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When registering new ppp interface via PPPIOCNEWUNIT ioctl then kernel has
to choose interface name as this ioctl API does not support specifying it.
Kernel in this case register new interface with name "ppp<id>" where <id>
is the ppp unit id, which can be obtained via PPPIOCGUNIT ioctl. This
applies also in the case when registering new ppp interface via rtnl
without supplying IFLA_IFNAME.
PPPIOCNEWUNIT ioctl allows to specify own ppp unit id which will kernel
assign to ppp interface, in case this ppp id is not already used by other
ppp interface.
In case user does not specify ppp unit id then kernel choose the first free
ppp unit id. This applies also for case when creating ppp interface via
rtnl method as it does not provide a way for specifying own ppp unit id.
If some network interface (does not have to be ppp) has name "ppp<id>"
with this first free ppp id then PPPIOCNEWUNIT ioctl or rtnl call fails.
And registering new ppp interface is not possible anymore, until interface
which holds conflicting name is renamed. Or when using rtnl method with
custom interface name in IFLA_IFNAME.
As list of allocated / used ppp unit ids is not possible to retrieve from
kernel to userspace, userspace has no idea what happens nor which interface
is doing this conflict.
So change the algorithm how ppp unit id is generated. And choose the first
number which is not neither used as ppp unit id nor in some network
interface with pattern "ppp<id>".
This issue can be simply reproduced by following pppd call when there is no
ppp interface registered and also no interface with name pattern "ppp<id>":
pppd ifname ppp1 +ipv6 noip noauth nolock local nodetach pty "pppd +ipv6 noip noauth nolock local nodetach notty"
Or by creating the one ppp interface (which gets assigned ppp unit id 0),
renaming it to "ppp1" and then trying to create a new ppp interface (which
will always fails as next free ppp unit id is 1, but network interface with
name "ppp1" exists).
This patch fixes above described issue by generating new and new ppp unit
id until some non-conflicting id with network interfaces is generated.
Signed-off-by: Pali Rohár <pali@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.