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[OCaml] Fix a possible crash in llvm_struct_name

Authored by jberdine on Mar 25 2021, 4:34 PM.

Description

[OCaml] Fix a possible crash in llvm_struct_name

The implementation of llvm_struct_name before this diff calls
caml_copy_string, which allocates, while the result local variable
points to a block allocated by caml_alloc_small that has not yet
been initialized. If the allocation in caml_copy_string triggers a
garbage collection, then the GC root result contains a pointer to
uninitialized data, which may crash the GC or lead to a memory
corruption.

This diff fixes this by allocating and initializing the string first
and then allocating and initializing the option, thereby leaving no
dangling pointers when allocations are made.

The conversion from a C string to an OCaml string option is refactored
into a function, cstr_to_string_option. This function is also used
to simplify the definitions of llvm_get_mdstring and
llvm_string_of_const.

Differential Revision: https://reviews.llvm.org/D99393

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