analyze: allow overriding dataflow for specific permissions #1088
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ahomescu
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ahomescu
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ahomescu
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This branch implements rewriting of nullable pointers (those lacking `PermissionSet::NON_NULL`) to `Option`. This includes the following kinds of rewrites: * Type annotations: `Option<&mut T>` instead of `&mut T` * Casts between nullable and non-nullable via `p.unwrap()` and `Some(p)`. For most permissions, we implement only one direction because the other is invalid/nonsensical, but here we implement both in order to support "unsound" rewrites involving overridden `NON_NULL` flags (as in #1088). * Borrows: when casting `p: Option<&mut T>`, we use `p.as_deref_mut().unwrap()` instead of `p.unwrap()` to avoid consuming the original `p`. (In the code, this is called a "downgrade", since it allows borrowing `Option<Box<T>>` as `Option<&T>` and similar.) * Pointer projections on nullable pointers. Where `NON_NULL` pointers would use `&p[0]`, nullable ones use `p.map(|ptr| &ptr[0])`. Internally, this is represented similar to `Some(&p.unwrap()[0])`, but it's handled specially by `rewrite::expr::convert` to produce a `map` call instead, which passes through `None` without a panic. * `unwrap()` calls on derefs. `*p` is rewritten to `*p.unwrap()`, or to `*p.as_deref().unwrap()` if a downgrade/borrow is necessary to avoid moving `p`. * `ptr::null()` and `0 as *const _` to `None`, and `p.is_null()` to `p.is_none()`. The new `non_null_rewrites.rs` test case passes, and the rewritten code compiles.
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This adds a new internal feature for overriding dataflow analysis for specific permissions of specific pointers. The
propagatemethod ofdataflownow takes an additionalupdates_forbiddenset, which has aPermissionSetmask for everyPointerId, and avoids adding or removing a permission for aptrif the corresponding bit is set inupdates_forbidden[ptr]. Whenupdates_forbiddenis used, the resulting permissions after runningdataflowmight not actually satisfy the dataflow constraints.This is designed to support the PDG "
NON_NULLoverride" feature, where information about nullability from the PDG can override static analysis results, though that feature is not part of the current PR.