Re: [PATCHv10 man-pages 5/5] execveat.2: initial man page for execveat(2)

[Rich Felker]

On Mon, Jan 12, 2015 at 11:33:49AM +0000, David Drysdale wrote:
> On Sat, Jan 10, 2015 at 1:33 AM, Rich Felker <dalias@xxxxxxxxxx> wrote:
> > On Fri, Jan 09, 2015 at 07:17:41PM -0600, Eric W. Biederman wrote:
> >> Rich Felker <dalias@xxxxxxxxxx> writes:
> >>
> >> > I'm not proposing code because I'm a libc developer not a kernel
> >> > developer. I know what's needed for userspace to provide a conforming
> >> > fexecve to applications, not how to implement that on the kernel side,
> >> > although I'm trying to provide constructive ideas. The hostility is
> >> > really not necessary.
> >>
> >> Conforming to what?
> >>
> >> The open group fexecve says nothing about requiring a file descriptor
> >> passed to fexecve to have O_CLOEXEC.
> >
> > It doesn't require it but it allows it, and in multithreaded programs
> > that might run child processes (or library code that might be used in
> > such situations), O_CLOEXEC is mandatory everywhere to avoid fd leaks.
> 
> As a naive idea related to Andy's suggestion elsewhere, could you
> just have an environment convention for fexecve-ing scripts?  That
> would reduce FD leaks without any need for kernel involvement/changes.
> 
> For example, set _FEXECVED_VIA_FD=4 but don't set
> O_CLOEXEC before fexecve, and the interpreter reads then
> closes that FD.  Or just get the interpreter to spot scripts named
> "/dev/fd/%d" and read-then-close the FD that way, cf. Eric's suggestion
> at https://lkml.org/lkml/2014/10/22/652.

No. Any omission of O_CLOEXEC even momentarily is a potentially
dangerous fd leak. This is the case whenever the process is
multithreaded and it's possible that other threads might fork and
exec. Think of the case of a privileged daemon re-execing itself (e.g.
to switch to an updated version) while there are potentially other
threads spawning non-privileged processes.

Rich
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