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[PAC][lld] Use braa instr in PAC PLT sequence with valid PAuth core info #113945

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Merged
merged 3 commits into from
Nov 18, 2024
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[PAC][lld] Use braa instr in PAC PLT sequence with valid PAuth core info #113945

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[PAC][lld] Use braa instr in PAC PLT sequence with valid PAuth core info
kovdan01 Oct 28, 2024
13875e7
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kovdan01 Nov 1, 2024
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kovdan01 Nov 10, 2024
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36 changes: 29 additions & 7 deletions lld/ELF/Arch/AArch64.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -999,7 +999,11 @@ class AArch64BtiPac final : public AArch64 {

private:
bool btiHeader; // bti instruction needed in PLT Header and Entry
bool pacEntry; // autia1716 instruction needed in PLT Entry
enum {
PEK_NoAuth,
PEK_AuthHint, // use autia1716 instr for authenticated branch in PLT entry
PEK_Auth, // use braa instr for authenticated branch in PLT entry
} pacEntryKind;
};
} // namespace

Expand All @@ -1014,9 +1018,21 @@ AArch64BtiPac::AArch64BtiPac(Ctx &ctx) : AArch64(ctx) {
// relocations.
// The PAC PLT entries require dynamic loader support and this isn't known
// from properties in the objects, so we use the command line flag.
pacEntry = ctx.arg.zPacPlt;
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It could be worth adding to the comment here. Something like:

By default we only use hint-space instructions, but if we detect the PAuthABI, which requires v8.3-A we can use the non-hint space instructions.

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Added, thanks, see b189f51

// By default we only use hint-space instructions, but if we detect the
// PAuthABI, which requires v8.3-A, we can use the non-hint space
// instructions.

if (ctx.arg.zPacPlt) {
if (llvm::any_of(ctx.aarch64PauthAbiCoreInfo,
[](uint8_t c) { return c != 0; }))
pacEntryKind = PEK_Auth;
else
pacEntryKind = PEK_AuthHint;
} else {
pacEntryKind = PEK_NoAuth;
}

if (btiHeader || pacEntry) {
if (btiHeader || (pacEntryKind != PEK_NoAuth)) {
pltEntrySize = 24;
ipltEntrySize = 24;
}
Expand Down Expand Up @@ -1066,9 +1082,13 @@ void AArch64BtiPac::writePlt(uint8_t *buf, const Symbol &sym,
0x11, 0x02, 0x40, 0xf9, // ldr x17, [x16, Offset(&(.got.plt[n]))]
0x10, 0x02, 0x00, 0x91 // add x16, x16, Offset(&(.got.plt[n]))
};
const uint8_t pacHintBr[] = {
0x9f, 0x21, 0x03, 0xd5, // autia1716
0x20, 0x02, 0x1f, 0xd6 // br x17
};
const uint8_t pacBr[] = {
0x9f, 0x21, 0x03, 0xd5, // autia1716
0x20, 0x02, 0x1f, 0xd6 // br x17
0x30, 0x0a, 0x1f, 0xd7, // braa x17, x16
0x1f, 0x20, 0x03, 0xd5 // nop
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maybe UDF instruction is a safer choice instead of NOP. also may not have any significance.

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I think it's a nice suggestion, but I'll apply it in a separate PR not to mix different changes - nop is present both in pacBr and stdBr, while the current PR is intended to change only pacBr.

};
const uint8_t stdBr[] = {
0x20, 0x02, 0x1f, 0xd6, // br x17
Expand Down Expand Up @@ -1096,8 +1116,10 @@ void AArch64BtiPac::writePlt(uint8_t *buf, const Symbol &sym,
relocateNoSym(buf + 4, R_AARCH64_LDST64_ABS_LO12_NC, gotPltEntryAddr);
relocateNoSym(buf + 8, R_AARCH64_ADD_ABS_LO12_NC, gotPltEntryAddr);

if (pacEntry)
memcpy(buf + sizeof(addrInst), pacBr, sizeof(pacBr));
if (pacEntryKind != PEK_NoAuth)
memcpy(buf + sizeof(addrInst),
pacEntryKind == PEK_AuthHint ? pacHintBr : pacBr,
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Could be a possibility to make pacHintBr and pacBr into an array indexed by pacEntryKind.

memcpy(buf + sizeof(addrInst),
  pacBrInstrs[pacEntryKind],
  sizeof(pacBrInstrs[pacEntryKind]));

Not sure if it is worth the trouble though.

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Thanks for suggestion. I think I'll leave this "as is" w/o changes - it looks like that a pretty small decrease in code length here will require increase in complexity by introducing invariants on enum values and increase in code length somewhere else.

sizeof(pacEntryKind == PEK_AuthHint ? pacHintBr : pacBr));
else
memcpy(buf + sizeof(addrInst), stdBr, sizeof(stdBr));
if (!hasBti)
Expand Down
10 changes: 6 additions & 4 deletions lld/test/ELF/aarch64-feature-pauth.s
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Original file line number Diff line number Diff line change
Expand Up @@ -56,8 +56,8 @@

# PACPLTTAG: 0x0000000070000003 (AARCH64_PAC_PLT)

# RUN: llvm-objdump -d pacplt-nowarn | FileCheck --check-prefix PACPLT -DA=10380 -DB=478 -DC=480 %s
# RUN: llvm-objdump -d pacplt-warn | FileCheck --check-prefix PACPLT -DA=10390 -DB=488 -DC=490 %s
# RUN: llvm-objdump -d pacplt-nowarn | FileCheck --check-prefixes=PACPLT,NOHINT -DA=10380 -DB=478 -DC=480 %s
# RUN: llvm-objdump -d pacplt-warn | FileCheck --check-prefixes=PACPLT,HINT -DA=10390 -DB=488 -DC=490 %s

# PACPLT: Disassembly of section .text:
# PACPLT: <func2>:
Expand All @@ -77,8 +77,10 @@
# PACPLT-NEXT: adrp x16, 0x30000 <func3+0x30000>
# PACPLT-NEXT: ldr x17, [x16, #0x[[C]]]
# PACPLT-NEXT: add x16, x16, #0x[[C]]
# PACPLT-NEXT: autia1716
# PACPLT-NEXT: br x17
# NOHINT-NEXT: braa x17, x16
# NOHINT-NEXT: nop
# HINT-NEXT: autia1716
# HINT-NEXT: br x17
# PACPLT-NEXT: nop

#--- abi-tag-short.s
Expand Down
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