Re: [PATCH 4/8] linux_dep: Fix CLOSE_RANGE_UNSHARE availability handling

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 06, 2024 at 08:12:23PM +0100, Stefano Brivio wrote:
> On Wed,  6 Nov 2024 17:54:17 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > If CLOSE_RANGE_UNSHARE isn't defined, we define a fallback version of
> > close_range() which is a (successful) no-op.  This is broken in several
> > ways:
> >  * It doesn't actually fix compile if using old kernel headers, because
> >    the caller of close_range() still directly uses CLOSE_RANGE_UNSHARE
> >    unprotected by ifdefs
> 
> For users using distribution packages, that is, pretty much everybody
> not developing passt, this is generally not a concern, because build
> environments typically ship kernel headers matching the C library and
> the distribution version at hand.

Unlike some of the other things fixed recently, this one is not
related to compile time versus runtime checks.  This one is simply
broken compiling against an older kernel, regardless of the runtime
version.  Without this patch, close_open_files() directly uses
CLOSE_RANGE_UNSHARE unprotected by an #ifdef.

> >  * Even if it did fix the compile, it means inconsistent behaviour between
> >    a compile time failure to find the value (we silently don't close files)
> >    and a runtime failure (we die with an error from close_range())
> 
> Given that this is mostly relevant for stuff built against musl (and
> running on a musl-based distribution), that's not really a problem in
> practice. See 6e9ecf57410b ("util: Provide own version of close_range(),
> and no-op fallback").
> 
> But sure, I'm fine with these changes in general, as they're strictly
> speaking more correct than the current behaviour, minus the next point.
> 
> >  * Silently not closing the files we intend to close for security reasons
> >    is probably not a good idea in any case
> 
> It's arguably even worse to force users to run containers or guests as
> root. That's the reason for the no-op implementation.

Uh... what's the connection to running as root?

> I don't think that introducing a dependency on a >= 5.9 kernel is a
> good idea.

We already have a dependency on compiling against a >= 5.9 kernel, see
above.

> If this bothers you or cppcheck, I'd rather call close_range()
> (possibly the weakly aliased implementation), and log a warning on
> ENOSYS instead of failing.

We could do that.  We'd need to consider EINVAL as well as ENOSYS, for
the case that the syscall exists, but the CLOSE_RANGE_UNSHARE flag
isn't recognized.

> > As bonus this fixes a cppcheck error I see with some different options I'm
> > looking to apply in future.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  linux_dep.h | 12 ++++--------
> >  1 file changed, 4 insertions(+), 8 deletions(-)
> > 
> > diff --git a/linux_dep.h b/linux_dep.h
> > index 3a41e42..240f50a 100644
> > --- a/linux_dep.h
> > +++ b/linux_dep.h
> > @@ -127,22 +127,18 @@ struct tcp_info_linux {
> >  
> >  #include <linux/close_range.h>
> >  
> > -#ifdef CLOSE_RANGE_UNSHARE	/* Linux kernel >= 5.9 */
> >  /* glibc < 2.34 and musl as of 1.2.5 need these */
> >  #ifndef SYS_close_range
> >  #define SYS_close_range		436
> >  #endif
> > +#ifndef CLOSE_RANGE_UNSHARE	/* Linux kernel < 5.9 */
> > +#define CLOSE_RANGE_UNSHARE	(1U << 1)
> > +#endif
> > +
> >  __attribute__ ((weak))
> >  /* cppcheck-suppress funcArgNamesDifferent */
> >  int close_range(unsigned int first, unsigned int last, int flags) {
> >  	return syscall(SYS_close_range, first, last, flags);
> >  }
> > -#else
> > -/* No reasonable fallback option */
> > -/* cppcheck-suppress funcArgNamesDifferent */
> > -int close_range(unsigned int first, unsigned int last, int flags) {
> > -	return 0;
> > -}
> > -#endif
> >  
> >  #endif /* LINUX_DEP_H */
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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