arm64: Make stxr_status an input operand in vm_do_cheri_revoke() #2086
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bsdjhb
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Apr 17, 2024
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If the capability under test changes between the initial load (done to clear the tag before storing it) and the load-link, the initial comparison fails and the inline asm block is supposed to leave stxr_status set to 2. However, the lack of an input constraint meant that the compiler simply elided the initialization of stxr_status, which could lead to an infinite loop. This could be fixed by making stxr_status an input operand, but it seems safer to just initialize it in the asm block.
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In a hybrid kernel, it's a bit nicer to keep the LL/SC loop in assembly so that we don't have to swap instruction sets when the comparison fails. This loop also avoids needing to keep 3 different states in stxr_status.
bsdjhb
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Apr 19, 2024
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No test case at the moment, as I've had trouble reproducing this. It's very timing-sensitive. Verified by looking at the disassembly of
vm_cheri_revoke_page_rw()in kgdb: