This is a regression caused by valgrind svn r15823 which fixed Bug 359871 "sanitize signal mask in ppoll and pselect syscalls".

The issue is caused by this C library/kernel ABI differences according to http://man7.org/linux/man-pages/man2/pselect6.2.html

       The Linux pselect6() system call modifies its timeout argument.
       However, the glibc wrapper function hides this behavior by using a
       local variable for the timeout argument that is passed to the system
       call.  Thus, the glibc pselect() function does not modify its timeout
       argument; this is the behavior required by POSIX.1-2001.

       The final argument of the pselect6() system call is not a sigset_t *
       pointer, but is instead a structure of the form:

           struct {
               const sigset_t *ss;     /* Pointer to signal set */
               size_t          ss_len; /* Size (in bytes) of object pointed
                                          to by 'ss' */
           };

       This allows the system call to obtain both a pointer to the signal
       set and its size, while allowing for the fact that most architectures
       support a maximum of 6 arguments to a system call.

What we are seeing is glibc modifying the timeout argument (NULL) and passing it as a struct { NULL, 8 } (where 8 is the correct ss_len if ss wouldn't be NULL).

valgrind doesn't check whether ss is NULL before calling PRE_MEM_READ on it and so generates a bogus warning.

Reproducible: Always