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    SubjectRe: [PATCH -tip v14 03/12] kprobes: checks probe address is instruction boudary on x86
    On Thu, Aug 13, 2009 at 04:34:28PM -0400, Masami Hiramatsu wrote:
    > Ensure safeness of inserting kprobes by checking whether the specified
    > address is at the first byte of a instruction on x86.
    > This is done by decoding probed function from its head to the probe point.
    >
    > Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
    > Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
    > Cc: Avi Kivity <avi@redhat.com>
    > Cc: Andi Kleen <ak@linux.intel.com>
    > Cc: Christoph Hellwig <hch@infradead.org>
    > Cc: Frank Ch. Eigler <fche@redhat.com>
    > Cc: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: H. Peter Anvin <hpa@zytor.com>
    > Cc: Ingo Molnar <mingo@elte.hu>
    > Cc: Jason Baron <jbaron@redhat.com>
    > Cc: Jim Keniston <jkenisto@us.ibm.com>
    > Cc: K.Prasad <prasad@linux.vnet.ibm.com>
    > Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
    > Cc: Li Zefan <lizf@cn.fujitsu.com>
    > Cc: Przemysław Pawełczyk <przemyslaw@pawelczyk.it>
    > Cc: Roland McGrath <roland@redhat.com>
    > Cc: Sam Ravnborg <sam@ravnborg.org>
    > Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > Cc: Tom Zanussi <tzanussi@gmail.com>
    > Cc: Vegard Nossum <vegard.nossum@gmail.com>
    > ---
    >
    > arch/x86/kernel/kprobes.c | 69 +++++++++++++++++++++++++++++++++++++++++++++
    > 1 files changed, 69 insertions(+), 0 deletions(-)
    >
    > diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
    > index b5b1848..80d493f 100644
    > --- a/arch/x86/kernel/kprobes.c
    > +++ b/arch/x86/kernel/kprobes.c
    > @@ -48,6 +48,7 @@
    > #include <linux/preempt.h>
    > #include <linux/module.h>
    > #include <linux/kdebug.h>
    > +#include <linux/kallsyms.h>
    >
    > #include <asm/cacheflush.h>
    > #include <asm/desc.h>
    > @@ -55,6 +56,7 @@
    > #include <asm/uaccess.h>
    > #include <asm/alternative.h>
    > #include <asm/debugreg.h>
    > +#include <asm/insn.h>
    >
    > void jprobe_return_end(void);
    >
    > @@ -245,6 +247,71 @@ retry:
    > }
    > }
    >
    > +/* Recover the probed instruction at addr for further analysis. */
    > +static int recover_probed_instruction(kprobe_opcode_t *buf, unsigned long addr)
    > +{
    > + struct kprobe *kp;
    > + kp = get_kprobe((void *)addr);
    > + if (!kp)
    > + return -EINVAL;
    > +
    > + /*
    > + * Basically, kp->ainsn.insn has an original instruction.
    > + * However, RIP-relative instruction can not do single-stepping
    > + * at different place, fix_riprel() tweaks the displacement of
    > + * that instruction. In that case, we can't recover the instruction
    > + * from the kp->ainsn.insn.
    > + *
    > + * On the other hand, kp->opcode has a copy of the first byte of
    > + * the probed instruction, which is overwritten by int3. And
    > + * the instruction at kp->addr is not modified by kprobes except
    > + * for the first byte, we can recover the original instruction
    > + * from it and kp->opcode.
    > + */
    > + memcpy(buf, kp->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
    > + buf[0] = kp->opcode;
    > + return 0;
    > +}
    > +
    > +/* Dummy buffers for kallsyms_lookup */
    > +static char __dummy_buf[KSYM_NAME_LEN];
    > +
    > +/* Check if paddr is at an instruction boundary */
    > +static int __kprobes can_probe(unsigned long paddr)
    > +{
    > + int ret;
    > + unsigned long addr, offset = 0;
    > + struct insn insn;
    > + kprobe_opcode_t buf[MAX_INSN_SIZE];
    > +
    > + if (!kallsyms_lookup(paddr, NULL, &offset, NULL, __dummy_buf))
    > + return 0;
    > +
    > + /* Decode instructions */
    > + addr = paddr - offset;
    > + while (addr < paddr) {
    > + kernel_insn_init(&insn, (void *)addr);
    > + insn_get_opcode(&insn);
    > +
    > + /* Check if the instruction has been modified. */
    > + if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION) {
    > + ret = recover_probed_instruction(buf, addr);



    I'm confused about the reason of this recovering. Is it to remove
    kprobes behind the current setting one in the current function?

    If such cleanup is needed for whatever reason, I wonder what happens
    to the corresponding kprobe structure, why isn't it using the arch_disarm_
    helper to patch back?

    (Questions that may prove my solid misunderstanding of the kprobes code ;-)

    Frederic.

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