Re: [PATCH net v2] ppp: remove SC_MUST_COMP

[Paolo Abeni]

On 11/3/26 02:56, Qingfang Deng wrote:
> SC_MUST_COMP accidentally shares the same value with SC_DC_ERROR.
> PPPIOCGFLAGS combines configuration flags with transmit and receive
> state, so userspace cannot distinguish them, and a read-modify-write
> sequence can promote a decompression error into SC_MUST_COMP.
>
> With SC_MUST_COMP set, intentional or not, a fatal decompression error
> (SC_DC_FERROR) causes the receive path to silently drop the failed
> packet before delivering it to userspace. pppd therefore cannot detect
> the error and reset the compressor's state. Subsequent incoming IP and
> CCP packets hit the same check and are also dropped, preventing normal
> recovery.
>
> The flag entered Linux in 2005 with commit b3f9b92a6ec1 ("[PPP]: add PPP
> MPPE encryption module"). The original patch submission [1] said pppd
> would set it when CCP came up with MPPE enabled. That intended
> integration [2] appears never to have landed upstream.
>
> Remove the broken and unused SC_MUST_COMP from SC_MASK and SC_FLAG_BITS,
> and remove its transmit and receive drop checks. Keep the uAPI macro and
> mark it as obsolete. PPPIOCSFLAGS will discard this bit while
> PPPIOCGFLAGS can still report it as SC_DC_ERROR.
>
> [1] https://lore.kernel.org/netdev/20050814141046.GB20944@xxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/netdev/20050814190529.GA26802@xxxxxxxxxxxxxxxxx/
> Fixes: b3f9b92a6ec1 ("[PPP]: add PPP MPPE encryption module")
> Signed-off-by: Qingfang Deng <qingfang.deng@xxxxxxxxx>
Given the 'broken from beginning' status and the hopefully low impact, I'm
applying this to net-next.

/P