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Date: Tue, 28 Feb 2023 23:15:06 +0300
From: Alexey Izbyshev <>
Subject: Re: [PATCH] accept4: don't fall back to accept if we got
 unknown flags

On 2023-02-28 20:25, Rich Felker wrote:
> 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.
Indeed, I missed that, thanks.

> These should fix both..

The patches look good to me.

But looking at dup3 more closely, I've noticed another bug: fcntl is 
called even if SYS_dup2 fails. So on kernels where SYS_dup3 is 
unavailable, dup3(-1, fd, O_CLOEXEC) will wrongly try to set FD_CLOEXEC 
on fd.


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