Re: [PATCH v2 2/3] fuzz: configure a sparse-mem device, by default

[Darren Kenny <darren.kenny@oracle.com>]

On Saturday, 2021-03-13 at 18:18:58 -05, Alexander Bulekov wrote:
> The generic-fuzzer often provides randomized DMA addresses to
> virtual-devices. For a 64-bit address-space, the chance of these
> randomized addresses coinciding with RAM regions, is fairly small. Even
> though the fuzzer's instrumentation eventually finds valid addresses,
> this can take some-time, and slows-down fuzzing progress (especially,
> when multiple DMA buffers are involved). To work around this, create
> "fake" sparse-memory that spans all of the 64-bit address-space. Adjust
> the DMA call-back to populate this sparse memory, correspondingly
>
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

> ---
>  tests/qtest/fuzz/generic_fuzz.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c
> index 387ae2020a..b5fe27aae1 100644
> --- a/tests/qtest/fuzz/generic_fuzz.c
> +++ b/tests/qtest/fuzz/generic_fuzz.c
> @@ -28,6 +28,7 @@
>  #include "hw/pci/pci.h"
>  #include "hw/boards.h"
>  #include "generic_fuzz_configs.h"
> +#include "hw/mem/sparse-mem.h"
>  
>  /*
>   * SEPARATOR is used to separate "operations" in the fuzz input
> @@ -64,6 +65,8 @@ static useconds_t timeout = DEFAULT_TIMEOUT_US;
>  
>  static bool qtest_log_enabled;
>  
> +MemoryRegion *sparse_mem_mr;
> +
>  /*
>   * A pattern used to populate a DMA region or perform a memwrite. This is
>   * useful for e.g. populating tables of unique addresses.
> @@ -191,8 +194,7 @@ void fuzz_dma_read_cb(size_t addr, size_t len, MemoryRegion *mr)
>       */
>      if (dma_patterns->len == 0
>          || len == 0
> -        || mr != current_machine->ram
> -        || addr > current_machine->ram_size) {
> +        || (mr != current_machine->ram && mr != sparse_mem_mr)) {
>          return;
>      }
>  
> @@ -238,7 +240,7 @@ void fuzz_dma_read_cb(size_t addr, size_t len, MemoryRegion *mr)
>                                        MEMTXATTRS_UNSPECIFIED);
>  
>          if (!(memory_region_is_ram(mr1) ||
> -              memory_region_is_romd(mr1))) {
> +              memory_region_is_romd(mr1)) && mr1 != sparse_mem_mr) {
>              l = memory_access_size(mr1, l, addr1);
>          } else {
>              /* ROM/RAM case */
> @@ -814,6 +816,12 @@ static void generic_pre_fuzz(QTestState *s)
>      }
>      qts_global = s;
>  
> +    /*
> +     * Create a special device that we can use to back DMA buffers at very
> +     * high memory addresses
> +     */
> +    sparse_mem_mr = sparse_mem_init(0, UINT64_MAX);
> +
>      dma_regions = g_array_new(false, false, sizeof(address_range));
>      dma_patterns = g_array_new(false, false, sizeof(pattern));
>  
> -- 
> 2.28.0