Re: [Xen-devel] [PATCH v2] tools: libxl: make it illegal to pass libxl__realloc(gc) a non-gc ptr

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Feb 11, 2016 at 09:23:54AM +0000, Ian Campbell wrote:
> That is, if gc is not NOGC and ptr is not NULL then ptr must be
> associated gc.
> 

"associated with gc"?

Anyway, I get the idea.

> Currently in this case the new_ptr would not be registered with any
> gc, which Coverity rightly points out (in various different places)
> would be a memory leak.
> 
> It would also be possible to fix this by adding a libxl__ptr_add() at
> the same point, however semantically it seems like a programming error
> to gc-realloc a pointer which is not associated with the gc in
> question, so treat it as such.
> 
> Compile tested only, this change could expose latent bugs.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

> ---
> v2: Check we actually didn't find the ptr in the gc
>     Correct log message and shorten since LOG will inject the
>        function name.
> ---
>  tools/libxl/libxl_internal.c | 4 ++++
>  tools/libxl/libxl_internal.h | 4 +++-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/libxl/libxl_internal.c b/tools/libxl/libxl_internal.c
> index 328046b..fc81130 100644
> --- a/tools/libxl/libxl_internal.c
> +++ b/tools/libxl/libxl_internal.c
> @@ -122,6 +122,10 @@ void *libxl__realloc(libxl__gc *gc, void *ptr, size_t 
> new_size)
>                  break;
>              }
>          }
> +        if (i == gc->alloc_maxsize) {
> +            LOG(CRITICAL, "pointer is not tracked by the given gc");
> +            abort();
> +        }
>      }
>  
>      return new_ptr;
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index fc1b558..650a958 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -617,7 +617,9 @@ _hidden void *libxl__zalloc(libxl__gc *gc_opt, size_t 
> size) NN1;
>  _hidden void *libxl__calloc(libxl__gc *gc_opt, size_t nmemb, size_t size) 
> NN1;
>  /* change the size of the memory block pointed to by @ptr to @new_size bytes.
>   * unlike other allocation functions here any additional space between the
> - * oldsize and @new_size is not initialised (similar to a gc'd realloc(3)). 
> */
> + * oldsize and @new_size is not initialised (similar to a gc'd realloc(3)).
> + * if @ptr is non-NULL and @gc_opt is not nogc_gc then @ptr must have been
> + * registered with @gc_opt previously. */
>  _hidden void *libxl__realloc(libxl__gc *gc_opt, void *ptr, size_t new_size) 
> NN1;
>  /* print @fmt into an allocated string large enoughto contain the result.
>   * (similar to gc'd asprintf(3)). */
> -- 
> 2.1.4
> 

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