The select and pselect6 syscall handlers may read invalid memory

[stevenengler]

Shadow's select and pselect6 syscall handlers read file descriptor sets as fixed-sized fd_sets for the read, write, and err fd lists, but the syscalls should actually read them as a file descriptor sets with a length derived from nfds.

The Linux kernel allows file descriptor sets of arbitrary size, determining the length of the sets to be checked from the value of nfds. However, in the glibc implementation, the fd_set type is fixed in size.

If the application uses a buffer smaller than the size of fd_set, shadow will read invalid memory. Most applications use glibc so this is unlikely to be an issue in most cases.