ARM: net: bpf: fix tail call jumps

When a tail call fails, it is documented that the tail call should
continue execution at the following instruction.  An example tail call
sequence is:

  12: (85) call bpf_tail_call#12
  13: (b7) r0 = 0
  14: (95) exit

The ARM assembler for the tail call in this case ends up branching to
instruction 14 instead of instruction 13, resulting in the BPF filter
returning a non-zero value:

  178:	ldr	r8, [sp, torvalds#588]	; insn 12
  17c:	ldr	r6, [r8, r6]
  180:	ldr	r8, [sp, torvalds#580]
  184:	cmp	r8, r6
  188:	bcs	0x1e8
  18c:	ldr	r6, [sp, torvalds#524]
  190:	ldr	r7, [sp, torvalds#528]
  194:	cmp	r7, #0
  198:	cmpeq	r6, torvalds#32
  19c:	bhi	0x1e8
  1a0:	adds	r6, r6, #1
  1a4:	adc	r7, r7, #0
  1a8:	str	r6, [sp, torvalds#524]
  1ac:	str	r7, [sp, torvalds#528]
  1b0:	mov	r6, torvalds#104
  1b4:	ldr	r8, [sp, torvalds#588]
  1b8:	add	r6, r8, r6
  1bc:	ldr	r8, [sp, torvalds#580]
  1c0:	lsl	r7, r8, #2
  1c4:	ldr	r6, [r6, r7]
  1c8:	cmp	r6, #0
  1cc:	beq	0x1e8
  1d0:	mov	r8, torvalds#32
  1d4:	ldr	r6, [r6, r8]
  1d8:	add	r6, r6, torvalds#44
  1dc:	bx	r6
  1e0:	mov	r0, #0		; insn 13
  1e4:	mov	r1, #0
  1e8:	add	sp, sp, torvalds#596	; insn 14
  1ec:	pop	{r4, r5, r6, r7, r8, sl, pc}

For other sequences, the tail call could end up branching midway through
the following BPF instructions, or maybe off the end of the function,
leading to unknown behaviours.

Fixes: 39c13c2 ("arm: eBPF JIT compiler")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/net/bpf_jit_32.c

@@ -949,7 +949,7 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx)
 	const u8 *tcc = bpf2a32[TCALL_CNT];
 	const int idx0 = ctx->idx;
 #define cur_offset (ctx->idx - idx0)
-#define jmp_offset (out_offset - (cur_offset))
+#define jmp_offset (out_offset - (cur_offset) - 2)
 	u32 off, lo, hi;
 
 	/* if (index >= array->map.max_entries)