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

[Masami Hiramatsu]

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@xxxxxxxxxx>
>> Acked-by: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
>> Cc: Avi Kivity <avi@xxxxxxxxxx>
>> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
>> Cc: Frank Ch. Eigler <fche@xxxxxxxxxx>
>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxx>
>> Cc: Jason Baron <jbaron@xxxxxxxxxx>
>> Cc: Jim Keniston <jkenisto@xxxxxxxxxx>
>> Cc: K.Prasad <prasad@xxxxxxxxxxxxxxxxxx>
>> Cc: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
>> Cc: PrzemysÅaw PaweÅczyk <przemyslaw@xxxxxxxxxxxx>
>> Cc: Roland McGrath <roland@xxxxxxxxxx>
>> Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
>> Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> Cc: Tom Zanussi <tzanussi@xxxxxxxxx>
>> Cc: Vegard Nossum <vegard.nossum@xxxxxxxxx>
>> ---
>>
>>  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,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

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