From: Arvind Sankar
> Sent: 02 September 2020 16:34
>
> The CRn accessor functions use __force_order as a dummy operand to
> prevent the compiler from reordering the inline asm.
>
> The fact that the asm is volatile should be enough to prevent this
> already, however older versions of GCC had a bug that could sometimes
> result in reordering. This was fixed in 8.1, 7.3 and 6.5. Versions prior
> to these, including 5.x and 4.9.x, may reorder volatile asm.
>
> There are some issues with __force_order as implemented:
> - It is used only as an input operand for the write functions, and hence
> doesn't do anything additional to prevent reordering writes.
> - It allows memory accesses to be cached/reordered across write
> functions, but CRn writes affect the semantics of memory accesses, so
> this could be dangerous.
> - __force_order is not actually defined in the kernel proper, but the
> LLVM toolchain can in some cases require a definition: LLVM (as well
> as GCC 4.9) requires it for PIE code, which is why the compressed
> kernel has a definition, but also the clang integrated assembler may
> consider the address of __force_order to be significant, resulting in
> a reference that requires a definition.
>
> Fix this by:
> - Using a memory clobber for the write functions to additionally prevent
> caching/reordering memory accesses across CRn writes.
> - Using a dummy input operand with an arbitrary constant address for the
> read functions, instead of a global variable. This will prevent reads
> from being reordered across writes, while allowing memory loads to be
> cached/reordered across CRn reads, which should be safe.
How much does using a full memory clobber for the reads cost?
It would remove any chance that the compiler decides it needs to
get the address of the 'dummy' location into a register so that
it can be used as a memory reference in a generated instruction
(which is probably what was happening for PIE compiles).
David
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