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This enables further work on implementing KASLR by adding relocation
support to the pre-kernel and updating the kernel to be less dependent
on specific virtual memory layouts.
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These are currently unused by the prekernel and ld used the same flags
by default - except for the .ksyms section which was marked as
read-write.
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Co-authored-by: Gunnar Beutner <gbeutner@serenityos.org>
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This implements a simple bootloader that is capable of loading ELF64
kernel images. It does this by using QEMU/GRUB to load the kernel image
from disk and pass it to our bootloader as a Multiboot module.
The bootloader then parses the ELF image and sets it up appropriately.
The kernel's entry point is a C++ function with architecture-native
code.
Co-authored-by: Liav A <liavalb@gmail.com>
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Without this we won't be able to detect whether .ksyms overlaps the end
of the page table we set up for the kernel image.
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This adds a new section .ksyms at the end of the linker map, reserves
5MiB for it (which are after end_of_kernel_image so they get re-used
once MemoryManager is initialized) and then embeds the symbol map into
the kernel binary with objcopy. This also shrinks the .ksyms section to
the real size of the symbol file (around 900KiB at the moment).
By doing this we can make the symbol map available much earlier in the
boot process, i.e. even before VFS is available.
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This doesn't really matter in terms of writability for the kernel text
because we set up proper page mappings anyway which prohibit writing
to the text segment. However, this makes the profiler happy which
previously died when validating the kernel's ELF program headers.
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This ensures that pages returned by
MM.allocate_supervisor_physical_page() have a physical address that
is in the bottom 16MB and can thus be used by the SB16 driver for DMA.
Fixes #8092.
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Slightly nicer than saying "0xc0000000" over and over.
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You can now declare functions with UNMAP_AFTER_INIT and they'll get
segregated into a separate kernel section that gets completely
unmapped at the end of initialization.
This can be used for anything we don't need to call once we've booted
into userspace.
There are two nice things about this mechanism:
- It allows us to free up entire pages of memory for other use.
(Note that this patch does not actually make use of the freed
pages yet, but in the future we totally could!)
- It allows us to get rid of obviously dangerous gadgets like
write-to-CR0 and write-to-CR4 which are very useful for an attacker
trying to disable SMAP/SMEP/etc.
I've also made sure to include a helpful panic message in case you
hit a kernel crash because of this protection. :^)
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You can now use the READONLY_AFTER_INIT macro when declaring a variable
and we will put it in a special ".ro_after_init" section in the kernel.
Data in that section remains writable during the boot and init process,
and is then marked read-only just before launching the SystemServer.
This is based on an idea from the Linux kernel. :^)
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Also add an assertion to make sure the safemem sections are never
discarded by the linker.
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The kernel ignored the first 8 MiB of RAM while parsing the memory map
because the kmalloc heaps and the super physical pages lived here. Move
all that stuff inside the .bss segment so that those memory regions are
accounted for, otherwise we risk overwriting boot modules placed next
to the kernel.
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This allows us to perform atomic operations on potentially unsafe
user space pointers.
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By having a separate list of constructors for the kernel heap
code, we can properly use constructors without re-running them
after the heap was already initialized. This solves some problems
where values were wiped out because they were overwritten by
running their constructors later in the initialization process.
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Closes https://github.com/SerenityOS/serenity/issues/2080
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The kernel and its static data structures are no longer identity-mapped
in the bottom 8MB of the address space, but instead move above 3GB.
The first 8MB above 3GB are pseudo-identity-mapped to the bottom 8MB of
the physical address space. But things don't have to stay this way!
Thanks to Jesse who made an earlier attempt at this, it was really easy
to get device drivers working once the page tables were in place! :^)
Fixes #734.
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We now map the kernel's text and rodata segments read+execute.
We also make the data and bss segments non-executable.
Thanks to q3k for the idea! :^)
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This reverts commit bd33c6627394b2166e1419965dd3b2d2dc0c401f.
This broke the network card drivers, since they depended on kmalloc
addresses being identity-mapped.
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The kernel is now no longer identity mapped to the bottom 8MiB of
memory, and is now mapped at the higher address of `0xc0000000`.
The lower ~1MiB of memory (from GRUB's mmap), however is still
identity mapped to provide an easy way for the kernel to get
physical pages for things such as DMA etc. These could later be
mapped to the higher address too, as I'm not too sure how to
go about doing this elegantly without a lot of address subtractions.
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Now the kernel page directory and the page tables are located at a
safe address, to prevent from paging data colliding with garbage.
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Move the kernel image to the 1 MB physical mark. This prevents it from
colliding with stuff like the VGA memory. This was causing us to end
up with the BIOS screen contents sneaking into kernel memory sometimes.
This patch also bumps the kmalloc heap size from 1 MB to 3 MB. It's not
the perfect permanent solution (obviously) but it should get the OOM
monkey off our backs for a while.
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Add text.startup to the .text block, add .ctors as well.
Use them in init.cpp to call global constructors after
gtd and idt init. That way any funky constructors should be ok.
Also defines some Itanium C++ ABI methods that probably shouldn't be,
but without them the linker gets very angry.
If the code ever actually tries to use __dso_handle or call
__cxa_atexit, there's bigger problems with the kernel.
Bit of a hack would be an understatement but hey. It works :)
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The old bootloader was hilariously complicated, requiring a floppy disk with
the kernel on it, and a hard drive with the file system. This patch removes
the floppy disk from the equation and replaces it with a multiboot header.
This means the kernel can now be booted with qemu-system-i386 -kernel kernel
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It's a lot crappier than I remembered it. It's gonna need a lot of work.
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