Re: [Qemu-devel] [PATCH 3/6] fw_cfg: assertion to detect memory leak when adding new data blob

[Laszlo Ersek <lersek@redhat.com>]

On 03/16/15 15:15, Gabriel L. Somlo wrote:
> Currently, fw_cfg_add_bytes_read_callback() does not deal with
> the possibility that the data pointer at the requested key position
> has previously been set, and assumes it will be called exactly once
> for each key value.
> 
> This patch introduces an assertion to codify this assumption, and
> insure the data pointer about to be set is NULL at the time the
> function is called, which will prevent the inadvertent leaking of
> data blobs by erroneous multiple calls using the same key value.
> 
> Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
> ---
>  hw/nvram/fw_cfg.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 86090f3..5501a97 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -399,6 +399,7 @@ static void fw_cfg_add_bytes_read_callback(FWCfgState *s, uint16_t key,
>      key &= FW_CFG_ENTRY_MASK;
>  
>      assert(key < FW_CFG_MAX_ENTRY && len < UINT32_MAX);
> +    assert(s->entries[arch][key].data == NULL); /* prevent memory leak */
>  
>      s->entries[arch][key].data = data;
>      s->entries[arch][key].len = (uint32_t)len;
> 

I think I agree with the patch (the assert itself), but the comment
could be more precise. I'd simply say "avoid selector key conflict" or
some such. The predicate we want to assert here primarily is "single
assignment of selector key", right? Calling
fw_cfg_add_bytes_read_callback() with the same key is an error
regardless of any leaks (you could call it with a pointer to a static
storage duration object, and it would remain an error just the same).

Thanks
Laszlo