x86: skip singlestep where possible

Check and skip singlestepping underlying instructions where possible.

For now handles single byte as well as few multibyte nop instructions.
However can be extended to other instructions too.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 3f0eb4e66a46352588972b1c47efb4730f724eb3..0d06976000b542723f2897c551c620d8e317f62e 100644 (file)
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -602,3 +602,50 @@ void abort_xol(struct pt_regs *regs, struct uprobe *uprobe)
        handle_riprel_post_xol(uprobe, regs, NULL);
        set_instruction_pointer(regs, utask->vaddr);
 }
+
+/*
+ * Skip these instructions:
+ *
+ * 0f 19 90 90 90 90 90                nopl   -0x6f6f6f70(%rax)
+ * 0f 1f 00                    nopl (%rax)
+ * 0f 1f 40 00                 nopl 0x0(%rax)
+ * 0f 1f 44 00 00              nopl 0x0(%rax,%rax,1)
+ * 0f 1f 80 00 00 00 00                nopl 0x0(%rax)
+ * 0f 1f 84 00 00 00 00                nopl 0x0(%rax,%rax,1)
+ * 66 0f 1f 44 00 00 00                nopw 0x0(%rax,%rax,1)
+ * 66 0f 1f 84 00 00 00                nopw 0x0(%rax,%rax,1)
+ * 66 87 c0                    xchg %eax,%eax
+ * 66 90                       nop
+ * 87 c0                       xchg %eax,%eax
+ * 90                          nop
+ */
+
+bool can_skip_xol(struct pt_regs *regs, struct uprobe *u)
+{
+       int i;
+
+       for (i = 0; i < MAX_UINSN_BYTES; i++) {
+               if ((u->insn[i] == 0x66))
+                       continue;
+
+               if (u->insn[i] == 0x90)
+                       return true;
+
+               if (i == (MAX_UINSN_BYTES - 1))
+                       break;
+
+               if ((u->insn[i] == 0x0f) && (u->insn[i+1] == 0x1f))
+                       return true;
+
+               if ((u->insn[i] == 0x0f) && (u->insn[i+1] == 0x19))
+                       return true;
+
+               if ((u->insn[i] == 0x87) && (u->insn[i+1] == 0xc0))
+                       return true;
+
+               break;
+       }
+
+       u->flags &= ~UPROBES_SKIP_SSTEP;
+       return false;
+}