[BPF] Return fail if disassembled insn registers out of range
Daniel reported a llvm-objdump segfault like below:
$ llvm-objdump -D bpf_xdp.o
...
0000000000000000 <.strtab>:
0: 00 63 69 6c 69 75 6d 5f <unknown>
1: 6c 62 36 5f 61 66 66 69 w2 <<= w6
...
(llvm-objdump: lib/Target/BPF/BPFGenAsmWriter.inc:1087: static const char*
llvm::BPFInstPrinter::getRegisterName(unsigned int): Assertion
`RegNo && RegNo < 25 && "Invalid register number!"' failed.
Stack dump:
0. Program arguments: llvm-objdump -D bpf_xdp.o
...
abort
...
llvm::BPFInstPrinter::getRegisterName(unsigned int)
llvm::BPFInstPrinter::printMemOperand(llvm::MCInst const*,
int, llvm::raw_ostream&, char const*)
llvm::BPFInstPrinter::printInstruction(llvm::MCInst const*,
unsigned long, llvm::raw_ostream&)
llvm::BPFInstPrinter::printInst(llvm::MCInst const*,
unsigned long, llvm::StringRef, llvm::MCSubtargetInfo const&,
llvm::raw_ostream&)
...
Basically, since -D enables disassembly for all sections, .strtab is also disassembled,
but some strings are decoded as legal instructions but with illegal register numbers.
When llvm-objdump tries to print register name for these illegal register numbers,
assertion and segfault happens.
The patch fixed the issue by returning fail for a disassembled insn if
that insn contains a reg operand with illegal reg number.
The insn will be printed as "<unknown>" instead of causing an assertion.