arm64: ftrace: fix branch range checks

The branch range checks in ftrace_make_call() and ftrace_make_nop() are
incorrect, erroneously permitting a forwards branch of 128M and
erroneously rejecting a backwards branch of 128M.

This is because both functions calculate the offset backwards,
calculating the offset *from* the target *to* the branch, rather than
the other way around as the later comparisons expect.

If an out-of-range branch were erroeously permitted, this would later be
rejected by aarch64_insn_gen_branch_imm() as branch_imm_common() checks
the bounds correctly, resulting in warnings and the placement of a BRK
instruction. Note that this can only happen for a forwards branch of
exactly 128M, and so the caller would need to be exactly 128M bytes
below the relevant ftrace trampoline.

If an in-range branch were erroeously rejected, then:

* For modules when CONFIG_ARM64_MODULE_PLTS=y, this would result in the
  use of a PLT entry, which is benign.

  Note that this is the common case, as this is selected by
  CONFIG_RANDOMIZE_BASE (and therefore RANDOMIZE_MODULE_REGION_FULL),
  which distributions typically seelct. This is also selected by
  CONFIG_ARM64_ERRATUM_843419.

* For modules when CONFIG_ARM64_MODULE_PLTS=n, this would result in
  internal ftrace failures.

* For core kernel text, this would result in internal ftrace failues.

  Note that for this to happen, the kernel text would need to be at
  least 128M bytes in size, and typical configurations are smaller tha
  this.

Fix this by calculating the offset *from* the branch *to* the target in
both functions.

Fixes: f8af0b36 ("arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()")
Fixes: e71a4e1b ("arm64: ftrace: add support for far branches to dynamic ftrace")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Tested-by: "Ivan T. Ivanov" <iivanov@suse.de>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220614080944.1349146-2-mark.rutland@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>