[RFC PATCH v5 8/9] libcamera: process: Use `close_range()` when available

[Kieran Bingham]

Quoting Barnabás Pőcze (2025-04-24 12:41:02)
> Use the `close_range()` system call when available as it is simpler
> and faster than iterating `/proc/self/fd/`.
> 
> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  meson.build               |  4 ++++
>  src/libcamera/process.cpp | 25 +++++++++++++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/meson.build b/meson.build
> index 9ba5e2ca9..ae13727eb 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -74,6 +74,10 @@ cc = meson.get_compiler('c')
>  cxx = meson.get_compiler('cpp')
>  config_h = configuration_data()
>  
> +if cc.has_header_symbol('unistd.h', 'close_range', prefix : '#define _GNU_SOURCE')
> +    config_h.set('HAVE_CLOSE_RANGE', 1)
> +endif
> +
>  if cc.has_header_symbol('fcntl.h', 'F_ADD_SEALS', prefix : '#define _GNU_SOURCE')
>      config_h.set('HAVE_FILE_SEALS', 1)
>  endif
> diff --git a/src/libcamera/process.cpp b/src/libcamera/process.cpp
> index 369fdb12d..42b56374d 100644
> --- a/src/libcamera/process.cpp
> +++ b/src/libcamera/process.cpp
> @@ -70,6 +70,31 @@ void closeAllFdsExcept(Span<const int> fds)
>  
>         ASSERT(v.empty() || v.front() >= 0);
>  
> +#if HAVE_CLOSE_RANGE
> +       /*
> +        * At the moment libcamera does not require at least Linux 5.9,
> +        * which introduced the `close_range()` system call, so a runtime
> +        * check is also needed to make sure that it is supported.
> +        */
> +       static const bool hasCloseRange = [] {
> +               return close_range(~0u, 0, 0) < 0 && errno == EINVAL;
> +       }();
> +
> +       if (hasCloseRange) {
> +               unsigned int prev = 0;
> +
> +               for (unsigned int curr : v) {
> +                       ASSERT(prev <= curr + 1);
> +                       if (prev < curr)
> +                               close_range(prev, curr - 1, 0);
> +                       prev = curr + 1;

Aha, I see it now. I was lost thinking this patch was a change in
behaviour - but it looks sane to me.


Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> +               }
> +
> +               close_range(prev, ~0u, 0);
> +               return;
> +       }
> +#endif
> +
>         DIR *dir = opendir("/proc/self/fd");
>         if (!dir)
>                 return;
> -- 
> 2.49.0
>