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From:   Florian Weimer <fweimer@redhat.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Mark Harris <mark.hsj@gmail.com>, libc-alpha@sourceware.org,
        linux-crypto@vger.kernel.org
Subject: Re: [PATCH v2] arc4random: simplify design for better safety
References: <20220725225728.824128-1-Jason@zx2c4.com>
        <20220725232810.843433-1-Jason@zx2c4.com>
        <CAMdZqKH=9mDhoW_gpL-pUEQAGuN=orc1doudyAuHdoPc7O53RQ@mail.gmail.com>
        <Yt/EySjdJjYW/EcB@zx2c4.com>
Date:   Tue, 26 Jul 2022 13:06:31 +0200
In-Reply-To: <Yt/EySjdJjYW/EcB@zx2c4.com> (Jason A. Donenfeld's message of
        "Tue, 26 Jul 2022 12:41:13 +0200")
Message-ID: <87bktci154.fsf@oldenburg.str.redhat.com>
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* 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