On Fri, Dec 18, 2015 at 07:39:19PM -0800, Nicolas S. Dade wrote: > This supercedes the previous pselect patch from 16 Dec 2015. > > Linux has a pselect syscall since 2.6.something. Using it > rather than emulating it with sigprocmask+select+sigprocmask > is smaller code, and works properly. (The emulation has > race conditions when unblocked signals arrive before or > after the select) > > The tv.nsec >= 1E9 handling comes from uclibc's linux select() > implementation, which itself uses pselect() internally if the > pselect syscall exists. I though it would be good to do the > same here. > > Note that although the libc pselect() API has 6 arguments, > the linux kernel syscall as 7 arguments. There is an extra, > somewhat vestigial, sizeof the signal mask argument. > > Signed-off-by: Nicolas S. Dade <[email protected]> > --- > libc/sysdeps/linux/common/pselect.c | 52 > +++++++++++++++++++++++++++++++++++++ > 1 file changed, 52 insertions(+) > > diff --git a/libc/sysdeps/linux/common/pselect.c > b/libc/sysdeps/linux/common/pselect.c > index bf19ce3..3f1dd28 100644 > --- a/libc/sysdeps/linux/common/pselect.c > +++ b/libc/sysdeps/linux/common/pselect.c > @@ -30,6 +30,57 @@ static int __NC(pselect)(int nfds, fd_set *readfds, fd_set > *writefds, > fd_set *exceptfds, const struct timespec *timeout, > const sigset_t *sigmask) > { > +#ifdef __NR_pselect6 > +#define NSEC_PER_SEC 1000000000L > + struct timespec _ts, *ts = 0; > + if (timeout) { > + /* The Linux kernel can in some situations update the timeout > value. > + * We do not want that so use a local variable. > + */ > + _ts = *timeout; > + > + /* GNU extension: allow for timespec values where the sub-sec > + * field is equal to or more than 1 second. The kernel will > + * reject this on us, so take care of the time shift ourself. > + * Some applications (like readline and linphone) do this. > + * See 'clarification on select() type calls and invalid > timeouts' > + * on the POSIX general list for more information. > + */ > + if (_ts.tv_nsec >= NSEC_PER_SEC) { > + _ts.tv_sec += _ts.tv_nsec / NSEC_PER_SEC; > + _ts.tv_nsec %= NSEC_PER_SEC; > + } > + > + ts = &_ts; > + } > + > + /* The pselect6 syscall API is strange. It wants a 7th arg to be > + * the sizeof(*sigmask). However syscalls with > 6 arguments aren't > + * supported on linux. So arguments 6 and 7 are stuffed in a struct > + * and a pointer to that struct is passed as the 6th argument to > + * the syscall. > + * Glibc stuffs arguments 6 and 7 in a ulong[2]. Linux reads > + * them as if there were a struct { sigset_t*; size_t } in > + * userspace. There woudl be trouble if userspace and the kernel are > + * compiled differently enough that size_t isn't the same as ulong, > + * but not enough to trigger the compat layer in linux. I can't > + * think of such a case, so I'm using linux's struct. > + * Furthermore Glibc sets the sigsetsize to _NSIG/8. However linux > + * checks for sizeof(sigset_t), which internally is a ulong array. > + * This means that if _NSIG isn't a multiple of BITS_PER_LONG then > + * linux will refuse glibc's value. So I prefer sizeof(sigset_t) for > + * the value of sigsetsize. > + */ > + struct { > + const sigset_t *sigmask; > + size_t sigsetsize; > + } args67 = { > + sigmask, > + sizeof(sigset_t), > + };
This might work if uclibc defines sigset_t not to have the ridiculous expansion space for 1024 signals like glibc does, but it still seems fragile. I'd use _NSIG/8. That's what we do in musl. (If the size you pass to the kernel is wrong you get EINVAL or something like that.) Rich _______________________________________________ uClibc mailing list [email protected] http://lists.busybox.net/mailman/listinfo/uclibc
