[PATCH 3/6] uprobes/x86: Accept more NOP forms

[Peter Zijlstra]

Instead of only accepting the x86_64 nop5 chosen by the kernel, accept
any x86_64 NOP or NOPL instruction that is 5 bytes.

Notably, the x86_64 nop5 pattern is valid in 32bit apps and could get
compiler generated when build for i686 (which introduced NOPL). Since
the trampoline is x86_64 only, make sure to limit to x86_64 code.

Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 arch/x86/kernel/uprobes.c |   37 ++++++++++++++++++++++++++++++++-----
 1 file changed, 32 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -1157,10 +1157,37 @@ void arch_uprobe_optimize(struct arch_up
        mmap_write_unlock(mm);
 }
 
-static bool can_optimize(struct arch_uprobe *auprobe, unsigned long vaddr)
+static bool insn_is_nop(struct insn *insn)
 {
-       if (memcmp(&auprobe->insn, x86_nops[5], 5))
+       return insn->opcode.nbytes == 1 && insn->opcode.bytes[0] == 0x90;
+}
+
+static bool insn_is_nopl(struct insn *insn)
+{
+       if (insn->opcode.nbytes != 2)
+               return false;
+
+       if (insn->opcode.bytes[0] != 0x0f || insn->opcode.bytes[1] != 0x1f)
+               return false;
+
+       if (!insn->modrm.nbytes)
+               return false;
+
+       if (X86_MODRM_REG(insn->modrm.bytes[0]) != 0)
+               return false;
+
+       /* 0f 1f /0 - NOPL */
+       return true;
+}
+
+static bool can_optimize(struct insn *insn, unsigned long vaddr)
+{
+       if (!insn->x86_64 || insn->length != 5)
                return false;
+
+       if (!insn_is_nop(insn) && !insn_is_nopl(insn))
+               return false;
+
        /* We can't do cross page atomic writes yet. */
        return PAGE_SIZE - (vaddr & ~PAGE_MASK) >= 5;
 }
@@ -1177,7 +1204,7 @@ static void riprel_pre_xol(struct arch_u
 static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
 {
 }
-static bool can_optimize(struct arch_uprobe *auprobe, unsigned long vaddr)
+static bool can_optimize(struct insn *insn, unsigned long vaddr)
 {
        return false;
 }
@@ -1539,15 +1566,15 @@ static int push_setup_xol_ops(struct arc
  */
 int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct 
*mm, unsigned long addr)
 {
-       struct insn insn;
        u8 fix_ip_or_call = UPROBE_FIX_IP;
+       struct insn insn;
        int ret;
 
        ret = uprobe_init_insn(auprobe, &insn, is_64bit_mm(mm));
        if (ret)
                return ret;
 
-       if (can_optimize(auprobe, addr))
+       if (can_optimize(&insn, addr))
                set_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags);
 
        ret = branch_setup_xol_ops(auprobe, &insn);