[Qemu-devel] should we try to stop using variable length arrays?

[Peter Maydell]

Currently QEMU has 9 uses of variable length arrays
(found using -Wvla):

hw/block/dataplane/virtio-blk.c:62:25: warning: variable length array
used [-Wvla]
    unsigned long bitmap[BITS_TO_LONGS(nvqs)];
                        ^
hw/i386/multiboot.c:364:18: warning: variable length array used [-Wvla]
    char kcmdline[strlen(kernel_filename) + strlen(kernel_cmdline) + 2];
                 ^
hw/i386/xen/xen-hvm.c:618:25: warning: variable length array used [-Wvla]
    unsigned long bitmap[DIV_ROUND_UP(npages, width)];
                        ^
hw/intc/xics.c:604:18: warning: variable length array used [-Wvla]
    uint8_t flags[ics->nr_irqs];
                 ^
hw/net/fsl_etsec/rings.c:383:18: warning: variable length array used [-Wvla]
    uint8_t  padd[etsec->rx_padding];
                 ^
hw/ppc/pnv.c:130:26: warning: variable length array used [-Wvla]
    uint32_t servers_prop[smt_threads];
                         ^
hw/ppc/spapr.c:162:26: warning: variable length array used [-Wvla]
    uint32_t servers_prop[smt_threads];
                         ^
hw/ppc/spapr.c:163:27: warning: variable length array used [-Wvla]
    uint32_t gservers_prop[smt_threads * 2];
                          ^
linux-user/syscall.c:3478:23: warning: variable length array used [-Wvla]
    struct sembuf sops[nsops];
                      ^

Should we be looking to get rid of these and turn on the -Wvla
warning? I know the Linux kernel has recently decided to do this
(some rationale at the start of https://lwn.net/Articles/749064/).
Now that doesn't necessarily apply to us as a userspace program,
but on the other hand if any of these were allowing the guest to
determine the size of an on-stack array that would not be great.
(The linux-user one is bogus in that way, though not a security issue
as the guest code there has full control anyway.)

Opinions? I admit that to some extent this is just my sense of
tidiness thinking that if we only have a handful of uses of
something we should squash that down to zero :-)

thanks
-- PMM