Commits on Jul 8, 2024

1. Fix segfault with custom page sizes on aarch64 …

   This commit fixes an issue with static memory initialization and custom
   page sizes interacting together on aarch64 Linux. (is that specific
   enough?)
   
   When static memory initialization is enabled chunks of memory to
   initialize the linear memory are made in host-page-size increments of
   memory. This is done to enable page-mapping via copy-on-write if
   customized. With the custom page sizes proposal, however, for the first
   time it's possible for a linear memory to be smaller than this chunk of
   memory. This means that a virtual memory allocation of a single host
   page can be made which is smaller than the initialization chunk.
   
   This currently only happens on aarch64 Linux where we coarsely
   approximate that the host page size is 64k but many hosts run with 4k
   pages. This means that a 64k initializer is created but the host only
   allocates 4k for a linear memory. This means that memory initialization
   can crash when a 64k initializer is copied into a 4k memory.
   
   This was not caught via fuzzing because fuzzing only runs on x86_64.
   This was not caught via CI because on CI guard pages are disabled
   entirely on QEMU and we got lucky in that a number of virtual memory
   allocations were all placed next to each other meaning that this copy
   was probably corrupting some other memory. Locally this was found by
   running tests on `main` as-is on AArch64 Linux (by bjorn3).
   
   This commit implements a few safeguards and a fix for this issue:
   
   * On CI with QEMU modestly-size guard pages are now enabled to catch
     this sooner in development should it happen again in the future.
   * An `assert!` is added during memory initialization that the memory
     copy is indeed valid. This causes the tests to fail as-is on `main`
     even on x86_64.
   * The issue itself is fixed by bailing out of static memory
     initialization should the host page size exceed the wasm page size
     which can now happen on aarch64 Linux with smaller page sizes.

   

   alexcrichton committed Jul 8, 2024
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