Messages in this thread | | | From | Nikolaus Rath <> | Subject | Re: commit d7afaec0b564f0609e116f5: fuse: add FUSE_NO_OPEN_SUPPORT flag to INIT | Date | Fri, 11 Nov 2016 08:28:44 -0800 |
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On Nov 11 2016, Miklos Szeredi <mszeredi@redhat.com> wrote: > On Fri, Nov 11, 2016 at 5:57 AM, Nikolaus Rath <Nikolaus@rath.org> wrote: >> On Nov 11 2016, Miklos Szeredi <mszeredi@redhat.com> wrote: >>> On Thu, Nov 10, 2016 at 11:31 PM, Nikolaus Rath <Nikolaus@rath.org> wrote: >>>> Hi Andrew, >>>> >>>> In commit d7afaec0b564f0609e116f5 you added a new FUSE_NO_OPEN_SUPPORT >>>> flag. But as far as I can tell, the flag is simply accepted without >>>> having any effect (including in libfuse). >>>> >>>> I tried to find related later commits, but did not find anything either. >>>> >>>> Am I missing something? >>> >>> Hmm, if fuse fs detects this flag, then it can return ENOSYS from open >>> resulting in this and subsequent opens succeeding without further >>> calls to userspace. If fuse fs doesn't detect this flag, it should >>> not return -ENOSYS, as that will result in the open failing, it should >>> instead implement a no-op open method. >> >> That doesn't sound like a good approach to me. That way, the file system >> has to *know* that this flag has been introduced in order to behave >> correctly, i.e. filesystems that predate the introduction of the flag >> will suddenly behave differently. >> >> I think the correct behavior would be to for the kernel to check if >> userspace passed the flag, and treat ENOSYS specially if and only if the >> flag was passed. > > ENOSYS is not a valid return value for any existing syscall. Fuse > uses that fact to attach this special meaning to ENOSYS. So > compatibility is not an issue here, old filesystems should never > return ENOSYS from open.
Oh, I was under the impression that any FUSE handler is free to return any error it wants. Where does one have to look to determine which return values are valid?
Best, -Nikolaus
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