[Mesa-dev] [PATCH v4 2/5] util: Get program name based on path when possible

[Eric Engestrom]

On Thursday, 2018-10-11 12:43:11 -0400, Nicholas Kazlauskas wrote:
> Some programs start with the path and command line arguments in
> argv[0] (program_invocation_name). Chromium is an example of
> an application using mesa that does this.
> 
> This tries to query the real path for the symbolic link /proc/self/exe
> to find the program name instead. It only uses the realpath if it
> was a prefix of the invocation to avoid breaking wine programs.
> 
> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
> ---
>  src/util/u_process.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/src/util/u_process.c b/src/util/u_process.c
> index 5e5927678d..cd16521ab3 100644
> --- a/src/util/u_process.c
> +++ b/src/util/u_process.c
> @@ -41,8 +41,24 @@ static const char *
>  __getProgramName()
>  {
>     char * arg = strrchr(program_invocation_name, '/');
> -   if (arg)
> +   if (arg) {
> +      /* If the / character was found this is likely a linux path or
> +       * an invocation path for a 64-bit wine program.
> +       *
> +       * However, some programs pass command line arguments into argv[0].
> +       * Strip these arguments out by using the realpath only if it was
> +       * a prefix of the invocation name.
> +       */
> +      static char *path;
> +
> +      if (!path)
> +         path = realpath("/proc/self/exe", NULL);
> +
> +      if (path && strncmp(path, program_invocation_name, strlen(path)) == 0)
> +         return path + (arg - program_invocation_name + 1);

You're doing pointer arithmetics with two unrelated strings here
(`program_invocation_name` and its substring `arg`, and `path`), which
looks really off.

I think I understand what you're trying to do (return the basename of
the realpath), but it would be much clearer if you used an additional
variable:

  if (...) {
    size_t basename_offset = arg + 1 - program_invocation_name;
    return path + basename_offset;
  }

We've also been walking these strings so many times by now that one more
time doesn't make any timing difference, but makes the code way more
intuitive:

  if (...)
    return strrchr(path, '/') + 1;

You can always cache it like you're caching `path` ;)

With either of those (preference for the latter):
Reviewed-by: Eric Engestrom <eric.engestrom at intel.com>

> +
>        return arg+1;
> +   }
>  
>     /* If there was no '/' at all we likely have a windows like path from
>      * a wine application.
> -- 
> 2.19.1
> 
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