Re: [Qemu-devel] [PATCH] util/mmap-alloc: check parameter before using

[Thomas Huth <thuth@redhat.com>]

On 26.10.2016 09:18, Cao jin wrote:
> Also refactor some code hunk for readability
> 
> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
> ---
>  util/mmap-alloc.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 5a85aa3..92c123a 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -41,6 +41,11 @@ size_t qemu_fd_getpagesize(int fd)
>  
>  void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
>  {
> +    /* Make sure align is a power of 2 */
> +    assert(!(align & (align - 1)));
> +    /* Always align to host page size */
> +    assert(align >= getpagesize());

Now you've moved code before the declaration of the local variables.
That's also some kind of ugly (and might not work with older versions of
C compilers?)...

>      /*
>       * Note: this always allocates at least one extra page of virtual address
>       * space, even if size is already aligned.
> @@ -68,11 +73,6 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
>          return MAP_FAILED;
>      }
>  
> -    /* Make sure align is a power of 2 */
> -    assert(!(align & (align - 1)));
> -    /* Always align to host page size */
> -    assert(align >= getpagesize());

... so maybe simply move the setting of "offset" (which requires
"align") here right in front of the mmap() instead?

>      ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
>                  MAP_FIXED |
>                  (fd == -1 ? MAP_ANONYMOUS : 0) |
> @@ -83,22 +83,20 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
>          return MAP_FAILED;
>      }
>  
> -    ptr += offset;
> -    total -= offset;
> -
>      if (offset > 0) {
> -        munmap(ptr - offset, offset);
> +        munmap(ptr, offset);
>      }
>  
>      /*
>       * Leave a single PROT_NONE page allocated after the RAM block, to serve as
>       * a guard page guarding against potential buffer overflows.
>       */
> +    total -= offset;
>      if (total > size + getpagesize()) {
> -        munmap(ptr + size + getpagesize(), total - size - getpagesize());
> +        munmap(ptr1 + size + getpagesize(), total - size - getpagesize());
>      }
>  
> -    return ptr;
> +    return ptr1;
>  }

 Thomas