I have a tool which downloads photos from a digital camera which appears as a removable USB device.  It includes the following code:

        my $u = $<;
        setuid(0) || die "Cannot change UID from $u to 0";
        system(split(' ','/sbin/modprobe usb-storage'));
        setuid($u) || die "Cannot drop privileges";
        system(split(' ','/sbin/mount /mnt/usb/flash'));

        [do some file manipulation in here]

        chdir ('/');
        system(split(' ','/sbin/umount /mnt/usb/flash'));
        sleep (2);        # this is pure raging paranoia
        setuid(0) || die "Cannot change UID from $u to 0";
        system(split(' ','/sbin/modprobe -r usb-storage'));
        setuid($u) || die "Cannot drop privileges";

[download]

The tool is mode 6775 root:console.  Invariably, the first setuid(0) succeeds, as does the first setuid($u).  Also invariably, the second setuid(0) fails, as in the following example:

babylon5:alaric:~:17 $ getpics
Will copy images from camera canon
/mnt/usb/flash/dcim is mounted
/mnt/usb/flash/dcim/104canon/img_0458.jpg -> /minbar/camera/canon/img_
+0458.jpg
/mnt/usb/flash/dcim/104canon/img_0459.jpg -> /minbar/camera/canon/img_
+0459.jpg
/mnt/usb/flash/dcim/104canon/img_0460.jpg -> /minbar/camera/canon/img_
+0460.jpg
Cannot become root from UID 1000! at /usr/local/bin/getpics line 133.
babylon5:alaric:~:18 $

[download]

Can anyone offer any explanation of why this might happen?

In reply to An odd failure of setuid(0) by Llew_Llaw_Gyffes

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