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Kernel+UE: Add MAP_FIXED_NOREPLACE mmap() flag
This feature was introduced in version 4.17 of the Linux kernel, and while it's not specified by POSIX, I think it will be a nice addition to our system. MAP_FIXED_NOREPLACE provides a less error-prone alternative to MAP_FIXED: while regular fixed mappings would cause any intersecting ranges to be unmapped, MAP_FIXED_NOREPLACE returns EEXIST instead. This ensures that we don't corrupt our process's address space if something is already at the requested address. Note that the more portable way to do this is to use regular MAP_ANONYMOUS, and check afterwards whether the returned address matches what we wanted. This, however, has a large performance impact on programs like Wine which try to reserve large portions of the address space at once, as the non-matching addresses have to be unmapped separately.
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6 changed files with 18 additions and 15 deletions
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@ -36,8 +36,8 @@ NonnullOwnPtr<MmapRegion> MmapRegion::create_anonymous(u32 base, u32 size, u32 p
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NonnullOwnPtr<MmapRegion> MmapRegion::create_file_backed(u32 base, u32 size, u32 prot, int flags, int fd, off_t offset, String name)
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{
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// Since we put the memory to an arbitrary location, do not pass MAP_FIXED to the Kernel.
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auto real_flags = flags & ~MAP_FIXED;
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// Since we put the memory to an arbitrary location, do not pass MAP_FIXED and MAP_FIXED_NOREPLACE to the Kernel.
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auto real_flags = flags & ~(MAP_FIXED | MAP_FIXED_NOREPLACE);
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auto* data = (u8*)mmap_with_name(nullptr, size, prot, real_flags, fd, offset, name.is_empty() ? nullptr : String::formatted("(UE) {}", name).characters());
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VERIFY(data != MAP_FAILED);
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auto* shadow_data = (u8*)mmap_initialized(size, 1, "MmapRegion ShadowData");
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