[PATCH 1/3] copyfd: reinstate proper guard around munmap()

[Denys Vlasenko]

On Fri, Jul 14, 2017 at 4:11 PM, Johannes Schindelin
<johannes.schindelin at gmx.de> wrote:
> In 4c1392296 (Introduce FEATURE_COPYBUF_KB, 2007-12-02), a feature was
> introduced where a large stack was allocated via mmap(), and
> consequently released via munmap(). Since this is overkill for small
> stacks, the mmap()/munmap() code was guarded inside an #if
> CONFIG_FEATURE_COPYBUF_KB > 4 ... #endif guard.
>
> In pure Win32 API, there is no support to abuse mmap() to allocate a
> stack that way, therefore the code is simply disabled in busybox-w32.
>
> But 8d75d794e (libbb: use sendfile() to copy data between file
> descriptors, 2014-11-27) removed that guard around the munmap() call.
> That was most likely a mistake, as the corresponding mmap() call is
> *still* inside an equivalent #if ... #endif guard.
>
> Let's revert the mistaken change.
>
> Signed-off-by: Johannes Schindelin <johannes.schindelin at gmx.de>
> ---
>  libbb/copyfd.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/libbb/copyfd.c b/libbb/copyfd.c
> index 7e3531903..eedf03790 100644
> --- a/libbb/copyfd.c
> +++ b/libbb/copyfd.c
> @@ -119,8 +119,10 @@ static off_t bb_full_fd_action(int src_fd, int dst_fd, off_t size)
>         }
>   out:
>
> +#if CONFIG_FEATURE_COPYBUF_KB > 4
>         if (buffer_size > 4 * 1024)
>                 munmap(buffer, buffer_size);
> +#endif

If CONFIG_FEATURE_COPYBUF_KB <= 4, then buffer_size is a constant < 4k
and compiler will eliminate the code anyway.