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Date: Tue, 28 Feb 2023 12:25:03 -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 12:21:01PM -0500, Rich Felker wrote:
> On Tue, Feb 28, 2023 at 02:42:39AM +0300, 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.
> > 
> > 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.
> The dup3 situation is even worse than you thought. The dup3 syscall is
> only attempted if O_CLOEXEC is set in flags. If not, the rest of flags
> are ignored and the dup2 syscall is made. I'll make a fix.

These should fix both..

View attachment "0001-fix-pipe2-silently-ignoring-unknown-flags-on-old-ker.patch" of type "text/plain" (966 bytes)

View attachment "0002-fix-dup3-ignoring-all-flags-but-O_CLOEXEC-on-archs-w.patch" of type "text/plain" (1382 bytes)

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