scripts/decodecode: make faulting insn ptr more robust

scripts/decodecode: make faulting insn ptr more robust

It can accidentally happen that the faulting insn (the exact instruction
bytes) is repeated a little further on in the trace.  This causes that
same instruction to be tagged twice, see example below.

What we want to do, however, is to track back from the end of the whole
disassembly so many lines as the slice which starts with the faulting
instruction is long.  This leads us to the actual faulting instruction
and *then* we tag it.

While we're at it, we can drop the sed "g" flag because we address only
this one line.

Also, if we point to an instruction which changes decoding depending on
the slice being objdumped, like a Jcc insn, for example, we do not even
tag it as a faulting instruction because the instruction decode changes
in the second slice but we use that second format as a regex on the
fsrst disassembled buffer and more often than not that instruction
doesn't match.

Again, simply tag the line which is deduced from the original "<>"
marking we've received from the kernel.

This also solves the pathologic issue of multiple tagging like this:

  29:*  0f 0b                   ud2         <-- trapping instruction
  2b:*  0f 0b                   ud2         <-- trapping instruction
  2d:*  0f 0b                   ud2         <-- trapping instruction

Double tagging example:

Code: 34 dd 40 30 ad 81 48 c7 c0 80 f6 00 00 48 8b 3c 30 48 01 c6 b8 ff ff ff ff 48 8d 57 f0 48 39 f7 74 2f 49 8b 4c 24 08 48 8b 47 f0 <48> 39 48 08 75 0e eb 2a 66 90 48 8b 40 f0 48 39 48 08 74 1e 48
All code
========
   0:   34 dd                   xor    $0xdd,%al
   2:   40 30 ad 81 48 c7 c0    xor    %bpl,-0x3f38b77f(%rbp)
   9:   80 f6 00                xor    $0x0,%dh
   c:   00 48 8b                add    %cl,-0x75(%rax)
   f:   3c 30                   cmp    $0x30,%al
  11:   48 01 c6                add    %rax,%rsi
  14:   b8 ff ff ff ff          mov    $0xffffffff,%eax
  19:   48 8d 57 f0             lea    -0x10(%rdi),%rdx
  1d:   48 39 f7                cmp    %rsi,%rdi
  20:   74 2f                   je     0x51
  22:   49 8b 4c 24 08          mov    0x8(%r12),%rcx
  27:   48 8b 47 f0             mov    -0x10(%rdi),%rax
  2b:*  48 39 48 08             cmp    %rcx,0x8(%rax)     <-- trapping instruction
  2f:   75 0e                   jne    0x3f
  31:   eb 2a                   jmp    0x5d
  33:   66 90                   xchg   %ax,%ax
  35:   48 8b 40 f0             mov    -0x10(%rax),%rax
  39:*  48 39 48 08             cmp    %rcx,0x8(%rax)     <-- trapping instruction
  3d:   74 1e                   je     0x5d
  3f:   48                      rex.W

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>