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The C++ standard specifies that `free` and `operator delete` should
be callable with nullptr. The non-aligned `kfree` already handles this,
but because of the pointer arithmetic to obtain the allocation start
pointer, the aligned version would produce undefined behavior.
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Now that we have a significant amount of code paths handling OOM, lets
enable kmalloc and friends to actually return nullptr. This way we can
start stressing these paths and validating all of they work as expected.
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This is a much more ergonomic option than getting a
`VERIFY_NOT_REACHED()` failure at run-time. I encountered this issue
with Clang, where sized deallocation is not the default due to ABI
breakage concerns.
Note that we can't simply just not declare these functions, because the
C++ standard states:
> If this function with size parameter is defined, the program shall
> also define the version without the size parameter.
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The compiler will use these to allocate objects that have alignment
requirements greater than that of our normal `operator new` (4/8 byte
aligned).
This means we can now use smart pointers for over-aligned types.
Fixes a FIXME.
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This was only used by a single class (AK::ByteBuffer) in the kernel
and not in an OOM-safe way.
Now that ByteBuffer no longer uses it, there's no need for the kernel
heap to burden itself with supporting this.
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C++14 gave us sized operator delete, but we haven't been taking
advantage of it. Let's get to a point where it can help us by
adding kfree_sized(void*, size_t).
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In standard C++, operators `new` and `new[]` are guaranteed to return a
valid (non-null) pointer and throw an exception if the allocation
couldn't be performed. Based on this, compilers did not check the
returned pointer before attempting to use them for object construction.
To avoid this, the allocator operators were changed to be `noexcept` in
PR #7026, which made GCC emit the desired null checks. Unfortunately,
this is a non-standard feature which meant that Clang would not accept
these function definitions, as it did not match its expected
declaration.
To make compiling using Clang possible, the special "nothrow" versions
of `new` are implemented in this commit. These take a tag type of
`std::nothrow_t` (used for disambiguating from placement new/etc.), and
are allowed by the standard to return null. There is a global variable,
`std::nothrow`, declared with this type, which is also exported into the
global namespace.
To perform fallible allocations, the following syntax should be used:
```cpp
auto ptr = new (nothrow) T;
```
As we don't support exceptions in the kernel, the only way of uphold the
"throwing" new's guarantee is to abort if the allocation couldn't be
performed. Once we have proper OOM handling in the kernel, this should
only be used for critical allocations, where we wouldn't be able to
recover from allocation failures anyway.
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We have that information in LibC, lets use that instead
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This implements the macOS API malloc_good_size() which returns the
true allocation size for a given requested allocation size. This
allows us to make use of all the available memory in a malloc chunk.
For example, for a malloc request of 35 bytes our malloc would
internally use a chunk of size 64, however the remaining 29 bytes
would be unused.
Knowing the true allocation size allows us to request more usable
memory that would otherwise be wasted and make that available for
Vector, HashTable and potentially other callers in the future.
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For Kernel OOM hardening to work correctly, we need to be able to
call a "nothrow" version of operator new. Unfortunately the default
"throwing" version of operator new assumes that the allocation will
never return on failure and will always throw an exception. This isn't
true in the Kernel, as we don't have exceptions. So if we call the
normal/throwing new and kmalloc returns NULL, the generated code will
happily go and dereference that NULL pointer by invoking the constructor
before we have a chance to handle the failure.
To fix this we declare operator new as noexcept in the Kernel headers,
which will allow the caller to actually handle allocation failure.
The delete implementations need to match the prototype of the new which
allocated them, so we need define delete as noexcept as well. GCC then
errors out declaring that you should implement sized delete as well, so
this change provides those stubs in order to compile cleanly.
Finally the new operator definitions have been standardized as being
declared with [[nodiscard]] to avoid potential memory leaks. So lets
declares the kernel versions that way as well.
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SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
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This macro inserts operator new/delete into a class, allowing you
to very easily specify a specific heap alignment.
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This allows us to get kmalloc() memory aligned to the VM page size.
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This was a thing back when the system was so little that any kernel
allocation above 1 MiB was basically guaranteed to be a bug. :^)
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The following script was used to make these changes:
#!/bin/bash
set -e
tmp=$(mktemp -d)
echo "tmp=$tmp"
find Kernel \( -name '*.cpp' -o -name '*.h' \) | sort > $tmp/Kernel.files
find . \( -path ./Toolchain -prune -o -path ./Build -prune -o -path ./Kernel -prune \) -o \( -name '*.cpp' -o -name '*.h' \) -print | sort > $tmp/EverythingExceptKernel.files
cat $tmp/Kernel.files | xargs grep -Eho '[A-Z0-9_]+_DEBUG' | sort | uniq > $tmp/Kernel.macros
cat $tmp/EverythingExceptKernel.files | xargs grep -Eho '[A-Z0-9_]+_DEBUG' | sort | uniq > $tmp/EverythingExceptKernel.macros
comm -23 $tmp/Kernel.macros $tmp/EverythingExceptKernel.macros > $tmp/Kernel.unique
comm -1 $tmp/Kernel.macros $tmp/EverythingExceptKernel.macros > $tmp/EverythingExceptKernel.unique
cat $tmp/Kernel.unique | awk '{ print "#cmakedefine01 "$1 }' > $tmp/Kernel.header
cat $tmp/EverythingExceptKernel.unique | awk '{ print "#cmakedefine01 "$1 }' > $tmp/EverythingExceptKernel.header
for macro in $(cat $tmp/Kernel.unique)
do
cat $tmp/Kernel.files | xargs grep -l $macro >> $tmp/Kernel.new-includes ||:
done
cat $tmp/Kernel.new-includes | sort > $tmp/Kernel.new-includes.sorted
for macro in $(cat $tmp/EverythingExceptKernel.unique)
do
cat $tmp/Kernel.files | xargs grep -l $macro >> $tmp/Kernel.old-includes ||:
done
cat $tmp/Kernel.old-includes | sort > $tmp/Kernel.old-includes.sorted
comm -23 $tmp/Kernel.new-includes.sorted $tmp/Kernel.old-includes.sorted > $tmp/Kernel.includes.new
comm -13 $tmp/Kernel.new-includes.sorted $tmp/Kernel.old-includes.sorted > $tmp/Kernel.includes.old
comm -12 $tmp/Kernel.new-includes.sorted $tmp/Kernel.old-includes.sorted > $tmp/Kernel.includes.mixed
for file in $(cat $tmp/Kernel.includes.new)
do
sed -i -E 's/#include <AK\/Debug\.h>/#include <Kernel\/Debug\.h>/' $file
done
for file in $(cat $tmp/Kernel.includes.mixed)
do
echo "mixed include in $file, requires manual editing."
done
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It would be tempting to uncomment these statements, but that won't work
with the new changes.
This was done with the following commands:
find . \( -name '*.cpp' -o -name '*.h' -o -name '*.in' \) -not -path './Toolchain/*' -not -path './Build/*' -exec awk -i inplace '$0 !~ /\/\/#define/ { if (!toggle) { print; } else { toggle = !toggle } } ; $0 ~/\/\/#define/ { toggle = 1 }' {} \;
find . \( -name '*.cpp' -o -name '*.h' -o -name '*.in' \) -not -path './Toolchain/*' -not -path './Build/*' -exec awk -i inplace '$0 !~ /\/\/ #define/ { if (!toggle) { print; } else { toggle = !toggle } } ; $0 ~/\/\/ #define/ { toggle = 1 }' {} \;
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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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Add an ExpandableHeap and switch kmalloc to use it, which allows
for the kmalloc heap to grow as needed.
In order to make heap expansion to work, we keep around a 1 MiB backup
memory region, because creating a region would require space in the
same heap. This means, the heap will grow as soon as the reported
utilization is less than 1 MiB. It will also return memory if an entire
subheap is no longer needed, although that is rarely possible.
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Rather than hardcoding where the kmalloc pool should be, place
it at the end of the kernel image instead. This avoids corrupting
global variables or other parts of the kernel as it grows.
Fixes #3257
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It's tedious to write (and look at) [[gnu::always_inline]] etc. :^)
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Since we scrub both kmalloc() and kfree() with predictable values, we
can log a helpful message when hitting a crash that looks like it might
be a dereference of such scrubbed data.
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Memory validation is used to verify that user syscalls are allowed to
access a given memory range. Ring 0 threads never make syscalls, and
so will never end up in validation anyway.
The reason we were allowing kmalloc memory accesses is because kernel
thread stacks used to be allocated in kmalloc memory. Since that's no
longer the case, we can stop making exceptions for kmalloc in the
validation code.
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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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Turns out we can use abi::__cxa_demangle() for this, and all we need to
provide is sprintf(), realloc() and free(), so this patch exposes them.
We now have fully demangled C++ backtraces :^)
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