the
Capability.
| Permission | Encoding |
| Executable | 0x8000000000013DBCULL |
| Read-Write Data | 0x8FBEULL |
| Read-Only Data | 0x1BFBEULL |
When a Morello-capable assembler sees a .capinit instruction, it reserves a
16-byte (128 bits) location (fragment) and generates a
R_MORELLO_CAPINIT relocation for the linker to create a capability in the
fragment. The assembler may use the fragment with the following format
to give out size hints for the linker to use before processing the relocation:
| 64-bit: empty | 64-bit: size |
This size hint will be incorporated into the capdesc size field, if not
superseded by more accurate information.
In case of position independent code (PIC), the assembler will generate a
R_MORELLO_LD128_GOT_LO12_NC relocation, which causes the linker to generate
a 16-byte aligned, 16-byte sized entry in the .got that will be initialised
by a capdesc entry in a capability descriptions table with the address of
the .got entry as its location field. All information required to
initialize the capability is self-contained in the capdesc entry, so the
linker is not required to provide any size hints in the .got entry.
When dynamic linking, capability initialization is done by the dynamic linker as
a result of processing one of the dynamic relocations listed in
Dynamic relocations table. For R_MORELLO_RELATIVE and
R_MORELLO_IRELATIVE relocations, the static linker must write the following
information to the fragment identified by r_offset.
| 64-bit: address | 56-bits: length | 8-bits: permissions |
The 8-bit permission field of the fragment encodes the symbol permissions as below.
| Permission | Encoding |
| Executable | 0x4ULL |
| Read-Write Data | 0x2ULL |
| Read-Only Data | 0x1ULL |
As in Static linking with Morello, the linker creates a 16-byte aligned,
16-byte sized entry in the .got for the R_MORELLO_LD128_GOT_LO12_NC
relocation generated by the assembler. However, a capability descriptions table
is not generated to initialize the .got entry. Instead it is expected that
the dynamic linker generates the table itself based on the R_MORELLO_GLOB_DAT and
R_MORELLO_JUMP_SLOT relocations created by the static linker. The dynamic linker
writes the generated capabilities back into the .got entry.
The status of this appendix is informative.
The following code is sample runtime initialization code that initializes global capabilities created by an LLVM-based Morello toolchain.
__init_global_caps:
mrs c2, DDC /* Default data capability */
adrp c0, __cap_relocs_start
add c0, c0, #:lo12:__cap_relocs_start
adrp c1, __cap_relocs_end
add c1, c1, #:lo12:__cap_relocs_end
gcvalue x1, c1
gcvalue x0, c0
cmp x0, x1
b.eq .CapInitEnd
sub x5, x1, x0 /* __cap_relocs_size */
scvalue c0, c2, x0
scvalue c1, c2, x1
/* Clear permissions that we're not going to want on global capabilities. */
ldr x5, =(BIT_07 | \ /* Compartment ID */
BIT_08 | \ /* Branch Unseal */
BIT_10 | \ /* Unseal */
BIT_11 ) /* Seal */
clrperm c2, c2, x5
.CapInit:
ldr x5, [c0], #8 /* Capability location */
ldr x24, [c0], #8 /* Object refered by the capability */
cbnz c24, .CapNonNull
add c0, c0, #24
mov x4, #0 /* c4 <- nullptr */
b .CapCont
.CapNonNull:
ldr x25, [c0], #8 /* Offset in the object */
ldr x26, [c0], #8 /* Size */
ldr x9, [c0], #8 /* Permissions */
/* Set the executive permission for executable capabilities */
scvalue c4, c2, x24 /* Set capability base */
scbndse c4, c4, x26 /* Set size */
scoff c4, c4, x25 /* Add offset */
clrperm c4, c4, x9 /* Clear permission bits set in __cap_desc_ */
.CapCont:
scvalue c5, c2, x5
str c4, [c5]
cmp c0, c1
b.ne .CapInit
.CapInitEnd:
ret
For C64 to A64 interworking, the following veneer is used:
adrp c16, sym
add c16, c16, :lo12:sym
br c16
For A64 to C64 interworking, and for C64 to C64 Range Extension, the following veneer is used. The BX changes the execution state from A64 to C64:
bx #4
adrp c16, sym
add c16, c16, :lo12:sym
br c16
The design is based on TLSDESC, with the purpose of minimizing the performance differences between A64 and C64, while providing strict bounds when resolving TLS globals.
The static block layout is the same used in AArch64 (Variant 1, see [TLS]), with the only exception that TCB and the DTV pointer are capabilities.
The thread pointer is a capability, held in CTPIDR_EL0. The thread pointer
needs to have the read, write, read capability and write capability permissions
and bounds such that the entire TLS static block is accessible.
A resolver function takes arguments in c0 (address of the TLS GOT slot), and c2 (a copy of the thread pointer) and returns a pointer to the TLS global in c0. The resolver function has a custom calling convention that must preserve all registers except c0 and c1.
Considerations:
- Any dynamically loaded modules will be placed outside of the bounds of the thread pointer, so a resolver function cannot return an offset from the thread pointer, but rather needs to return a pointer (capability).
- To minimize reading of
CTPIDR_EL0, the resolver functions take a copy ofCTPIDR_EL0as an argument and preserve it.
If the TLS variable is in the static block, while resolving the
R_MORELLO_TLSDESC relocation, the dynamic linker will place in the two GOT
slots associated with this variable:
- A capability to the static TLS block resolver function at offset 0.
- The offset of the variable in the static TLS block at offset 16 (8 bytes).
- The size of the variable at offset 24 (8 bytes).
An implementation of the static block resolver could be the following:
ldp x0, x1, [c0, #16]
add c0, c2, x0
scbnds c0, c0, x1
ret c30
The capability to the TLS variable is derived from CTPIDR_EL0. There
are no requirements on how this is performed or the registers used, except
that the sequence doesn't produce a dynamic relocation. A possible
instruction sequence could be:
mrs c0, CTPIDR_EL0
movz x8, #:tprel_g1:local_exec_var
movk x8, #:tprel_g0_nc:local_exec_var
movz x9, #:size_g1:local_exec_var
movk x9, #:size_g0_nc:local_exec_var
add c0, c0, x8, uxtx
scbnds c0, c0, x9
The capability to the TLS variable is derived from CTPIDR_EL0. The size
and offset of the TLS variable is stored in a GOT slot (first 8 bytes contains the
offset and the second 8 bytes the size). This GOT slot is initialized by a
R_MORELLO_TPREL128 dynamic relocation. The access must use the
R_MORELLO_TLSIE_ADR_GOTTPREL_PAGE20 and R_MORELLO_TLSIE_ADD_LO12
relocations in order to allow relaxation to Local Exec. There are no other
requirements on how this is performed or the registers used. A possible
instruction sequence could be:
adrp c0, :gottprel:initial_exec_var
add c0, c0, :gottprel_lo12:initial_exec_var
ldp x0, x8, [c0]
mrs c1, CTPIDR_EL0
add c0, c1, x0, uxtx
scbnds c0, c0, x8
The linker will generate 16 bytes in a read-only section, containing the offset in the static TLS block in the first 8 bytes and the size of the symbol in the next 8 bytes:
.section .rodata
_sym_data:
.xword tlsoffset(sym)
.xword sizeof(sym)
Note
tlsoffset(sym) denotes the offset in the static TLS block of the symbol
sym, while sizeof(sym) denotes the size of the symbol sym. These are
not valid assembler directives.
The relaxation is performed by:
- changing the
R_MORELLO_TLSIE_ADR_GOTTPREL_PAGE20relocation on the symbolsymto aR_MORELLO_ADR_PREL_PG_HI20with the symbol_sym_data - changing the
R_MORELLO_TLSIE_ADD_LO12relocation on symbol thesymto aR_AARCH64_ADD_ABS_LO12_NCrelocation with the symbol_sym_data.
Note
The symbol and section names in the example above are only used for explanation purposes. An implementation does not need to create an additional symbol when performing this relaxation. There is no constraint on the name of the read-only section where the data is placed.
The instruction sequence used for the General Dynamic access model is similar to that of other TLSDESC implementations, with the exception that the result doesn't need to be added to the thread pointer. However c2 needs to contain the thread pointer. The instruction sequence contains an additional NOP instruction in order to permit the static linker to perform a relaxation to Local Exec or Initial Exec.
The General Dynamic access sequence must be output in the following form to allow correct linker relaxation:
adrp c0, :tlsdesc:sym
ldr c1, [c0, :tlsdesc_lo12:sym]
add c0, c0, :tlsdesc_lo12:sym
nop
.tlsdesccall sym
blr c1
The relaxed sequence is:
adrp c0, :gottprel:sym
add c0, c0, :gottprel_lo12:sym
ldp x0, x1, [c0]
add c0, c2, x0
scbnds c0, c0, x1
The linker will generate 16 bytes in a read-only section, containing the offset in the static TLS block in the first 8 bytes and the size of the symbol in the next 8 bytes:
.section .rodata
_sym_data:
.xword tlsoffset(sym)
.xword sizeof(sym)
Note
tlsoffset(sym) denotes the offset in the static TLS block of the symbol
sym, while sizeof(sym) denotes the size of the symbol sym. These are
not valid assembler directives.
The relaxed sequence is:
adrp c0, _sym_data
add c0, c0, :lo12:_sym_data
ldp x0, x1, [c0]
add c0, c2, x0
scbnds c0, c0, x1