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List:       busybox
Subject:    Re: [PATCH] makedevs: make special node creation idempotent
From:       Denys Vlasenko <vda.linux () googlemail ! com>
Date:       2016-11-27 22:29:47
Message-ID: CAK1hOcML774_xzX0Tynu0hofdjRLYxea0jGfPfdYd4e7RJxHAQ () mail ! gmail ! com
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On Sat, Nov 5, 2016 at 2:37 PM, Arnout Vandecappelle (Essensium/Mind)
<arnout@mind.be> wrote:
> When makedevs is called for a second time with the same device file, it
> will fail because the files already exist and mknod() gives -EEXISTS.
>
> To avoid this, check if the file already exists and if so, if it is of
> the right type. The chmod and chown changes are still done.

Another applet, mdev, deals with this problem by simply checking EEXIST
and if it happens, ignore it.

Sure, this is suboptimal (this way the existing object may be not a device
node at all), but usually only root can create entries in /dev,
so in practice this is okayish.

And it's small:

function                                             old     new   delta
makedevs_main                                        246     256     +10

> function                                             old     new   delta
> makedevs_main                                       1086    1201    +115
> .rodata                                           138731  138832    +101
> ------------------------------------------------------------------------------
> (add/remove: 0/0 grow/shrink: 2/0 up/down: 216/0)             Total: 216 bytes
>    text    data     bss     dec     hex filename
>  856021   15958    1968  873947   d55db busybox_old
>  856237   15958    1968  874163   d56b3 busybox_unstripped

I fixed it in git by adding this errno check:

mknod(...) != 0 && errno != EEXIST
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