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This is exposed via the non-standard serenity_mmap() call in userspace.
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We should never end up deallocating an empty range, or a range that
ends before it begins.
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This made the allocator perform worse, so here's another second off of
the Kernel/Process.cpp compile time from a simple bugfix! (31s to 30s)
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Previously, when deallocating a range of VM, we would sort and merge
the range list. This was quite slow for large processes.
This patch optimizes VM deallocation in the following ways:
- Use binary search instead of linear scan to find the place to insert
the deallocated range.
- Insert at the right place immediately, removing the need to sort.
- Merge the inserted range with any adjacent range(s) in-line instead
of doing a separate merge pass into a list copy.
- Add Traits<Range> to inform Vector that Range objects are trivial
and can be moved using memmove().
I've also added an assertion that deallocated ranges are actually part
of the RangeAllocator's initial address range.
I've benchmarked this using g++ to compile Kernel/Process.cpp.
With these changes, compilation goes from ~41 sec to ~35 sec.
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As suggested by Joshua, this commit adds the 2-clause BSD license as a
comment block to the top of every source file.
For the first pass, I've just added myself for simplicity. I encourage
everyone to add themselves as copyright holders of any file they've
added or modified in some significant way. If I've added myself in
error somewhere, feel free to replace it with the appropriate copyright
holder instead.
Going forward, all new source files should include a license header.
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This is not ASLR, but it does de-trivialize exploiting the ELF loader
which would previously always parse executables at 0x01001000 in every
single exec(). I've taken advantage of this multiple times in my own
toy exploits and it's starting to feel cheesy. :^)
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This might be useful for debugging since guard pages introduce a fair
amount of noise in the virtual address space.
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Put one unused page on each side of VM allocations to make invalid
accesses more likely to generate crashes.
Note that we will not add this guard padding for mmap() at a specific
memory address, only to "mmap it anywhere" requests.
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Otherwise we'll start handing out addresses that are very likely already in
use by existing ranges.
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This replaces the previous virtual address allocator which was basically
just "m_next_address += size;"
With this in place, virtual addresses can get reused, which cuts down on
the number of page tables created. When we implement ASLR some day, we'll
probably have to do page table deallocation, but for now page tables are
only deallocated once the process dies.
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