misc/108211: potentially a bug for inet_aton in sys/netinet/libalias/alias_proxy.c

[Bruce Evans]

The following reply was made to PR kern/108211; it has been noted by GNATS.

From: Bruce Evans <bde at zeta.org.au>
To: Yong Tang <yong.599 at gmail.com>
Cc: freebsd-gnats-submit at FreeBSD.org, freebsd-bugs at FreeBSD.org, 
    green at freebsd.org
Subject: Re: misc/108211: potentially a bug for inet_aton in
 sys/netinet/libalias/alias_proxy.c
Date: Tue, 23 Jan 2007 17:11:27 +1100 (EST)

 On Mon, 22 Jan 2007, Yong Tang wrote:
 
 >> Description:
 > In sys/netinet/libalias/alias_proxy.c,
 > The following code exist.
 >
 > 158 #ifdef  _KERNEL
 > 159 static int
 > 160 inet_aton(cp, addr)
 > 161         const char *cp;
 > 162         struct in_addr *addr;
 > 163 {
 >
 >
 > 180                 l = strtoul(c, &endptr, 0);
 > 181
 > 182                 if (l == ULONG_MAX || l == 0)
 > 183                         return (0);
 >
 > However, if the input cp is "0.0.0.0", then it seems this function will return (0) which is considered as an error.
 
 Bleah, yet another example of how not to check for errors in the strto*
 family.
 
 > The reason is because 180:
 > l = strtoul(c, &endptr, 0);
 > l will return a 0 when the c is "0".
 >
 > Not quite sure if this is done purposely in FreeBSD but I have never experience similiar cases in other unix-platforms.
 >
 > Possible solution:
 >
 > change
 > 182 (l == ULONG_MAX || l == 0)
 >
 > into
 > 182 (l == ULONG_MAX || (l == 0 && (endptr == c))
 
 Why not just use libc's inet_aton()?  Well, there is a problem -- this
 is for the kernel, so inet_aton() must be duplicated, but it can't be
 duplicated literally since the strto* family is fundamentally broken
 in the kernel.  strto*'s API depends on errno for reporting errors
 correctly, and errno doesn't exist in the kernel.  This kernel version
 is a literal duplicate of the libc version except for massive bitrot.
 It is identical to an old libc version (libc/net/inet_addr.c 1.16)
 except for small changes to avoid avoid checking errno and thus implement
 the bug, and minor bitrot in the libc version (const poisoning has
 only been done in the kernel).
 
 The massive bitrot in the current libc version (libc/inet/inet_addr.c
 1.3) is mainly to implement a home made broken version of strtoul()
 inline.  Perhaps strtoul() is too inflexible, but I doubt it.  The
 must obvious bug in the new version is the usual overflow in home made
 implementations of atoi/strto*: "val = val * base + 'c' - '0'" may
 overflow, but there is no overflow checking.  Thus the garbage input
 "4294967297.1.1.1" is now treated as "1.1.1.1" on systems with 32-bit
 unsigned longs although it is detected as being garbage on systems
 with 64-bit unsigned longs.  Some style bugs are also obvious.
 
 Possibly simpler solution for 1 kernel version and 2 userland versions:
 - remove all traces of errno and related bogus checks for (l == ULONG_MAX)
    and (l == 0).  Checking (errno == ERANGE) isn't wrong, but it isn't
    needed since the value will be ULONG_MAX after a range error, and since
    ULONG_MAX is too large for part of a dotted quad address it will be
    detected as an error later.  Checking (l == ULONG_MAX) is wrong in
    general since it might not be a the result of a range error, but here
    the range is smaller so the check isn't wrong.  However, the check is
    unnecessary since all values between 256 and ULONG_MAX inclusive will
    be dected as errors later.  Checking (l == 0) is just a bug since 0
    is within range.
 - remove broken home made strtol() in libc version, and otherwise merge
    versions and fix bitrot.  The "new" libc version is actually not newer,
    but just bitrot back to the vendor version.  FreeBSD fixed "old" version
    to use strtoul() in rev.1.8 in 1999, but the changes were lost when the
    vendor-version was reimported in a different directory in 2006.  FreeBSD
    also made a few fixes to the old version between 1.8 and 1.16.  The
    main one is related: 1.8 showed yet another way of how not to check
    for errors in strto* -- it checked (l == ERANGE) instead of
    (errno == ERANGE).  Some of its other changes are not quite right.
 
 Bruce