mailing list of musl libc
 help / color / mirror / code / Atom feed
From: Alexey Izbyshev <>
Subject: Re: [musl] [PATCH] accept4: don't fall back to accept if we got unknown flags
Date: Tue, 28 Feb 2023 02:51:42 +0300	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 2023-02-28 02:42, Alexey Izbyshev wrote:
> 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.

But actually it doesn't. The error should always be EINVAL in case of 
unknown flags for that, but the patch propagates ENOSYS on arches 
without socketcall.


  reply	other threads:[~2023-02-27 23:51 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-27 19:46 Alexey Izbyshev
2023-02-27 22:38 ` Rich Felker
2023-02-27 23:42   ` Alexey Izbyshev
2023-02-27 23:51     ` Alexey Izbyshev [this message]
2023-02-27 23:53       ` Rich Felker
2023-02-28 17:21     ` Rich Felker
2023-02-28 17:25       ` Rich Felker
2023-02-28 20:15         ` Alexey Izbyshev
2023-02-28 20:51           ` Rich Felker

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).