[Buildroot] Allowing user to run ldconfig in post-build script

[Eric Le Bihan <eric.le.bihan.dev@free.fr>]

Le Thu, 16 Jun 2016 22:28:54 +0200,
Thomas Petazzoni <thomas.petazzoni@free-electrons.com> a ?crit :

> On Thu, 16 Jun 2016 21:45:22 +0200, Eric Le Bihan wrote:
> 
> > > > Not being able to run ldconfig and generate the ld.so cache has
> > > > two drawbacks:
> > > > 
> > > >  - it prevents the user from installing some libraries in other
> > > > locations than /lib and /usr/lib (e.g. /opt/foo/lib). This can
> > > > be solved with symlinks, though.      
> > > 
> > > Does the dynamic linker uses /etc/ld.so.conf at runtime to find
> > > other libraries, even if there is no /etc/ld.so.cache? If that's
> > > the case, then our check for ld.so.conf being absent is somewhat
> > > wrong, as it would be valid to have, independently of whether
> > > ldconfig has created ld.so.cache or not.    
> > 
> > Inspecting the glibc source code shows that:
> > 
> >  - /etc/ld.so.conf is only read by ldconfig (see elf/ldconfig.c). 
> >  - /etc/ld.so.cache is only read by the dynamic loader (see
> > elf/dl-cache.c).  
> 
> OK.
> 
> > This is confirmed when using strace. So, IMHO, this sanity check is
> > of no use.  
> 
> If what you say above is true, then your conclusion is completely
> wrong: the sanity check is *absolutely* mandatory.
> 
> Since Buildroot does *NOT* generate ld.so.cache, and ld.so.conf is not
> read by the dynamic loader, having a ld.so.conf in a Buildroot
> filesystem is a mistake, as the user might thing that the dynamic
> linker will search in the directories listed in ld.so.conf, but it is
> not true: the dynamic linker only searches in the libraries referenced
> in ld.so.cache, and ignores ld.so.conf.

OK. So the check is needed to get a clean target root file system,
devoid of files which may mislead the user. However, it makes sense to
provide a configuration entry to disable it, for the ld.so-savvy users
who want to create the cache on their own, no?

-- 
ELB