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[BOLT] Fix support for PIC jump tables

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Summary:
BOLT heuristics failed to work if false PIC jump table entries were
accepted when they were pointing inside a function, but not at
an instruction boundary.

This fix checks if the destination falls at instruction boundary, and
if it does not, it truncates the jump table. This, of course, still does not
guarantee that the entry corresponds to a real destination, and we can
have "false positive" entry(ies). However, it shouldn't affect
correctness of the function, but the CFG may have edges that are never
taken. We may update an incorrect jump table entry, corresponding to an
unrelated data, and for that reason we force moving of jump tables if a
PIC jump table was detected.

(cherry picked from FBD8559588)
This commit is contained in:
Maksim Panchenko
2018-06-20 21:43:22 -07:00
parent 35c09dc4dd
commit 3ab2929b36
1 changed files with 22 additions and 5 deletions
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27
bolt/src/BinaryFunction.cpp
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@@ -854,12 +854,13 @@ IndirectBranchType BinaryFunction::processIndirectBranch(MCInst &Instruction,
Type = IndirectBranchType::POSSIBLE_JUMP_TABLE;
continue;
}
// Potentially a switch table can contain __builtin_unreachable() entry
// Potentially a switch table can contain __builtin_unreachable() entry
// pointing just right after the function. In this case we have to check
// another entry. Otherwise the entry is outside of this function scope
// and it's not a switch table.
if (Value == getAddress() + getSize()) {
JTOffsetCandidates.push_back(Value - getAddress());
JTOffsetCandidates.push_back(getSize());
IgnoredBranches.emplace_back(Offset, getSize());
} else {
break;
}
@@ -1388,16 +1389,32 @@ add_instruction:
addInstruction(Offset, std::move(Instruction));
}
postProcessJumpTables();
updateState(State::Disassembled);
postProcessJumpTables();
}
void BinaryFunction::postProcessJumpTables() {
// Create labels for all entries.
for (auto &JTI : JumpTables) {
auto &JT = *JTI.second;
for (auto Offset : JT.OffsetEntries) {
if (JT.Type == JumpTable::JTT_PIC && opts::JumpTables == JTS_BASIC) {
opts::JumpTables = JTS_MOVE;
outs() << "BOLT-INFO: forcing -jump-tables=move as PIC jump table was "
"detected\n";
}
for (unsigned I = 0; I < JT.OffsetEntries.size(); ++I) {
auto Offset = JT.OffsetEntries[I];
if (Offset != getSize() && !getInstructionAtOffset(Offset)) {
DEBUG(dbgs() << "BOLT-DEBUG: truncating jump table " << JT.getName()
<< " at index " << I << " containing offset 0x"
<< Twine::utohexstr(Offset) << '\n');
assert(I > 1 && "jump table with a size smaller than 1 detected");
assert(JT.Type == JumpTable::JTT_PIC &&
"unexpected truncation of non-PIC jump table");
JT.OffsetEntries.resize(I);
break;
}
auto *Label = getOrCreateLocalLabel(getAddress() + Offset,
/*CreatePastEnd*/ true);
JT.Entries.push_back(Label);