bug#12632: file permissions checking mishandled when setuid

[Eli Zaretskii]

> Date: Mon, 15 Oct 2012 14:38:05 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 12632@debbugs.gnu.org
> 
> On 10/15/2012 10:31 AM, Eli Zaretskii wrote:
> > Well, actually I thought we should stay with 'stat'.
> 
> We cannot stay with 'stat' everywhere, since 'stat' does not tell us
> whether a file is readable or writeable or executable.  We must use
> faccessat (or something like it, e.g., euidaccess) when we're
> implementing functions like check_writable, check_executable, or
> basically any function that uses R_OK, W_OK, or X_OK.

I was talking about check_existing.

> It's true that we could use 'stat' instead of faccessat(..., F_OK,
> ...), but the question then arises, why bother to make a special
> case for F_OK?

Because it's more universally available, and I believe also more
efficient.

> > So 'stat' is still better, IMO, because it is very efficient
> 
> Why should 'stat' be more efficient than faccessat?

Because it's used a lot.