On Tue, Oct 01, 2024 at 05:12:38PM +0200, Christian Brauner wrote: > On Fri, Sep 27, 2024 at 03:21:59PM GMT, Edgecombe, Rick P wrote: > > Did you catch that a token can be at a different offsets location on the stack > > depending on args passed to map_shadow_stack? So userspace will need something > > like the code above, but that adjusts the 'shadow_stack_size' such that the > > kernel looks for the token in the right place. It will be even weirder if > > someone uses clone3 to switch to a stack that has already been used, and pivoted > > off of, such that a token was left in the middle of the stack. In that case > > userspace would have to come up with args disconnected from the actual size of > > the shadow stack such that the kernel would be cajoled into looking for the > > token in the right place. > > > > A shadow stack size is more symmetric on the surface, but I'm not sure it will > > be easier for userspace to handle. So I think we should just have a pointer to > > the token. But it will be a usable implementation either way. My suspicion would be that if we're doing the pivot to a previously used shadow stack we'd also be pivoting the regular stack along with it which would face similar issues with having an unusual method for specifying the stack top so I don't know how much we're really winning. Like we both keep saying either of the interfaces works though, it's just a taste question with both having downsides. > Maybe it's best to let glibc folks decide what is better/more ergonomic for them. The relevant people are on the thread I think. I've rebased onto v6.12-rc1, assuming I don't notice anything horrible in testing I'll post that with the ABI unchanged for now.
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