Document mmap safety #674
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Document mmap safety #674
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philipc
reviewed
Sep 22, 2024
| @@ -186,7 +186,9 @@ impl<'data> Context<'data> { | |||
| fn mmap(path: &Path) -> Option<Mmap> { | |||
| let file = File::open(path).ok()?; | |||
| let len = file.metadata().ok()?.len().try_into().ok()?; | |||
| unsafe { Mmap::map(&file, len, 0) } | |||
| // SAFETY: All files we mmap are mmaped by the dynamic linker or the kernel already for the | |||
| // executable code of the process. Modifying them would cause crashes or UB anyways. | |||
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This is true for some but not all uses of mmap in this crate. DWARF can be in a separate file that is not loaded by the dynamic linker or kernel (.dSYM on macOS, .debug/.dwz/.dwp on linux, or unpacked object files on both).
That doesn't mean we should use fs::read for those though. We don't need to read all of the DWARF, and doing so would affect performance and memory usage.
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Supercedes #667
Document safety preconditions and reasoning for why they're followed. I'd like to push the unsafety down another level, or just remove the
mmapfunction (makeMmapa common type with sys impls for the platform-specific bits), but those both are a lot more work.