On Fri, Aug 18, 2017 at 05:27:47PM -0700, Ricardo Neri wrote:
> Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
> Developer's Manual volume 2A states that when ModRM.mod !=11b and
> ModRM.rm = 100b indexed register-indirect addressing is used. In other
> words, a SIB byte follows the ModRM byte. In the specific case of
> SIB.index = 100b, the scale*index portion of the computation of the
> effective address is null. To signal callers of this particular situation,
> get_reg_offset() can return -EDOM (-EINVAL continues to indicate that an
> error when decoding the SIB byte).
>
> An example of this situation can be the following instruction:
>
> 8b 4c 23 80 mov -0x80(%rbx,%riz,1),%rcx
> ModRM: 0x4c [mod:1b][reg:1b][rm:100b]
> SIB: 0x23 [scale:0b][index:100b][base:11b]
> Displacement: 0x80 (1-byte, as per ModRM.mod = 1b)
>
> The %riz 'register' indicates a null index.
>
> In long mode, a REX prefix may be used. When a REX prefix is present,
> REX.X adds a fourth bit to the register selection of SIB.index. This gives
> the ability to refer to all the 16 general purpose registers. When REX.X is
> 1b and SIB.index is 100b, the index is indicated in %r12. In our example,
> this would look like:
>
> 42 8b 4c 23 80 mov -0x80(%rbx,%r12,1),%rcx
> REX: 0x42 [W:0b][R:0b][X:1b][B:0b]
> ModRM: 0x4c [mod:1b][reg:1b][rm:100b]
> SIB: 0x23 [scale:0b][.X: 1b, index:100b][.B:0b, base:11b]
> Displacement: 0x80 (1-byte, as per ModRM.mod = 1b)
>
> %r12 is a valid register to use in the scale*index part of the effective
> address computation.
>
> Cc: Borislav Petkov <b...@suse.de>
> Cc: Andy Lutomirski <l...@kernel.org>
> Cc: Dave Hansen <dave.han...@linux.intel.com>
> Cc: Adam Buchbinder <adam.buchbin...@gmail.com>
> Cc: Colin Ian King <colin.k...@canonical.com>
> Cc: Lorenzo Stoakes <lstoa...@gmail.com>
> Cc: Qiaowei Ren <qiaowei....@intel.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Nathan Howard <liverl...@gmail.com>
> Cc: Adan Hawthorn <adanhawth...@gmail.com>
> Cc: Joe Perches <j...@perches.com>
> Cc: Ravi V. Shankar <ravi.v.shan...@intel.com>
> Cc: x...@kernel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calde...@linux.intel.com>
> ---
> arch/x86/mm/mpx.c | 22 ++++++++++++++++++++--
> 1 file changed, 20 insertions(+), 2 deletions(-)
Reviewed-by: Borislav Petkov <b...@suse.de>
--
Regards/Gruss,
Boris.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284
(AG Nürnberg)
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