Re: [PATCH -tip v14 03/12] kprobes: checks probe address is instruction boudary on x86

Masami Hiramatsu
2009-08-18T23:17:20Z

Frederic Weisbecker wrote:
> 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?

No, it recovers just an instruction which is probed by a kprobe,
because we need to know the first byte of this instruction for
decoding it.

Perhaps we'd better to have more generic interface (text_peek?)
for it because another subsystem (e.g. kgdb) may want to insert int3...

Thank you,