Date: Tue, 28 Feb 2023 12:25:03 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com 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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