[Bug 12236] ash hangs after wait returns EINTR

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=12236

--- Comment #2 from Denys Vlasenko <vda.linux at googlemail.com> ---
Yes, looks like the bug is here:
        /*
         * If we were from a system call, check for system call restarting...
         */
        if (regs->orig_r2 >= 0) {
                continue_addr = regs->ea;
                restart_addr = continue_addr - 4;
                retval = regs->r2;

                /*
                 * Prepare for system call restart. We do this here so that a
                 * debugger will see the already changed PC.
                 */
                switch (retval) {
                case ERESTART_RESTARTBLOCK:
                        restart = -2;
                case ERESTARTNOHAND:
                case ERESTARTSYS:
                case ERESTARTNOINTR:
                        restart++;
                        regs->r2 = regs->orig_r2;
                        regs->r7 = regs->orig_r7;
                        regs->ea = restart_addr;
                        break;
                }
        }

The above code examines r2, the "errno" register on the exit from syscall, but
it DOES NOT check that it is indeed an error! 

linux/arch/nios2/kernel/entry.S says:

        /* Execute the system call */
        callr   r1

        /* If the syscall returns a negative result:
         *   Set r7 to 1 to indicate error,
         *   Negate r2 to get a positive error code
         * If the syscall returns zero or a positive value:
         *   Set r7 to 0.
         * The sigreturn system calls will skip the code below by
         * adding to register ra. To avoid destroying registers
         */
translate_rc_and_ret:
        movi    r1, 0
        bge     r2, zero, 3f
        sub     r2, zero, r2
        movi    r1, 1
3:

IOW: the code in do_signal() should have checked that r7 == 1 before assuming
that r2 == 512 means "ERESTARTSYS".

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