Re: [PATCH v4 4/7] powerpc/kprobes: Use safer string functions in kprobe_lookup_name()

[Paul Clarke <pc@us.ibm.com>]

Sent too soon.  The suggestions don't guarantee null termination.  Refined, below. (Sorry for the noise.)

On 04/21/2017 08:33 AM, Paul Clarke wrote:
> On 04/21/2017 07:33 AM, Naveen N. Rao wrote:
>> Convert usage of strchr()/strncpy()/strncat() to
>> strnchr()/memcpy()/strlcat() for simpler and safer string manipulation.

>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>> index 97b5eed1f76d..c73fb6e3b43f 100644
>> --- a/arch/powerpc/kernel/kprobes.c
>> +++ b/arch/powerpc/kernel/kprobes.c
>> @@ -65,28 +65,27 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>>  	char dot_name[MODULE_NAME_LEN + 1 + KSYM_NAME_LEN];
>>  	const char *modsym;
>>  	bool dot_appended = false;
>> -	if ((modsym = strchr(name, ':')) != NULL) {
>> +	if ((modsym = strnchr(name, ':', MODULE_NAME_LEN)) != NULL) {
>>  		modsym++;
>>  		if (*modsym != '\0' && *modsym != '.') {
>>  			/* Convert to <module:.symbol> */
>> -			strncpy(dot_name, name, modsym - name);
>> +			memcpy(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);
>> +			strlcat(dot_name, modsym, sizeof(dot_name));
> 
> Would it be more efficient here to replace this:
> --
> dot_name[modsym - name + 1] = '\0';
> strlcat(dot_name, modsym, sizeof(dot_name));
> --
> 
> with this:
> strncpy(&dot_name[modsym - name + 1], modsym, KSYM_NAME_LEN);

keep the null termination, and just adjust the starting point for strlcat:
--
dot_name[modsym - name + 1] = '\0';
strlcat(&dot_name[modsym - name + 1], modsym, KSYM_NAME_LEN);
--

> (So you aren't rescanning dot_name to find the end, when you already know the end position?)
> 
>>  			dot_appended = true;
>>  		} else {
>>  			dot_name[0] = '\0';
>> -			strncat(dot_name, name, sizeof(dot_name) - 1);
>> +			strlcat(dot_name, name, sizeof(dot_name));
> 
> and here do:
> strncpy(dot_name, name, sizeof(dot_name));
> 
> (and remove the null termination immediately above)

nevermind.  :-)

> 
>>  		}
>>  	} else if (name[0] != '.') {
>>  		dot_name[0] = '.';
>>  		dot_name[1] = '\0';
>> -		strncat(dot_name, name, KSYM_NAME_LEN - 2);
>> +		strlcat(dot_name, name, sizeof(dot_name));
> 
> and here do:
> strncpy(&dot_name[1], name, sizeof(dot_name));
> 
> (and remove the null termination immediately above)
> 

nevermind.  :-)

>>  		dot_appended = true;
>>  	} else {
>>  		dot_name[0] = '\0';
>> -		strncat(dot_name, name, KSYM_NAME_LEN - 1);
>> +		strlcat(dot_name, name, sizeof(dot_name));
> 
> and here do:
> strncpy(dot_name, name, sizeof(dot_name));
> 
> (and remove the null termination immediately above)
> 
>>  	}

nevermind.  :-)

PC