From: Daniel Shahaf <d.s@daniel.shahaf.name>
To: Peter Stephenson <p.w.stephenson@ntlworld.com>
Cc: zsh-workers@zsh.org
Subject: Re: Any way to allow clobbering empty files when noclobber is set?
Date: Mon, 8 Jun 2020 03:27:30 +0000 [thread overview]
Message-ID: <20200608032730.614e197c@tarpaulin.shahaf.local2> (raw)
In-Reply-To: <15816a8422ec6889dda0f62bf80b11a88163ea3b.camel@ntlworld.com>
Peter Stephenson wrote on Sun, 07 Jun 2020 18:00 +0100:
> On Sun, 2020-06-07 at 11:55 +0000, Daniel Shahaf wrote:
> > My point here is really just the one I already made in 45976, and
> > wasn't answered there: can't we avoid the close()-then-open() sequence
> > that 45968 does? That one seems to be an _avoidable_ race condition,
> > unlike the above ones.
>
> I think what it boils down to here is either you test using fstat() the
> the file is empty, or you re-open using O_TRUNC, both of which ensure
> the file is empty at that point. Then at some later point, the file
> will be written to. Between the two it's there but empty, which is
> unavoidable.
*nod*
> So my last change, closing and opening with O_TRUNC,
> doesn't really gain anything over leaving it open after checking the
> size with fstat(), I don't think, in which case the extra open() is
> redundant and best removed (and that puts us in the fortuitous position
> where we haven't actually added any system calls in adding
> CLOBBER_EMPTY). But I don't think taking it out actually removes a
> race.
The race is:
1. zsh runs «: > existing-empty-file».
2. zsh calls open(O_EXCL) which fails, then open() and fstat(), which both succeeds.
3. Some other process writes data to the file.
4. zsh opens the file with O_TRUNC.
5. bin_true() returns, not having written anything to the file.
6. zsh closes the file.
Step #4 will have deleted the other process's data. If we remove the
close()/open() pair, the step #4 data will survive.
It's an edge case, yes.
Cheers,
Daniel
next prev parent reply other threads:[~2020-06-08 3:28 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CGME20200603020919eucas1p13e26ebcbb335784d14bfb97b137f385a@eucas1p1.samsung.com>
2020-06-03 2:08 ` Martin Tournoij
2020-06-03 12:04 ` Peter Stephenson
2020-06-04 1:48 ` Daniel Shahaf
2020-06-04 2:43 ` Bart Schaefer
2020-06-04 4:06 ` Daniel Shahaf
2020-06-04 5:00 ` Bart Schaefer
2020-06-05 3:10 ` Daniel Shahaf
2020-06-05 3:18 ` Daniel Shahaf
2020-06-06 1:07 ` Bart Schaefer
2020-06-06 4:48 ` Daniel Shahaf
2020-06-06 7:04 ` Bart Schaefer
2020-06-04 6:31 ` Martin Tournoij
2020-06-05 2:22 ` Daniel Shahaf
2020-06-04 2:13 ` Vin Shelton
2020-06-04 2:35 ` Bart Schaefer
2020-06-04 2:36 ` Daniel Shahaf
2020-06-04 11:57 ` Vin Shelton
2020-06-04 5:06 ` Bart Schaefer
2020-06-04 5:41 ` Roman Perepelitsa
2020-06-05 2:07 ` Daniel Shahaf
2020-06-05 4:38 ` Roman Perepelitsa
2020-06-06 1:41 ` Bart Schaefer
2020-06-06 4:55 ` Daniel Shahaf
2020-06-06 6:25 ` Roman Perepelitsa
2020-06-06 7:08 ` Bart Schaefer
2020-06-06 8:03 ` Daniel Shahaf
[not found] ` <1941572212.466119.1591360860372@mail2.virginmedia.com>
[not found] ` <e7f7dfe2-eb4a-457b-85fb-091935a74c0e@www.fastmail.com>
2020-06-06 11:57 ` Peter Stephenson
2020-06-06 12:48 ` Roman Perepelitsa
2020-06-06 15:24 ` Bart Schaefer
2020-06-06 16:24 ` Peter Stephenson
2020-06-07 11:55 ` Daniel Shahaf
2020-06-07 17:00 ` Peter Stephenson
2020-06-08 3:27 ` Daniel Shahaf [this message]
2020-06-08 9:30 ` Peter Stephenson
2020-06-07 17:20 ` Bart Schaefer
2020-06-06 15:09 ` Bart Schaefer
2020-06-04 6:47 ` Martin Tournoij
2020-06-04 9:42 ` Peter Stephenson
2020-06-04 12:20 ` Roman Perepelitsa
2020-06-04 12:26 ` Peter Stephenson
2020-06-04 12:15 ` Peter Stephenson
2020-06-04 20:35 ` Bart Schaefer
2020-06-05 1:59 ` Daniel Shahaf
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