Date: Sun, 11 Aug 2019 22:19:57 +0000 From: Rick Macklem <rmacklem@uoguelph.ca> To: Ian Lepore <ian@freebsd.org>, Alan Somers <asomers@freebsd.org> Cc: "gljennjohn@gmail.com" <gljennjohn@gmail.com>, "freebsd-current@FreeBSD.org" <freebsd-current@FreeBSD.org> Subject: Re: RFC: should lseek(SEEK_DATA/SEEK_HOLE) return ENOTTY? Message-ID: <YTBPR01MB3616E699F9A7298BA0A3D6AFDDD00@YTBPR01MB3616.CANPRD01.PROD.OUTLOOK.COM> In-Reply-To: <c4472ee6c3f0602c616be594158c5f658129a24a.camel@freebsd.org> References: <YTBPR01MB3616B6F068199B6A3329432CDDD00@YTBPR01MB3616.CANPRD01.PROD.OUTLOOK.COM> <20190811090405.50cc49b1@ernst.home> <fe075daa384006c2056bb844cbccb6454c56fc3b.camel@freebsd.org> <CAOtMX2g=6cQdBpD2c7wS1ZaVFz08TDz-uauvCh_sS3VynLkR0g@mail.gmail.com>, <c4472ee6c3f0602c616be594158c5f658129a24a.camel@freebsd.org>
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Ian Lepore wrote: >On Sun, 2019-08-11 at 09:12 -0600, Alan Somers wrote: >> On Sun, Aug 11, 2019 at 8:57 AM Ian Lepore <ian@freebsd.org> wrote: >> > >> > On Sun, 2019-08-11 at 09:04 +0200, Gary Jennejohn wrote: >> > > On Sun, 11 Aug 2019 02:03:10 +0000 >> > > Rick Macklem <rmacklem@uoguelph.ca> wrote: >> > > >> > > > Hi, >> > > > >> > > > I've noticed that, if you do a lseek(SEEK_DATA/SEEK_HOLE) on a >> > > > file >> > > > that >> > > > resides in a file system that does not support holes, ENOTTY is >> > > > returned. >> > > > >> > > > This error isn't listed for lseek() and seems a liitle weird. >> > > > >> > > >> > > ENOTTY is the standard error return for an unimplemented >> > > ioctl(2), >> > > and SEEK_HOLE ultimately becomes a call to fo_ioctl(). That's true and explains why it returns ENOTTY. However, lseek(2) is not io= ctl(2) and it doesn't list ENOTTY as an error. (Just to make things confusing, lseek(2) using SEEK_DATA/SEEK_HOLE appears = to be only a POSIX draft at this point, so POSIX doesn't really help w.r.t. w= hat errors should be returned for this case.) >> > > >> > > > I can see a couple of alternatives to this: >> > > > 1 - Return a different error. Maybe ENXIO? >> > > > or >> > > > 2 - Have lseek() do the trivial implementation when the >> > > > VOP_IOCTL() >> > > > fails. >> > > > - For SEEK_DATA, just return the offset given as argument >> > > > and >> > > > for SEEK_HOLE >> > > > return the file's size as the offset. >> > > > >> > > > What do others think? rick >> > > > ps: The man page should be updated, whatever is done w.r.t. >> > > > this. >> > > > >> > > >> > > I also vote for option 2 >> > > >> > >> > If SEEK_DATA and SEEK_HOLE don't return the standard "ioctl not >> > supported" error code and return a fake result, how are you >> > supposed to >> > determine at runtime whether SEEK_HOLE is supported or not? >> > >> > -- Ian >> >> pathconf(2) will tell you. >> > >Ahh, I wasn't aware of that. > >For option 2, lseek() has to not just return the info, but must also >actually set the file position accordingly, and has to treat offset >=3D >filesize as an error. Yes, this check is done below the VOP_IOCTL() layer for the file system (using vn_bmap_seekhole() or custom code). I think the easiest way to implement #2 is create a vop_stdioctl() and put = it into sys/kern/vfs_default.c. It would need to do this check. Interestingly, I had assumed the discussion would have been between leaving the errno alone vs changing the errno. I only threw in #2 for completeness sake. --> Now, it appears that #2 is the favourite. I'll wait for more responses before I propose a patch. Thanks for the comments, rick
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