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Date: Mon, 27 Feb 2023 18:53:22 -0500
From: Rich Felker <>
Subject: Re: [PATCH] accept4: don't fall back to accept if we got
 unknown flags

On Tue, Feb 28, 2023 at 02:51:42AM +0300, Alexey Izbyshev wrote:
> 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.

Yeah, I was just about to say that. I think that's wrong. The patch
should be something like:

+       if (flg & ~(SOCK_CLOEXEC|SOCK_NONBLOCK)) return __syscall_ret(-EINVAL);

or explicitly assigning to errno.


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