[Qemu-devel] pc_q35/i440fx: uses uninitialized variable 'ram_memory' if !xen_enabled()

[Peter Maydell]

Building QEMU with clang 3.3 results in the following warning:

    hw/i386/pc_q35.c:115:9: error: variable 'ram_memory' is used
          uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]
        if (!xen_enabled()) {
            ^~~~~~~~~~~~~~
    hw/i386/pc_q35.c:134:32: note: uninitialized use occurs here
        q35_host->mch.ram_memory = ram_memory;
                                   ^~~~~~~~~~
    hw/i386/pc_q35.c:115:5: note: remove the 'if' if its condition is always
          true
        if (!xen_enabled()) {
        ^~~~~~~~~~~~~~~~~~~~
    hw/i386/pc_q35.c:71:29: note: initialize the variable 'ram_memory' to
          silence this warning
        MemoryRegion *ram_memory;
                                ^
                                 = NULL
    1 error generated.


This looks correct -- if xen_enabled() is true, we skip the call
to pc_memory_init() which is what initializes ram_memory, but
then later on we still stuff it into the q35_host->mch field,
and as far as I can tell hw/pci-host/q35.c:mch_init() then uses
the ram_memory value whether xen is enabled or not.

There also seems to be a similar use-of-uninitialized which
clang doesn't spot in i440fx: hw/i386_pc_piix.c:pc_init1()
doesn't init ram_memory if xen_enabled() is true, but we
then pass ram_memory through i440fx_init()->i440fx_common_init()
->init_pam() which then tries to use it.

Can anybody who knows more about the x86 hw models and/or Xen
suggest the correct fix for this?

thanks
-- PMM