Re: [Qemu-trivial] [PATCH v3] util/mmap-alloc: check parameter before using

[Michael Tokarev]

28.10.2016 11:56, Cao jin wrote:
> Also refactor a little bit for readability

See comments below...

> Signed-off-by: Cao jin <address@hidden>
> ---
>  util/mmap-alloc.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 5a85aa3..2f55f5e 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -12,6 +12,7 @@
>  
>  #include "qemu/osdep.h"
>  #include "qemu/mmap-alloc.h"
> +#include "qemu/host-utils.h"
>  
>  #define HUGETLBFS_MAGIC       0x958458f6
>  
> @@ -61,18 +62,18 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, 
> bool shared)
>  #else
>      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 
> 0);
>  #endif
> -    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> +    size_t offset;
>      void *ptr1;
>  
>      if (ptr == MAP_FAILED) {
>          return MAP_FAILED;
>      }
>  
> -    /* Make sure align is a power of 2 */
> -    assert(!(align & (align - 1)));
> +    assert(is_power_of_2(align));
>      /* Always align to host page size */
>      assert(align >= getpagesize());
>  
> +    offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
>      ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
>                  MAP_FIXED |
>                  (fd == -1 ? MAP_ANONYMOUS : 0) |
> @@ -83,22 +84,20 @@ 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;

Why did you change ptr to ptr1 here and above?

On linux, mmap(2) manpage says that address serves as hint, and the
system create the mapping at a nearby page boundary.  Generally, this
address is just a hint.  So I'm not really sure if this code is actually
right.. :)

At the very least, your commit comment is a bit misleading, as it says
about readability, but it also MAY change semantics.

Maybe just move BOTH "ptr+=, total-=" parts down the line and keep
using ptr instead of ptr1?

It'd be very good, in my opinion, to document how this whole thing
is supposed to work :)

Thanks,

/mjt