If the enclave is entered with CSSA=1, then the enclave starts executing the stage-1 signal handler asssembly code. This assembly code prepares the context for the stage-2 signal handler, which will be executed in regular context with CSSA=0 and using C code.
This stage-1 signal handler uses the RBX register as a base pointer to the SSA[0].GPRSGX region. Closer to the end of the stage-1 handler flow, in particular before the .Lcssa1_exception_eexit label, this SSA[0] base pointer is not needed anymore, so the RBX register is used for other purposes. In particular, RBX is used to hold the stashed RDX value (where-to-exit address in untrusted runtime).
However, a future commit that will introduce AEX-Notify flows inside the enclave needs to access the SSA[0].GPRSGX region at this stage (in the .Lcssa1_exception_eexit label). So the RBX register must not be overwritten. Thus, this preparatory commit does not clobber RBX, but instead stashes RDX into another unused register, R10.
Description of the changes
Part 4 in AEX-Notify series.
If the enclave is entered with CSSA=1, then the enclave starts executing the stage-1 signal handler asssembly code. This assembly code prepares the context for the stage-2 signal handler, which will be executed in regular context with CSSA=0 and using C code.
This stage-1 signal handler uses the RBX register as a base pointer to the SSA[0].GPRSGX region. Closer to the end of the stage-1 handler flow, in particular before the
.Lcssa1_exception_eexitlabel, this SSA[0] base pointer is not needed anymore, so the RBX register is used for other purposes. In particular, RBX is used to hold the stashed RDX value (where-to-exit address in untrusted runtime).However, a future commit that will introduce AEX-Notify flows inside the enclave needs to access the SSA[0].GPRSGX region at this stage (in the
.Lcssa1_exception_eexitlabel). So the RBX register must not be overwritten. Thus, this preparatory commit does not clobber RBX, but instead stashes RDX into another unused register, R10.See also related PRs and discussions:
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How to test this PR?
CI is enough.
This change is