Re: [PATCH net] ppp: remove SC_MUST_COMP

[Qingfang Deng]

On 10/31/2026 5:32 AM, netdev-bot+sashiko@xxxxxxxxxx wrote:
> [Severity: Medium]
> Does the fix need the define to be removed from the UAPI header?
>
> PPPIOCSFLAGS already masks the user value with the kernel-internal
> SC_FLAG_BITS:
>
> drivers/net/ppp/ppp_generic.c:ppp_ioctl() {
>      ...
> 		ppp->flags = val & SC_FLAG_BITS;
>      ...
> }
>
> Removing the bit from SC_FLAG_BITS (and SC_MASK), together with the two
> checks, looks like enough to stop SC_DC_ERROR from being stored as a
> config flag.
>
> Deleting the define is a separate change at the source level. Userspace
> that includes <linux/ppp-ioctl.h>, directly or through <linux/if_ppp.h>,
> and refers to SC_MUST_COMP would no longer build.
>
> The commit message calls the flag unused, but the evidence is that one
> pppd integration from 2005 never landed upstream. That does not rule out
> out-of-tree pppd MPPE forks or other daemons that use the name.
>
> With the Fixes: tag pointing at a 2005 commit, this header change will
> likely be backported to the stable kernel-headers as well.
>
> Would it work to keep the define, with a comment that the kernel ignores
> it and that it aliases SC_DC_ERROR, while still dropping it from
> SC_FLAG_BITS and SC_MASK and removing the checks?

I found no trace of existing users (searched GitHub). Even _if_ there 
are any, _and_ SC_MUST_COMP does not alias SC_DC_ERROR, the logic is 
still flawed - received packets are silently dropped without noticing 
the userspace.

If the goal is to prevent the removal from backports, I may split out 
the removal part into a net-next patch.