The glibc strlen begins by checking the first few bytes, if necessary, so that it can continue with the assumption that the string is 4-byte or 8-byte aligned.

The glibc strlen begins by checking the first few bytes, if necessary, so that it can continue with the assumption that the string is 4-byte or 8-byte aligned. So no, it's not horribly broken: all the reads are aligned, and you're not going to get a page boundary in the middle of the word. (Also, you're not going to be reading off the end of a malloc'ed block, because essentially all malloc implementations, including the one in glibc, return blocks whose start address and size are multiples of the architecture's word size.)