2006/11/10, Bruce Evans <[EMAIL PROTECTED]>:
On Fri, 10 Nov 2006, MQ wrote:
> 2006/11/5, Bruce Evans <[EMAIL PROTECTED]>:
>>
>> On Sat, 4 Nov 2006, Brooks Davis wrote:
>>
>> > On Sat, Nov 04, 2006 at 02:46:30AM +0000, MQ wrote:
>> >> 2006/11/3, Brooks Davis <[EMAIL PROTECTED]>:
>>
>> >>> The particular definition used is excedingly ugly. At a minimum
there
>> >>> needs to be a define or a constant "16" for the lenght rather than
the
>> >>> 4*sizeof("123") nonsense.
>>
>> The `4*sizeof "123"' is not nonsense. It is better than the userland
>> version
>> at the time it was committed. The userland version hard-coded the size
as
>> 18 (sic). The current userland version still hard-codes 18, but now
>> actually needs it to print an error message of length 17. The uglyness
in
>> `4*sizeof "123"' is just that it has 3 formatting style bugs (missing
>> ...
>> > I'd just use 16. The inet_ntoa function is frankly inane. It
attempts
>> > to support chars that aren't 8 bits which would break so much stuff
it
>> > isn't funny.
>>
>> No, it assumes 8-bit chars. It's masking with 0xff is apparently
copied
>> from an old implementation that used plain chars. The userland
>> implementation at the time it was committed does that, but uses a macro
>> to do the masking and is missing lots of style bugs.
>>
>> The userland version now calls inet_ntop(). This is missing the design
>> bug of using a static buffer. It calls inet_ntop4() for the ipv4 case.
>> This is closer to being non-ugly:
>>
>> % static const char *
>> % inet_ntop4(const u_char *src, char *dst, socklen_t size)
>> % {
>> % static const char fmt[] = "%u.%u.%u.%u";
>> % char tmp[sizeof "255.255.255.255"];
>> % int l;
>> %
>> % l = snprintf(tmp, sizeof(tmp), fmt, src[0], src[1], src[2],
>> src[3]);
>> % if (l <= 0 || (socklen_t) l >= size) {
>> % errno = ENOSPC;
>> % return (NULL);
>> % }
>> % strlcpy(dst, tmp, size);
>> % return (dst);
>> % }
>>
>> I would write this as:
>>
>> %%%
>> CTASSERT(CHAR_BIT == 8); /* else src would be misintepreted */
>>
>> static const char *
>> inet_ntop4(const u_char *src, char *dst, socklen_t size)
>> {
>> int n;
>>
>> n = snprintf(dst, size, "%u.%u.%u.%u", src[0], src[1], src[2],
>> src[3]);
>> assert(n >= 0); /* CHAR_BIT == 8 imples 0 < n <= 16 */
>> ...
> I don't know why the ret array in the userland version of the inet_ntoa
> should be 17. The length of the message itself is 17, and an \0 is
needed
> for the str* to work.
Yes, it needs to be 18 for the error message. I wrote "18 (sic)" because
18 is a surprising value. Anyone who knows what an inet address is would
expect a length of 16. But programmers shouldn't be counting bytes in
strings and mentally computing max(16, 18) and hard-coding that. The
magic 16 won't change, but the 18 might. Spelling 16/18 as a macro
and hard-coding 16/18 in the macro would be even worse, since the value is
only used onece.
> By the way, 4 * sizeof("123") chars should be always enough to contain
an
> IPv4 address, no matter how many bits consititute a char.
It's enough for an ipv4 address, but inet_ntop4() is a library routine
so it shouldn't crash on invalid input. With 8-bit chars, it happens
that there is no invalid input for u_char *src (except a src array with
less than 4 chars in it). With >= 10-bit chars, the result could be
"1023.1023.1023.1023", which isn't an ipv4 address and is too large
for a 16-18 byte buffer. inet_ntop4() needs to ensure that this and
some other errors don't occur. It uses mainly snprintf(), but with
snprintf() another set of errors and out-of-bounds values can occur
in theory, and inet_ntop4()'s handling of these is not quite right.
Bruce
I see. Though inet_ntoa in the userland will never result in a buffer
overflow, it may return a invalid result to the caller. The inet_ntoa in the
kernel use an and with 0xff to ensure the reuslt is always a IPv4 Address,
so in this aspect, it's better than that in the userland. I really agree
with you that the use of a hard-coded macro is a bad idea. It's still
confusing and less portable than that calculated by the compiler.
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