NULL safe set_boxed_mut_ptr/set_arc_mut_ptr #402
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Previously the `set_boxed_mut_ptr()` and `set_arc_mut_ptr()` helper fns used for assigning out parameters across the FFI boundary took `*mut *mut C` and `*mut *const C` for the destination argument `dst`. Using these safely required callers always verify that `dst != NULL`. In practice it's very easy to forget to do this and danger lurks! We could modify these helpers to do the check itself, but we tend to use these fns near the end of a function to assign a result in a success case and we would prefer `NULL` checking happen at the beginning of the function. One proposed solution is to modify these setter functions to take `&mut *mut C` and `&mut *const C`. By using new helper fns to carefully construct a `&mut` from the input double pointer we can front-load the `NULL` check and the assignment in the set fns can proceed knowing there's no possibility for a `NULL` outer pointer. This commit implements this strategy, updating the argument type of `set_boxed_mut_ptr` and `set_arc_mut_ptr` to take `&mut (*const|*mut) C`. New `try_mut_from_ptr_ptr` and `try_ref_from_ptr_ptr` macros allow converting from `*mut *mut C` and `*mut *const C` to the reference types, bailing early for `NULL`.
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I think the fn names and rustdoc could use some bike shedding but I wanted to get this up for input before I log off. 💤 |
jsha
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Mar 29, 2024
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ctz
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Apr 2, 2024
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Mental note to pull these into openssl-compat
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Thanks for taking a look.
#404 might be of interest in that area as well. |
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Previously the
set_boxed_mut_ptr()andset_arc_mut_ptr()helper fns used for assigning out parameters across the FFI boundary took*mut *mut Cand*mut *const Cfor the destination argumentdst. Using these safely required callers always verify thatdst != NULL. In practice it's very easy to forget to do this and danger lurks!We could modify these helpers to do
NULLchecking, but we tend to use them near the end of a function to assign a result in a success case and we would preferNULLchecking happen at the beginning of the function.One proposed solution is to modify these setter functions to take
&mut *mut Cand&mut *const C. By using new helper macros to carefully construct a&mutfrom the input double pointer we can front-load theNULLcheck and the assignment in the set fns can proceed knowing there's no possibility for aNULLouter pointer.This commit implements this strategy, updating the argument type of
set_boxed_mut_ptrandset_arc_mut_ptrto take&mut (*const|*mut) C. Newtry_mut_from_ptr_ptrandtry_ref_from_ptr_ptrmacros allow converting from*mut *mut Cand*mut *const Cto the reference types, bailing early forNULL.Resolves #380