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3fc86e1901
An attempt to do an exception-return (branch to one of the magic addresses) in linux-user mode for M-profile should behave like a normal branch, because linux-user mode is always going to be in 'handler' mode. This used to work, but we broke it when we added support for the M-profile security extension in commit d02a8698d7ae2bfed. In that commit we allowed even handler-mode calls to magic return values to be checked for and dealt with by causing an EXCP_EXCEPTION_EXIT exception to be taken, because this is needed for the FNC_RETURN return-from-non-secure-function-call handling. For system mode we added a check in do_v7m_exception_exit() to make any spurious calls from Handler mode behave correctly, but forgot that linux-user mode would also be affected. How an attempted return-from-non-secure-function-call in linux-user mode should be handled is not clear -- on real hardware it would result in return to secure code (not to the Linux kernel) which could then handle the error in any way it chose. For QEMU we take the simple approach of treating this erroneous return the same way it would be handled on a CPU without the security extensions -- treat it as a normal branch. The upshot of all this is that for linux-user mode we should never do any of the bx_excret magic, so the code change is simple. This ought to be a weird corner case that only affects broken guest code (because Linux user processes should never be attempting to do exception returns or NS function returns), except that the code that assigns addresses in RAM for the process and stack in our linux-user code does not attempt to avoid this magic address range, so legitimate code attempting to return to a trampoline routine on the stack can fall into this case. This change fixes those programs, but we should also look at restricting the range of memory we use for M-profile linux-user guests to the area that would be real RAM in hardware. Backports commit 9027d3fba605d8f6093342ebe4a1da450d374630 from qemu |
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