Re: [PATCH] linux-user: Fix pipe() vs. pipe2() usage for ALPHA, MIPS, SH4 and SPARC

[Helge Deller]

On 7/18/22 16:33, Peter Maydell wrote:
> On Mon, 18 Jul 2022 at 15:21, Helge Deller <deller@gmx.de> wrote:
>> On 7/18/22 14:51, Peter Maydell wrote:
>>> Why do we need to do this?
>>
>> Yep, we don't *need* to...
>>
>>> If the flags argument is 0,
>>> then pipe2() is the same as pipe(), so we can safely
>>> emulate it with the host pipe() call. It is, or at
>>> least was, worth doing that, so that guests that use
>>> pipe2(fds, 0) can still run on hosts that don't implement
>>> the pipe2 syscall.
>>
>> True, but only for pipe2(fds,0), not e.g. for pipe2(fds,1).
>> On the other side if a guest explicitly calls pipe2()
>> and if it isn't available, then IMHO -ENOSYS should be returned.
>> Let's assume userspace checks in configure/make scripts if pipe2()
>> is available and succeeds due to pipe2(fds,0), it will assume pipe2()
>> is generally available which isn't necessarily true.
>
> Fair point. Did you run into this in practice, or is it just a
> theoretical concern ?
>
> NB that any probing code that does that will also get the wrong
> answer on musl libc, though, because musl's pipe2() implementation
> always calls pipe() for a zero-flags call:
> https://git.musl-libc.org/cgit/musl/tree/src/unistd/pipe2.c
>
>>> There's probably a reasonable argument to be made that we don't
>>> care any more about hosts so old they don't have pipe2 and that
>>> we could just dump do_pipe2() and the CONFIG_PIPE2 check, and
>>> have do_pipe() unconditionally call pipe2(). Would just need
>>> somebody to check what kernel/glibc versions pipe2() came in.
>>
>> Yes.
>
> I just had a look, and the pipe2 syscall came in in Linux 2.6.27.
> musl has implemented pipe2() since at least 2013, and glibc must
> have had it for at least that long.
>
> In fact current glibc always implements pipe() in terms of pipe2():
> https://sourceware.org/git/?p=glibc.git;a=commit;h=efc6b2dbc47231dee7a7ac39beec808deb4e4d1f
>
> So my preference would be that we should just say "we can assume
> that pipe2 is always available and implemented on linux hosts" and
> remove the code that handles the possibility that it isn't.

Ok for me.
Do you want me to send a patch or will you do?

Helge