Reentrant problem with inet_ntoa in the kernel

[Max Laier]

On Thursday 02 November 2006 09:26, . wrote:
> Hi,
>
> I am confused by the use of inet_ntoa function in the kernel.
>
> The function inet_ntoa in the /sys/libkern/inet_ntoa.c uses a static
> array static char buf[4 * sizeof "123"];
> to store the result. And it returns the address of the array to the
> caller.
>
> I think this inet_ntoa is not reentrant, though there are several
> functions calling it. If two functions call it simultaneously, the
> result will be corrupted. Though I haven't really encountered this
> situation, it may occur someday, especially when using
> multi-processors.
>
> There is another reentrant version of inet_ntoa called inet_ntoa_r in
> the same file. It has been there for several years, but just used by
> ipfw2 for about four times in 7-CURRENT. In my patch, I replaced all
> the calls to inet_ntoa with calls to inet_ntoa_r.
>
> By the way, some of the original calls is written in this style:
> strcpy(buf, inet_ntoa(ip))
> The modified code is written in this style
> inet_ntoa_r(ip, buf)
> This change avoids a call to strcpy, and can save a little time.
>
> Here is the patch.
> http://people.freebsd.org/~delphij/misc/patch-itoa-by-nodummy-at-yeah-n
>et
>
> I've already sent to PR(kern/104738), but got no reply, maybe it should
> be discussed here first?

In general, correct IPs in logs and debugging messages are a good thing.  
I'm not sure, however, it is a good thing to put 17 bytes of buffer space 
on every function stack that might want to print an IP address.  I think 
it's less intrusive and equally good to have a hand full of static 
buffers available which are given out in a round-robin fashion - as 
attempted in ip6_sprintf.  Obviously the buffer rotation needs to be 
atomic, though.  If a caller needs the result for more than logging - or 
cares strongly - it can still allocate a private buffer and use the _r 
version.  A general replacement of all applications of inet_ntoa just 
seems bloat.

-- 
/"\  Best regards,                      | mlaier at freebsd.org
\ /  Max Laier                          | ICQ #67774661
 X   http://pf4freebsd.love2party.net/  | mlaier at EFnet
/ \  ASCII Ribbon Campaign              | Against HTML Mail and News
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 187 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-net/attachments/20061102/6e546462/attachment.pgp