Re: [PATCH] util/oslib-posix : qemu_init_exec_dir implementation for MacOS

[Roman Bolshakov]

On Tue, May 26, 2020 at 09:40:27PM +0100, David CARLIER wrote:
> From b24a6702beb2a4e2a9c1c03b69c6d1dd07d4cf08 Mon Sep 17 00:00:00 2001
> From: David Carlier <devnexen@gmail.com>
> Date: Tue, 26 May 2020 21:35:27 +0100
> Subject: [PATCH] util/oslib: current process full path resolution on MacOS
> 
> Using existing libproc to fill the path.
> 
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
>  util/oslib-posix.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 062236a1ab..96f0405ee6 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -55,6 +55,10 @@
>  #include <sys/sysctl.h>
>  #endif
> 
> +#ifdef __APPLE__
> +#include <libproc.h>
> +#endif
> +
>  #include "qemu/mmap-alloc.h"
> 
>  #ifdef CONFIG_DEBUG_STACK_USAGE
> @@ -366,6 +370,15 @@ void qemu_init_exec_dir(const char *argv0)
>              p = buf;
>          }
>      }
> +#elif defined(__APPLE__)
> +    {
> +        uint32_t len;
> +        len = proc_pidpath(getpid(), buf, sizeof(buf) - 1);

Hi David,

proc_pidpath handler in the xnu kernel is indirectly calling
build_path_with_parent [1] and it includes NULL terminator into
buffersize, i.e. the patch seems to limit path length to one less
character than OS allows.

> +        if (len > 0) {
> +            buf[len] = 0;
> +            p = buf;
> +        }
> +    }
>  #endif
>      /* If we don't have any way of figuring out the actual executable
>         location then try argv[0].  */
> -- 
> 2.26.2
> 


The change looks okay but it's not clear why it is needed [2].

Also, libproc.h [3] has this comment:

/*
 * This header file contains private interfaces to obtain process information.  
 * These interfaces are subject to change in future releases.
 */

But iTerm2 [4] an Chromium [5] are using it. An alternative (if it works
and IMO less appealing) would be to retrieve process path using AppKit [6].

1. 
https://opensource.apple.com/source/xnu/xnu-6153.81.5/bsd/vfs/vfs_cache.c.auto.html
2. 
https://wiki.qemu.org/Contribute/SubmitAPatch#Write_a_meaningful_commit_message
3. 
https://opensource.apple.com/source/xnu/xnu-6153.81.5/libsyscall/wrappers/libproc/libproc.h.auto.html
4. 
https://gitlab.com/gnachman/iterm2/blob/872e3d63fbcaf7b4235c4f3b7273e09a7ba4c1d1/sources/iTermLSOF.m#L31
5. 
https://github.com/chromium/chromium/blob/2ca8c5037021c9d2ecc00b787d58a31ed8fc8bcb/base/process/process_handle_mac.cc#L31
6. https://developer.apple.com/documentation/appkit/nsworkspace?language=objc

Regards,
Roman