Messages in this thread | | | Subject | Re: [PATCH 5/7] seccomp_filter: Document what seccomp_filter is and how it works. | From | Eric Paris <> | Date | Wed, 04 May 2011 13:55:33 -0400 |
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On Wed, 2011-05-04 at 19:03 +0200, Frederic Weisbecker wrote: > On Wed, May 04, 2011 at 12:22:40PM -0400, Eric Paris wrote: > So the following set of operations: > > > > SECCOMP_FILTER_SET, __NR_foo, "a=0" > > SECCOMP_FILTER_SET, __NR_read, "1" > > SECCOPM_FILTER_APPLY > > > > SECCOMP_FILTER_SET, __NR_foo, "b=0" > > SECCOMP_FILTER_APPLY > > > > SECCOMP_FILTER_SET, __NR_write, "1" > > SECCOMP_FILTER_APPLY > > > > Would return EPERM for the __NR_write entry since it was a new syscall > > after a set. I think we agree on all this. > > No, why? > > The default filter for a syscall, if none have been given for it, is "0". > > Thus, if you write "1" later, the entire filter is going to be: > > "0 && 1" > > Which is fine, we are not overriding already applied permissions there. > > So where is the need to return -EPERM in such a specific case? Is it > worth the corner case to check in the kernel, and to handle in userspace? > And for what reason?
I assumed without looking at the code (always a bad idea) that he wasn't going to explicitly create a rule with "0" and was going to implicitly deny anything without a rule. If there is an explicit "0" rule then you are right, i don't see a need to deny the set operation in the kernel. But if it is implicit in the non-existence of a filter then it should be easy to tell userspace it isn't allowed any more.
-Eric
-Eric
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