Generic strlen

Eric Blake
Fri Oct 29 18:14:00 GMT 2010

On 10/29/2010 11:49 AM, David A. Ramos wrote:
> Hi newlib maintainers,
> Our checking tools (KLEE) keeps complaining about newlib's generic strlen version. It looks like it was patched back in May 2008 to include a speed hack that violates ISO C. It attempts to first word align the pointer, and then read a word at a time to check for a NULL:
> libc/string/strlen.c:
> 73  /* If the string is word-aligned, we can check for the presence of                     
> 74     a null in each word-sized block.  */
> 75  aligned_addr = (unsigned long *)str;
> 76  while (!DETECTNULL (*aligned_addr))
> 77    aligned_addr++;
> Obviously, this can read out of bounds if the memory allocated to that string is less than a word in length. While on most architectures this wouldn't actually cause a segfault, I don't think that's a safe assumption for the generic version of a libc routine. The same patch included an i386 target containing the same algorithm, which may be perfectly acceptable.
> Thoughts?

As long as reading beyond the end of a string does not fault, you can't
detect the violation of the standard, so the as-if rule applies.  Prove
to me that there is an architecture that can fault on anything less than
a word boundary, and then we'll talk about changing the code.  Until
then, this implementation may violate strict C89, but it is by all means
portable to all possible platforms that newlib will ever target.  And
while you can indeed detect that you are reading beyond bounds by using
tools such as valgrind, I don't consider that as a valid argument to be
worth changing the code (you should instead be installing valgrind
exceptions to document the fact that we are knowingly reading beyond
bounds, but that the end result of that extraneous read does not affect
program execution because we are not basing any decisions off of that
extra data).

Eric Blake    +1-801-349-2682
Libvirt virtualization library

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 619 bytes
Desc: OpenPGP digital signature
URL: <>

More information about the Newlib mailing list