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[BOLT] Add auto parsing for Linux kernel .altinstructions #95068

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merged 2 commits into from
Jun 11, 2024
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[BOLT] Add auto parsing for Linux kernel .altinstructions #95068

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9dded7f
[BOLT] Add auto parsing for Linux kernel .altinstructions
maksfb Jun 11, 2024
932b57d
fixup! [BOLT] Add auto parsing for Linux kernel .altinstructions
maksfb Jun 11, 2024
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72 changes: 67 additions & 5 deletions bolt/lib/Rewrite/LinuxKernelRewriter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -273,6 +273,8 @@ class LinuxKernelRewriter final : public MetadataRewriter {

/// Handle alternative instruction info from .altinstructions.
Error readAltInstructions();
Error tryReadAltInstructions(uint32_t AltInstFeatureSize,
bool AltInstHasPadLen, bool ParseOnly);
Error rewriteAltInstructions();

/// Read .pci_fixup
Expand Down Expand Up @@ -1319,12 +1321,69 @@ Error LinuxKernelRewriter::rewriteBugTable() {
/// u8 padlen; // present in older kernels
/// } __packed;
///
/// Note the structures is packed.
/// Note that the structure is packed.
///
/// Since the size of the "feature" field could be either u16 or u32, and
/// "padlen" presence is unknown, we attempt to parse .altinstructions section
/// using all possible combinations (four at this time). Since we validate the
/// contents of the section and its size, the detection works quite well.
/// Still, we leave the user the opportunity to specify these features on the
/// command line and skip the guesswork.
Error LinuxKernelRewriter::readAltInstructions() {
AltInstrSection = BC.getUniqueSectionByName(".altinstructions");
if (!AltInstrSection)
return Error::success();

// Presence of "padlen" field.
std::vector<bool> PadLenVariants;
if (opts::AltInstHasPadLen.getNumOccurrences())
PadLenVariants.push_back(opts::AltInstHasPadLen);
else
PadLenVariants = {false, true};

// Size (in bytes) variants of "feature" field.
std::vector<uint32_t> FeatureSizeVariants;
if (opts::AltInstFeatureSize.getNumOccurrences())
FeatureSizeVariants.push_back(opts::AltInstFeatureSize);
else
FeatureSizeVariants = {2, 4};

for (bool AltInstHasPadLen : PadLenVariants) {
for (uint32_t AltInstFeatureSize : FeatureSizeVariants) {
LLVM_DEBUG({
dbgs() << "BOLT-DEBUG: trying AltInstHasPadLen = " << AltInstHasPadLen
<< "; AltInstFeatureSize = " << AltInstFeatureSize << ";\n";
});
if (Error E = tryReadAltInstructions(AltInstFeatureSize, AltInstHasPadLen,
/*ParseOnly*/ true)) {
consumeError(std::move(E));
continue;
}

LLVM_DEBUG(dbgs() << "Matched .altinstructions format\n");

if (!opts::AltInstHasPadLen.getNumOccurrences())
BC.outs() << "BOLT-INFO: setting --" << opts::AltInstHasPadLen.ArgStr
<< '=' << AltInstHasPadLen << '\n';

if (!opts::AltInstFeatureSize.getNumOccurrences())
BC.outs() << "BOLT-INFO: setting --" << opts::AltInstFeatureSize.ArgStr
<< '=' << AltInstFeatureSize << '\n';

return tryReadAltInstructions(AltInstFeatureSize, AltInstHasPadLen,
/*ParseOnly*/ false);
}
}

// We couldn't match the format. Read again to properly propagate the error
// to the user.
return tryReadAltInstructions(opts::AltInstFeatureSize,
opts::AltInstHasPadLen, /*ParseOnly*/ false);
}

Error LinuxKernelRewriter::tryReadAltInstructions(uint32_t AltInstFeatureSize,
bool AltInstHasPadLen,
bool ParseOnly) {
const uint64_t Address = AltInstrSection->getAddress();
DataExtractor DE = DataExtractor(AltInstrSection->getContents(),
BC.AsmInfo->isLittleEndian(),
Expand All @@ -1336,12 +1395,12 @@ Error LinuxKernelRewriter::readAltInstructions() {
Address + Cursor.tell() + (int32_t)DE.getU32(Cursor);
const uint64_t AltInstAddress =
Address + Cursor.tell() + (int32_t)DE.getU32(Cursor);
const uint64_t Feature = DE.getUnsigned(Cursor, opts::AltInstFeatureSize);
const uint64_t Feature = DE.getUnsigned(Cursor, AltInstFeatureSize);
const uint8_t OrgSize = DE.getU8(Cursor);
const uint8_t AltSize = DE.getU8(Cursor);

// Older kernels may have the padlen field.
const uint8_t PadLen = opts::AltInstHasPadLen ? DE.getU8(Cursor) : 0;
const uint8_t PadLen = AltInstHasPadLen ? DE.getU8(Cursor) : 0;

if (!Cursor)
return createStringError(
Expand All @@ -1358,7 +1417,7 @@ Error LinuxKernelRewriter::readAltInstructions() {
<< "\n\tFeature: 0x" << Twine::utohexstr(Feature)
<< "\n\tOrgSize: " << (int)OrgSize
<< "\n\tAltSize: " << (int)AltSize << '\n';
if (opts::AltInstHasPadLen)
if (AltInstHasPadLen)
BC.outs() << "\tPadLen: " << (int)PadLen << '\n';
}

Expand All @@ -1375,7 +1434,7 @@ Error LinuxKernelRewriter::readAltInstructions() {

BinaryFunction *AltBF =
BC.getBinaryFunctionContainingAddress(AltInstAddress);
if (AltBF && BC.shouldEmit(*AltBF)) {
if (!ParseOnly && AltBF && BC.shouldEmit(*AltBF)) {
BC.errs()
<< "BOLT-WARNING: alternative instruction sequence found in function "
<< *AltBF << '\n';
Expand All @@ -1397,6 +1456,9 @@ Error LinuxKernelRewriter::readAltInstructions() {
" referenced by .altinstructions entry %d",
OrgInstAddress, EntryID);

if (ParseOnly)
continue;

// There could be more than one alternative instruction sequences for the
// same original instruction. Annotate each alternative separately.
std::string AnnotationName = "AltInst";
Expand Down
20 changes: 13 additions & 7 deletions bolt/test/X86/linux-alt-instruction.s
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Original file line number Diff line number Diff line change
Expand Up @@ -12,24 +12,30 @@
## Older kernels used to have padlen field in alt_instr. Check compatibility.

# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --defsym PADLEN=1 \
# RUN: %s -o %t.o
# RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
# RUN: %s -o %t.padlen.o
# RUN: %clang %cflags -nostdlib %t.padlen.o -o %t.padlen.exe \
# RUN: -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-has-padlen -o %t.out \
# RUN: llvm-bolt %t.padlen.exe --print-normalized --alt-inst-has-padlen -o %t.padlen.out \
# RUN: | FileCheck %s

## Check with a larger size of "feature" field in alt_instr.

# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown \
# RUN: --defsym FEATURE_SIZE_4=1 %s -o %t.o
# RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
# RUN: --defsym FEATURE_SIZE_4=1 %s -o %t.fs4.o
# RUN: %clang %cflags -nostdlib %t.fs4.o -o %t.fs4.exe \
# RUN: -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=4 -o %t.out \
# RUN: llvm-bolt %t.fs4.exe --print-normalized --alt-inst-feature-size=4 -o %t.fs4.out \
# RUN: | FileCheck %s

## Check that out-of-bounds read is handled properly.

# RUN: not llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=2 -o %t.out
# RUN: not llvm-bolt %t.fs4.exe --alt-inst-feature-size=2 -o %t.fs4.out

## Check that BOLT automatically detects structure fields in .altinstructions.

# RUN: llvm-bolt %t.exe --print-normalized -o %t.out | FileCheck %s
# RUN: llvm-bolt %t.exe --print-normalized -o %t.padlen.out | FileCheck %s
# RUN: llvm-bolt %t.exe --print-normalized -o %t.fs4.out | FileCheck %s

# CHECK: BOLT-INFO: Linux kernel binary detected
# CHECK: BOLT-INFO: parsed 2 alternative instruction entries
Expand Down
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