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Date: Tue, 28 Feb 2023 02:51:42 +0300
From: Alexey Izbyshev <>
Subject: Re: [PATCH] accept4: don't fall back to accept if we got
 unknown flags

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.


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