Bug#1013299: linux-image-4.19.0-20-amd64: NULL pointer deref in qdisc_put() due to missing backport

[Thorsten Glaser <tg@mirbsd.de>]

Diederik de Haas dixit:

>It's a bit 'above my paygrade', but if qdisk_put() can accept a NULL pointer
>then I'm curious whether that would be allowed for other functions in that file
>as well ... there are several having "struct Qdisc *qdisc" as (only)
>parameter, but only qdisk_put() checks for NULL.
>That is also true for the current 'master' branch ...

AIUI the check was added because qdisc_destroy() could accept one,
and several qdiscs are using that, it’s like free(3) in that regard,
the other functions aren’t.

bye,
//mirabilos
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