execvpe limits PATH environment variable to PATH_MAX

[DJ Delorie]

Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
> And I don't think it would be a good practice to use a different
> semantic for execvpe, i.e, allow arbitrary size paths.

Does the standard allow arbitrary sized environment strings?  If so, we
should not limit our implementation. (it doesn't but it doesn't limit
$PATH to PATH_MAX either)

Also, PATH_MAX has nothing to do with environment strings - it's the
maximum size of a filesystem pathname.  Let's not abuse it.

If anything, we should use ARG_MAX [1][2].  I will note that PATH_MAX is
4,096 on my system, but ARG_MAX is 131,072.  That might break alloca.

[E2BIG]
  The number of bytes used by the new process image's argument list and
  environment list is greater than the system-imposed limit of {ARG_MAX}
  bytes.

The only mention of PATH_MAX in exec*() is

[ENAMETOOLONG]
  The length of the path argument or the length of the pathname
  constructed from the file argument exceeds {PATH_MAX}, or pathname
  resolution of a symbolic link produced an intermediate result with a
  length that exceeds {PATH_MAX}.

... which only refers to the pathname to the image to be exec'd, not the
arguments or environment.
  
[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html

[2] https://pubs.opengroup.org/onlinepubs/009695399/basedefs/limits.h.html
{ARG_MAX}
  Maximum length of argument to the exec functions including environment
  data.  Minimum Acceptable Value: {_POSIX_ARG_MAX}