[PATCH] powerpc/kprobes: refactor kprobe_lookup_name for safer string operations

[Naveen N. Rao]

Excerpts from Masami Hiramatsu's message of April 26, 2017 10:11:
> On Tue, 25 Apr 2017 21:37:11 +0530
> "Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> wrote:
> 
>> Use safer string manipulation functions when dealing with a
>> user-provided string in kprobe_lookup_name().
>> 
>> Reported-by: David Laight <David.Laight at ACULAB.COM>
>> Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/kernel/kprobes.c | 47 ++++++++++++++++++-------------------------
>>  1 file changed, 20 insertions(+), 27 deletions(-)
>> 
>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>> index 160ae0fa7d0d..5a17e6467be9 100644
>> --- a/arch/powerpc/kernel/kprobes.c
>> +++ b/arch/powerpc/kernel/kprobes.c
>> @@ -53,7 +53,7 @@ bool arch_within_kprobe_blacklist(unsigned long addr)
>>  
>>  kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>>  {
>> -	kprobe_opcode_t *addr;
>> +	kprobe_opcode_t *addr = NULL;
>>  
>>  #ifdef PPC64_ELF_ABI_v2
>>  	/* PPC64 ABIv2 needs local entry point */
>> @@ -85,36 +85,29 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>>  	 * Also handle <module:symbol> format.
>>  	 */
>>  	char dot_name[MODULE_NAME_LEN + 1 + KSYM_NAME_LEN];
>> -	const char *modsym;
>>  	bool dot_appended = false;
>> -	if ((modsym = strchr(name, ':')) != NULL) {
>> -		modsym++;
>> -		if (*modsym != '\0' && *modsym != '.') {
>> -			/* Convert to <module:.symbol> */
>> -			strncpy(dot_name, name, modsym - name);
>> -			dot_name[modsym - name] = '.';
>> -			dot_name[modsym - name + 1] = '\0';
>> -			strncat(dot_name, modsym,
>> -				sizeof(dot_name) - (modsym - name) - 2);
>> -			dot_appended = true;
>> -		} else {
>> -			dot_name[0] = '\0';
>> -			strncat(dot_name, name, sizeof(dot_name) - 1);
>> -		}
>> -	} else if (name[0] != '.') {
>> -		dot_name[0] = '.';
>> -		dot_name[1] = '\0';
>> -		strncat(dot_name, name, KSYM_NAME_LEN - 2);
>> +	const char *c;
>> +	ssize_t ret = 0;
>> +	int len = 0;
>> +
>> +	if ((c = strnchr(name, MODULE_NAME_LEN, ':')) != NULL) {
>> +		c++;
>> +		len = c - name;
>> +		memcpy(dot_name, name, len);
>> +	} else
>> +		c = name;
>> +
>> +	if (*c != '\0' && *c != '.') {
>> +		dot_name[len++] = '.';
>>  		dot_appended = true;
>> -	} else {
>> -		dot_name[0] = '\0';
>> -		strncat(dot_name, name, KSYM_NAME_LEN - 1);
>>  	}
>> -	addr = (kprobe_opcode_t *)kallsyms_lookup_name(dot_name);
>> -	if (!addr && dot_appended) {
>> -		/* Let's try the original non-dot symbol lookup	*/
>> +	ret = strscpy(dot_name + len, c, KSYM_NAME_LEN);
>> +	if (ret >= 0)
> 
> Here, maybe you can skip the case of ret == 0. (Or, would we have
> a symbol which only has "."?)

Ah, indeed. Good point. We just need the test to be (ret > 0).

Michael,
If the rest of the patch is fine by you, would you be ok to make the 
small change above? If not, please let me know and I'll re-spin. Thanks.

> 
> Others look good to me.

Thanks for the review,
- Naveen