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Linux kernel simply return current brk when request brk addr is not
feasible. The pk should probably do the same.
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The memory manager maintains the first free page as the page after the
`_end` synthetic emitted by the linker. This value is stored in a
translation unit local variable. This value is only ever written to
from `init_early_alloc` which is static and only ever invoked from
`pk_vm_init`. Furthermore, the value that `first_free_page` is ever set
to is computed as a rounding of the _address_ of `_end`. Because the
address of the symbol cannot change during execution of a normal
program, this is effectively a constant, making the computed value a
"constant" which can be re-materialized. Now, with the knowledge that
the value is effectively a constant that can be re-materialized and the
fact that the value is ever written to at a single position, we can
simply re-materialize the value if it was ever changed in
`free_page_addr`. This will allow the 8-byte value to be truncated to
1-byte.
Now, we can inline `__early_pgalloc_align`, and because the combination
of `__early_alloc` and `__early_pgalloc_align` is small, we can inline
that again at the two sites locally. This changes the
`__augment_page_freelist` to re-materialize the constant when needed for
the allocation.
The re-materialization however uses a pc-relative addressing, which now
computes a different value than expected - the address has become a VA
rather than a PA. This results in the address computed by
`free_page_addr` (which is the result of the `__early_pgalloc_align`) to
be a virtual address after the relocation, which then propagates through
`__early_alloc` to the value in `__augment_page_freelist`, which is then
consumed by `__page_alloc`, which will treat the now VA as a PA and
perform an additional translation to a VA.
Mark the value as `volatile` to indicate that the value must be read at
all points to thwart the size optimization of the compiler resulting in
a mis-compilation resulting in the eventual invalid memory access during
the `memset` that follows the allocation.
Thanks to @nzmichaelh for the help in tracking this down!
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The LLVM IAS currently does not support the older spelling for the CSR.
Update the references to the modern name.
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Previously, the pk would always run from virtual address MEM_START.
Instead, remap it into the negative virtual addresses, allowing user
processes to expand beyond MEM_START.
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Return the old brk if mmap fails, rather than just asserting out
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Estimate available memory and return -1 from mmap if not enough
is available, rather than assert-failing.
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This will improve flexibility going forward.
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Enforced with sstatus.SUM.
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f81b722bf004177eadaf6f1b4b9e699e20257521 is a regression. If a
read-only segment does not begin on a page boundary, it would cause
the ELF loader to blow up.
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Yuck.
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