On Tue, Mar 07, 2017 at 04:32:36PM -0800, 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 a SIB byte is used and the base of the SIB byte points to R/EBP (i.e., base = 5) and the mod part of the ModRM byte is zero, the value of such register will not be used as part of the address computation. To signal this, a -EDOM error is returned to indicate callers that they should ignore the value.
Also, for this particular case, a displacement of 32-bits should follow the SIB byte if the mod part of ModRM is equal to zero. The instruction decoder ensures that this is the case.
Cc: Dave Hansen dave.hansen@linux.intel.com Cc: Adam Buchbinder adam.buchbinder@gmail.com Cc: Colin Ian King colin.king@canonical.com Cc: Lorenzo Stoakes lstoakes@gmail.com Cc: Qiaowei Ren qiaowei.ren@intel.com Cc: Peter Zijlstra peterz@infradead.org Cc: Nathan Howard liverlint@gmail.com Cc: Adan Hawthorn adanhawthorn@gmail.com Cc: Joe Perches joe@perches.com Cc: Ravi V. Shankar ravi.v.shankar@intel.com Cc: x86@kernel.org Signed-off-by: Ricardo Neri ricardo.neri-calderon@linux.intel.com
arch/x86/mm/mpx.c | 29 ++++++++++++++++++++++------- 1 file changed, 22 insertions(+), 7 deletions(-)
diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c index d9e92d6..ef7eb67 100644 --- a/arch/x86/mm/mpx.c +++ b/arch/x86/mm/mpx.c @@ -121,6 +121,17 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
case REG_TYPE_BASE: regno = X86_SIB_BASE(insn->sib.value);
/** If mod is 0 and register R/EBP (regno=5) is indicated in the* base part of the SIB byte,
you can simply say here: "if SIB.base == 5, the base of the register-indirect addressing is 0."
the value of such register should
* not be used in the address computation. Also, a 32-bit
Not "Also" but "In this case, a 32-bit displacement..."
* displacement is expected in this case; the instruction* decoder takes care of it. This is true for both R13 and* R/EBP as REX.B will not be decoded.
You don't need that sentence as the only thing that matters is ModRM.mod being 0.
*/if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
The 0 test we normally do with the ! (also flip parts of if-condition):
if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
return -EDOM;- if (X86_REX_B(insn->rex_prefix.value)) regno += 8; break;
@@ -161,16 +172,21 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs) eff_addr = regs_get_register(regs, addr_offset); } else { if (insn->sib.nbytes) {
/** Negative values in the base and index offset means* an error when decoding the SIB byte. Except -EDOM,* which means that the registers should not be used* in the address computation.*/ base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
if (base_offset < 0)
if (unlikely(base_offset == -EDOM))base = 0;else if (unlikely(base_offset < 0))
Bah, unlikely's in something which is not really a hot path. They only encumber readability, no need for them.