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This is why the manifestation of memory UB on my kernel will be untrappable abnormal exit, not a catchable signal. 😊
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I wonder how many otherwise-legit apps catch SIGSEGV and friends? like, about the same as mmap the 0 page maybe?
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No, it's very common. It's widely used in garbage collectors (instead of write barriers) and JIT compilers (for changing the code and also just for halting code execution) among other places. It's so common that userfaultfd was added as a better approach:
man7.org/linux/man-page
makes sense, I didn't know GCs used this. previously I had only seen it used in weird niches such as transparent distributed shared memory.
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it seems like a crap design though. we don't want to handle every SIGSEGV, but rather only those that come from regions we've mmaped for very specific purposes.
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Firefox also has a custom linker using this. On Android, the traditional officially supported approach to native libraries has them stored compressed in the apk and automatically extracted by the package manager to a library directory not writable by the application.
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The more modern officially supported approach leaves them uncompressed in the apk (which only impacts installed size, since apk downloads should be compressed due to this approach for various kinds of resources, etc.) and aligned to page size. The linker knows how to map those.
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The other common use case is optimizing code size for NullPointerExceptions in managed languages: instead of littering every object access with a check and branch, just blindly dereference and fix things up in the signal handler.
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This predictably leads to fun stuff when you have multiple runtimes in the same process that try to do the same thing. Android has some somewhat terrible hacks to maintain a chain of signal handlers that always get run first before user-set ones: android.googlesource.com/platform/art/+
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