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Date: Tue, 28 Feb 2023 02:42:39 +0300
From: Alexey Izbyshev <>
Subject: Re: [PATCH] accept4: don't fall back to accept if we got
 unknown flags

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.


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