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Re: [PATCH 2/4] MIPS Kprobes: Deny probes on ll/sc instructions

To: "manesoni@cisco.com" <manesoni@cisco.com>
Subject: Re: [PATCH 2/4] MIPS Kprobes: Deny probes on ll/sc instructions
From: David Daney <david.daney@cavium.com>
Date: Tue, 08 Nov 2011 12:01:18 -0800
Cc: Ralf Baechle <ralf@linux-mips.org>, "ananth@in.ibm.com" <ananth@in.ibm.com>, "kamensky@cisco.com" <kamensky@cisco.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
In-reply-to: <20111108170535.GC16526@cisco.com>
References: <20111108170336.GA16526@cisco.com> <20111108170535.GC16526@cisco.com>
Sender: linux-mips-bounce@linux-mips.org
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On 11/08/2011 09:05 AM, Maneesh Soni wrote:

From: Maneesh Soni<manesoni@cisco.com>

Deny probes on ll/sc instructions for MIPS kprobes

As ll/sc instruction are for atomic read-modify-write operations, allowing
probes on top of these insturctions is a bad idea.


s/insturctions/instructions/

Not only is it a bad idea, it will probably make them fail 100% of the time.

It is also an equally bad idea to place a probe between any LL and SC instructions. How do you prevent that?

If you cannot prevent probes between LL and SC, why bother with this at all?

David Daney

Signed-off-by: Victor Kamensky<kamensky@cisco.com>
Signed-off-by: Maneesh Soni<manesoni@cisco.com>
---
  arch/mips/kernel/kprobes.c |   31 +++++++++++++++++++++++++++++++
  1 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/arch/mips/kernel/kprobes.c b/arch/mips/kernel/kprobes.c
index 9fb1876..0ab1a5f 100644
--- a/arch/mips/kernel/kprobes.c
+++ b/arch/mips/kernel/kprobes.c
@@ -113,6 +113,30 @@ insn_ok:
        return 0;
  }

+/*
+ * insn_has_ll_or_sc function checks whether instruction is ll or sc
+ * one; putting breakpoint on top of atomic ll/sc pair is bad idea;
+ * so we need to prevent it and refuse kprobes insertion for such
+ * instructions; cannot do much about breakpoint in the middle of
+ * ll/sc pair; it is upto user to avoid those places
+ */
+static int __kprobes insn_has_ll_or_sc(union mips_instruction insn)
+{
+       int ret = 0;
+
+       switch (insn.i_format.opcode) {
+       case ll_op:
+       case lld_op:
+       case sc_op:
+       case scd_op:
+               ret = 1;
+               break;
+       default:
+               break;
+       }
+       return ret;
+}
+
  int __kprobes arch_prepare_kprobe(struct kprobe *p)
  {
        union mips_instruction insn;
@@ -121,6 +145,13 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)

        insn = p->addr[0];

+       if (insn_has_ll_or_sc(insn)) {
+               pr_notice("Kprobes for ll and sc instructions are not"
+                         "supported\n");
+               ret = -EINVAL;
+               goto out;
+       }
+
        if (insn_has_delayslot(insn)) {
                pr_notice("Kprobes for branch and jump instructions are not"
                          "supported\n");


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