On 7/18/22 16:33, Peter Maydell wrote: > On Mon, 18 Jul 2022 at 15:21, Helge Deller <del...@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
