Re: [musl] [PATCH] accept4: don't fall back to accept if we got unknown flags

[Rich Felker <dalias@libc.org>]

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.

Rich