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macOS provides us with two flags that makes reading from an mmap'd file more resilient against crashes: - MAP_RESILIENT_CODESIGN avoids crashes when mapping a macho binary with an invalid signature. Apple has some weird shenanigans going on that makes it validate the signature of mmaped file (even when read-only) under some undocumented circumstances, and this flag avoids this. - MAP_RESILIENT_MEDIA avoids crashes when the underlying data becomes inaccessible - either because the file is no longer present (think removable media getting unplugged) or because the file got truncated to a smaller size. When this happens, it returns zeroes instead.
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@de-vri-es while you're at it, small up on this change that seems relevant too |
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macOS provides us with two flags that makes reading from an mmap'd file more resilient against crashes:
MAP_RESILIENT_CODESIGN avoids crashes when mapping a macho binary with an invalid signature. Apple has some weird shenanigans going on that makes it validate the signature of mmaped file (even when read-only) under some undocumented circumstances, and this flag avoids this. See Crash on macOS (
EXC_BAD_ACCESS (Code Signature Invalid)) VirusTotal/yara#1309 for more informationMAP_RESILIENT_MEDIA avoids crashes when the underlying data becomes inaccessible - either because the file is no longer present (think removable media getting unplugged) or because the file got truncated to a smaller size. When this happens, it returns zeroes instead.
Note that
MAP_RESILIENT_MEDIAonly works forMAP_PRIVATEmappings.These aren't very well documented, but can be found in the XNU source code: https://github.com/apple-oss-distributions/xnu/blob/xnu-12377.61.12/bsd/sys/mman.h
This PR adds both to every MMAP to make them more robust. Arguably, the second one (
MAP_RESILIENT_MEDIA) does change the semantics ofmmapcompared to how it works on every other platform, and as such we may want to move it to be a flag that the user must provide?