objtool: Support repeated uses of the same C jump table

This fixes objtool for both a GCC issue and a Clang issue:

1) GCC issue:

   kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x8d5: sibling call from callable instruction with modified stack frame

   With CONFIG_RETPOLINE=n, GCC is doing the following optimization in
   ___bpf_prog_run().

   Before:

           select_insn:
                   jmp *jumptable(,%rax,8)
                   ...
           ALU64_ADD_X:
                   ...
                   jmp select_insn
           ALU_ADD_X:
                   ...
                   jmp select_insn

   After:

           select_insn:
                   jmp *jumptable(, %rax, 8)
                   ...
           ALU64_ADD_X:
                   ...
                   jmp *jumptable(, %rax, 8)
           ALU_ADD_X:
                   ...
                   jmp *jumptable(, %rax, 8)

   This confuses objtool.  It has never seen multiple indirect jump
   sites which use the same jump table.

   For GCC switch tables, the only way of detecting the size of a table
   is by continuing to scan for more tables.  The size of the previous
   table can only be determined after another switch table is found, or
   when the scan reaches the end of the function.

   That logic was reused for C jump tables, and was based on the
   assumption that each jump table only has a single jump site.  The
   above optimization breaks that assumption.

2) Clang issue:

   drivers/usb/misc/sisusbvga/sisusb.o: warning: objtool: sisusb_write_mem_bulk()+0x588: can't find switch jump table

   With clang 9, code can be generated where a function contains two
   indirect jump instructions which use the same switch table.

The fix is the same for both issues: split the jump table parsing into
two passes.

In the first pass, locate the heads of all switch tables for the
function and mark their locations.

In the second pass, parse the switch tables and add them.

Fixes: e55a7325 ("bpf: Fix ORC unwinding in non-JIT BPF code")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/e995befaada9d4d8b2cf788ff3f566ba900d2b4d.1563413318.git.jpoimboe@redhat.com



Co-developed-by: Josh Poimboeuf <jpoimboe@redhat.com>