Generate .llvm_jump_table_sizes section

For example,
  [ 6] .rodata           PROGBITS        0000000000000000 000740 0000d6 00   A  0   0  8
  [ 7] .rel.rodata       REL             0000000000000000 003860 000080 10   I 39   6  8
  [ 8] .llvm_jump_table_sizes LLVM_JT_SIZES 0000000000000000 000816 000010 00      0   0  1
  [ 9] .rel.llvm_jump_table_sizes REL    0000000000000000 0038e0 000010 10   I 39   8  8
  ...
  [14] .llvm_jump_table_sizes LLVM_JT_SIZES 0000000000000000 000958 000010 00      0   0  1
  [15] .rel.llvm_jump_table_sizes REL    0000000000000000 003970 000010 10   I 39  14  8

With llvm-readelf dump section 8 and 14:
  $ llvm-readelf -x 8 user_ringbuf_success.bpf.o
  Hex dump of section '.llvm_jump_table_sizes':
  0x00000000 00000000 00000000 04000000 00000000 ................
  $ llvm-readelf -x 14 user_ringbuf_success.bpf.o
  Hex dump of section '.llvm_jump_table_sizes':
  0x00000000 20000000 00000000 04000000 00000000  ...............
You can see. There are two jump tables:
  jump table 1: offset 0, size 4 (4 labels)
  jump table 2: offset 0x20, size 4 (4 labels)

Check sections 9 and 15, we can find the corresponding section:
  Relocation section '.rel.llvm_jump_table_sizes' at offset 0x38e0 contains 1 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000000  0000000a00000002 R_BPF_64_ABS64         0000000000000000 .rodata
  Relocation section '.rel.llvm_jump_table_sizes' at offset 0x3970 contains 1 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000000  0000000a00000002 R_BPF_64_ABS64         0000000000000000 .rodata
and confirmed that the relocation is against '.rodata'.

Dump .rodata section:
  0x00000000 a8000000 00000000 10010000 00000000 ................
  0x00000010 b8000000 00000000 c8000000 00000000 ................
  0x00000020 28040000 00000000 00050000 00000000 (...............
  0x00000030 70040000 00000000 b8040000 00000000 p...............
  0x00000040 44726169 6e207265 7475726e 65643a20 Drain returned:

So we can get two jump tables:
  .rodata offset 0, # of lables 4:
  0x00000000 a8000000 00000000 10010000 00000000 ................
  0x00000010 b8000000 00000000 c8000000 00000000 ................
  .rodata offset 0x200, # of lables 4:
  0x00000020 28040000 00000000 00050000 00000000 (...............
  0x00000030 70040000 00000000 b8040000 00000000 p...............

This way, you just need to scan related code section. As long as it
matches one of jump tables (.rodata relocation, offset also matching),
you do not need to care about gotox at all in libbpf.

Author: Yonghong Song

llvm/include/llvm/CodeGen/AsmPrinter.h

@@ -26,13 +26,15 @@
 #include "llvm/CodeGen/StackMaps.h"
 #include "llvm/DebugInfo/CodeView/CodeView.h"
 #include "llvm/IR/InlineAsm.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cstdint>
 #include <memory>
 #include <utility>
 #include <vector>
 
 namespace llvm {
+extern cl::opt<bool> EmitJumpTableSizesSection;
 
 class AddrLabelMap;
 class AsmPrinterHandler;

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -168,7 +168,7 @@ static cl::opt<bool> BBAddrMapSkipEmitBBEntries(
              "unnecessary for some PGOAnalysisMap features."),
     cl::Hidden, cl::init(false));
 
-static cl::opt<bool> EmitJumpTableSizesSection(
+cl::opt<bool> llvm::EmitJumpTableSizesSection(
     "emit-jump-table-sizes-section",
     cl::desc("Emit a section containing jump table addresses and sizes"),
     cl::Hidden, cl::init(false));

llvm/lib/Target/BPF/BPFAsmPrinter.cpp

@@ -57,6 +57,7 @@ class BPFAsmPrinter : public AsmPrinter {
 } // namespace
 
 bool BPFAsmPrinter::doInitialization(Module &M) {
+  EmitJumpTableSizesSection = true;
   AsmPrinter::doInitialization(M);
 
   // Only emit BTF when debuginfo available.