ARM: 6892/1: handle ptrace requests to change PC during interrupted system calls
GDB's interrupt.exp test cases currenly fail on ARM. The problem is how do_signal handled restarting interrupted system calls: The entry.S assembler code determines that we come from a system call; and that information is passed as "syscall" parameter to do_signal. That routine then calls get_signal_to_deliver [*] and if a signal is to be delivered, calls into handle_signal. If a system call is to be restarted either after the signal handler returns, or if no handler is to be called in the first place, the PC is updated after the get_signal_to_deliver call, either in handle_signal (if we have a handler) or at the end of do_signal (otherwise). Now the problem is that during [*], the call to get_signal_to_deliver, a ptrace intercept may happen. During this intercept, the debugger may change registers, including the PC. This is done by GDB if it wants to execute an "inferior call", i.e. the execution of some code in the debugged program triggered by GDB. To this purpose, GDB will save all registers, allocate a stack frame, set up PC and arguments as appropriate for the call, and point the link register to a dummy breakpoint instruction. Once the process is restarted, it will execute the call and then trap back to the debugger, at which point GDB will restore all registers and continue original execution. This generally works fine. However, now consider what happens when GDB attempts to do exactly that while the process was interrupted during execution of a to-be- restarted system call: do_signal is called with the syscall flag set; it calls get_signal_to_deliver, at which point the debugger takes over and changes the PC to point to a completely different place. Now get_signal_to_deliver returns without a signal to deliver; but now do_signal decides it should be restarting a system call, and decrements the PC by 2 or 4 -- so it now points to 2 or 4 bytes before the function GDB wants to call -- which leads to a subsequent crash. To fix this problem, two things need to be supported: - do_signal must be able to recognize that get_signal_to_deliver changed the PC to a different location, and skip the restart-syscall sequence - once the debugger has restored all registers at the end of the inferior call sequence, do_signal must recognize that *now* it needs to restart the pending system call, even though it was now entered from a breakpoint instead of an actual svc instruction This set of issues is solved on other platforms, usually by one of two mechanisms: - The status information "do_signal is handling a system call that may need restarting" is itself carried in some register that can be accessed via ptrace. This is e.g. on Intel the "orig_eax" register; on Sparc the kernel defines a magic extra bit in the flags register for this purpose. This allows GDB to manage that state: reset it when doing an inferior call, and restore it after the call is finished. - On s390, do_signal transparently handles this problem without requiring GDB interaction, by performing system call restarting in the following way: first, adjust the PC as necessary for restarting the call. Then, call get_signal_to_deliver; and finally just continue execution at the PC. This way, if GDB does not change the PC, everything is as before. If GDB *does* change the PC, execution will simply continue there -- and once GDB restores the PC it saved at that point, it will automatically point to the *restarted* system call. (There is the minor twist how to handle system calls that do *not* need restarting -- do_signal will undo the PC change in this case, after get_signal_to_deliver has returned, and only if ptrace did not change the PC during that call.) Because there does not appear to be any obvious register to carry the syscall-restart information on ARM, we'd either have to introduce a new artificial ptrace register just for that purpose, or else handle the issue transparently like on s390. The patch below implements the second option; using this patch makes the interrupt.exp test cases pass on ARM, with no regression in the GDB test suite otherwise. Cc: patches@linaro.org Signed-off-by:Ulrich Weigand <ulrich.weigand@linaro.org> Signed-off-by:
Arnd Bergmann <arnd.bergmann@linaro.org> Signed-off-by:
Russell King <rmk+kernel@arm.linux.org.uk>
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