[PAUTHABIELF64] Add R_AARCH64_AUTH_GOT_ADR_PREL_LO21 relocation #259
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| Compared to the tiny code model without pointer authentication an | ||
| additonal adr is required to get the address of the GOT entry. |
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I'm not sure I am following here - all PLT sequences as currently defined(1) must compute the address of the GOT entry for the dynamic linker. So the ADR is required even without PAUTH, and it would be worth having the equivalent relocation if it doesn't already exist (given that just having PC-relative LDR won't work).
(1) It is possible to define PLT sequences that compute the address of the GOT entry in a different way.
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This is intended for something like:
int x;
int* f(void) {
return &x;
}
Which with the tiny code-model is
f:
ldr x0, :got:x
ret
There's no need for the adr, in the non PAuth case. In the PAuth case the address is needed for authentication.
All the current linker PLT implementations use the small code-model so they use adrp. I take the point that we could have a tiny PLT sequence, off the top of my head something like:
adr x16, <address of .got.plt entry>
ldr x17, [x16]
br x17
Which would need a non-Auth equivalent relocation like R_AARCH64_GOT_ADR_PREL_LO21. Although as the linker generates the .plt and the .got.plt so it could just implement the calculation without a relocation directive.
Is there a need for something like R_AARCH64_GOT_ADR_PREL_LO21 in user code?
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You're right, there doesn't seem a case for user code, so if linkers use internal relocations then there is no need to add one.
| | | | | value to bits [11:0] of | | ||
| | | | | X. No overflow check. | | ||
| +-------------+----------------------------------------+----------------------------------+--------------------------+ | ||
| | 0x811D | R\AARCH64\_AUTH\_GOT\_ADR\_PREL\_LO21 | G(ENCD(GDAT(S + A))) - P | Set the immediate | |
|
We double checked and aligned all names in the implementation according to the spec (see llvm/llvm-project@029ba55) |
With the tiny code model and a signed GOT, an adr instruction is needed to get the address of the GOT entry for input to the authenication. For example: adr x8, :got_auth: symbol ldr x0, [x8] // Authenticate to get unsigned pointer autia x0, x8 The adr requires a new relocation code where there isn't a direct equivalent in the main ABI as there is not need to take the address of the GOT slot when no authentication is required. We define R_AARCH64_AUTH_GOT_ADR_PREL21_LO21 for this purpose following the naming convention of R_<CLS>_ADR_PREL_LO21. which is its closest equivalent.
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Thanks for the confirmation. |
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With the tiny code model and a signed GOT, an adr instruction is needed to get the address of the GOT entry for input to the authenication. For example:
adr x8, :got_auth: symbol
ldr x0, [x8]
// Authenticate to get unsigned pointer
autia x0, x8
The adr requires a new relocation code where there isn't a direct equivalent in the main ABI as there is not need to take the address of the GOT slot when no authentication is required.
We define R_AARCH64_AUTH_GOT_ADR_PREL21_LO21 for this purpose following the naming convention of R__ADR_PREL_LO21. which is its closest equivalent.