Re: [PATCH v2] arc4random: simplify design for better safety

[Florian Weimer <fweimer@xxxxxxxxxx>]

* Jason A. Donenfeld:

> Not in current kernels, where it always returns at least PAGE_SIZE bytes
> before checking for pending signals. In older kernels, if there was a
> signal pending at the top, it would do no work and return -ERESTARTSYS,
> which I believe should then get restarted by glibc's syscaller?

glibc does not handle ERESTARTSYS, it's a kernel-internal error code
that's not exported in UAPI headers and must not leak to userspace
(except perhaps via ptrace).  I believe restarts are handled in the
kernel signal code, by tweaking the program counter.  Looking at that,
ERESTARTSYS gets translated to EINTR for !SA_RESTART system calls:

        /* Are we from a system call? */
        if (syscall_get_nr(current, regs) != -1) {
                /* If so, check system call restarting.. */
                switch (syscall_get_error(current, regs)) {
                case -ERESTART_RESTARTBLOCK:
                case -ERESTARTNOHAND:
                        regs->ax = -EINTR;
                        break;

                case -ERESTARTSYS:
                        if (!(ksig->ka.sa.sa_flags & SA_RESTART)) {
                                regs->ax = -EINTR;
                                break;
                        }
                        fallthrough;
                case -ERESTARTNOINTR:
                        regs->ax = regs->orig_ax;
                        regs->ip -= 2;
                        break;
                }
        }

(arch/x86/kernel/signal.c)

Thanks,
Florian