Messages in this thread | | | Date | Fri, 14 Apr 2023 03:55:15 +0100 | From | Al Viro <> | Subject | Re: Weirdness with bind mounts and moving files from tmpfs |
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On Thu, Apr 13, 2023 at 05:23:13PM -0700, Richard Cochran wrote:
> So far, so good. Now do the same, but with 'mv' instead of 'cp'... > > # echo three > /run/file > # mv /run/file /home/file > # cat /opt/file > two > > At this point, the contents of /opt/file are stuck forever with "two"... > > # echo three > /run/file > # cp /run/file /home/file > # cat /opt/file > two > # echo three > /home/file > # cat /opt/file > two > > What is going on here? Is this a bug or a feature?
strace and compare the traces... cp(1) opens the existing target, truncates and writes to it; mv(1) tries rename(), falling back to unlink()+creat()+write() in case rename() failed with -EXDEV.
The same behaviour can be seen without any mount(2) use:
; echo x > a ; ln a b ; echo y > c ; cat a x ; cat b x ; cat c y ; cp c a ; cat b y ; echo x > a ; cat a x ; cat b x ; mv c a ; cat a y ; cat b x ;
Exact same underlying cause - writing down what happens step-by-step might be enlightening...
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