From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-1.1 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_MSPIKE_H2 autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 24312 invoked from network); 27 Feb 2023 23:42:59 -0000 Received: from second.openwall.net (193.110.157.125) by inbox.vuxu.org with ESMTPUTF8; 27 Feb 2023 23:42:59 -0000 Received: (qmail 8174 invoked by uid 550); 27 Feb 2023 23:42:55 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 8139 invoked from network); 27 Feb 2023 23:42:54 -0000 DKIM-Filter: OpenDKIM Filter v2.11.0 mail.ispras.ru 755AF44C1001 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ispras.ru; s=default; t=1677541359; bh=aPdIe+t805YL1uvW6fL9+zNwpz62dXiWDLzsO3F3Q8k=; h=Date:From:To:Subject:Reply-To:In-Reply-To:References:From; b=Kj9zM5Ynjjob6E6qMzDk18t7llngCWM/L9cX10UpmOjhr5GiizcFQbI5s0namZb7M jPcrjPHU2uSUHsLrrQMQb+U7hhfOCKGHdn4eHSKuGgmi6mSjmdxS/f4CPLCYcGisCE bKMjCLS1c9aGMSrx1B6tc37tAFKK3scUK41VZUxo= MIME-Version: 1.0 Date: Tue, 28 Feb 2023 02:42:39 +0300 From: Alexey Izbyshev To: musl@lists.openwall.com Mail-Followup-To: musl@lists.openwall.com In-Reply-To: <20230227223822.GH4163@brightrain.aerifal.cx> References: <20230227194654.61114-1-izbyshev@ispras.ru> <20230227223822.GH4163@brightrain.aerifal.cx> User-Agent: Roundcube Webmail/1.4.4 Message-ID: <334e77d7dead0bfc41ba8a9eda6fdb0b@ispras.ru> X-Sender: izbyshev@ispras.ru Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [musl] [PATCH] accept4: don't fall back to accept if we got unknown flags On 2023-02-28 01:38, Rich Felker wrote: > On Mon, Feb 27, 2023 at 10:46:54PM +0300, Alexey Izbyshev wrote: >> accept4 emulation via accept ignores unknown flags, so it can >> spuriously >> succeed instead of failing (or succeed without doing the action >> implied >> by an unknown flag if it's added in a future kernel). Worse, unknown >> flags trigger the fallback code even on modern kernels if the real >> accept4 syscall returns EINVAL, because this is indistinguishable from >> socketcall returning EINVAL due to lack of accept4 support. Fix this >> by >> always propagating the syscall attempt failure if unknown flags are >> present. >> >> The behavior is still not ideal on old kernels lacking accept4 on >> arches >> with socketcall, where failing with ENOSYS instead of EINVAL returned >> by >> socketcall would be preferable, but at least modern kernels are now >> fine. > > Can you clarify what you mean about ENOSYS vs EINVAL here? I'm not > following. > Sorry for confusion, I meant the following. On arches with socketcall, if a program running on an old kernel that doesn't support accept4 in any form calls accept4 with unknown flags, musl's accept4 will fail with EINVAL after this patch. But the reason of failure remains unclear to the programmer: is it because some flag is not supported or because accept4 is not supported at all? So I thought it'd be better to fail with ENOSYS in this case instead, although I don't know a good way to do that: the EINVAL ambiguity exists at socketcall level too, so testing whether the kernel's socketcall supports __SC_accept4 or not would probably involve calling it with known-good arguments on a separately created socket, and I certainly don't propose to do that. On the other hand, it could be argued that a function that can emulate a certain baseline feature set of another function shouldn't fail with ENOSYS at all because the real function would never do that. The two cleanest options for possibly-not-supported functions seem to be either always failing with ENOSYS if the kernel doesn't support the syscall or failing with a reasonable error if the caller requests something unsupported by the emulation. And I think accept4 satisfies the latter with this patch. As an aside, note that dup3 and pipe2 currently also ignore unknown flags on old kernels, and for pipe2 there is a valid flag (O_DIRECT) that could be silently ignored because of that. But there is no issue on newer kernels supporting the syscalls, unlike for accept4. Alexey