Re: [PATCH v3 3/3] elf: Also try DT_RUNPATH for LD_AUDIT dlopen [BZ #28455]

[Carlos O'Donell <carlos@redhat.com>]

On 12/8/21 10:25, H.J. Lu wrote:
> DT_RUNPATH is only used to find the immediate dependencies of the
> executable or shared object containing the DT_RUNPATH entry.  Update
> the glibc LD_AUDIT dlopen call to try the DT_RUNPATH entry of the
> executable.  This partially fixes BZ #28455.

Please post v4 split out.

> ---
>  elf/Makefile       |  9 +++++++--
>  elf/dl-load.c      | 15 +++++++++++++++
>  elf/tst-audit14a.c |  1 +
>  3 files changed, 23 insertions(+), 2 deletions(-)
>  create mode 100644 elf/tst-audit14a.c
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index 4b5bdcb08e..aa6b24d790 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -247,10 +247,10 @@ ifneq ($(selinux-enabled),1)
>  tests-execstack-yes = tst-execstack tst-execstack-needed tst-execstack-prog
>  endif
>  ifeq ($(have-depaudit),yes)
> -tests += tst-audit14 tst-audit15 tst-audit16
> +tests += tst-audit14 tst-audit15 tst-audit16 tst-audit14a

Please split one test per line and sort.

>  ifeq ($(run-built-tests),yes)
>  tests-special += $(objpfx)tst-audit14-cmp.out $(objpfx)tst-audit15-cmp.out \
> -		 $(objpfx)tst-audit16-cmp.out
> +		 $(objpfx)tst-audit16-cmp.out $(objpfx)tst-audit14a-cmp.out

Please split one test per line and sort.

>  endif
>  endif
>  endif
> @@ -1521,6 +1521,8 @@ tst-auditmany-ENV = \
>  LDFLAGS-tst-audit14 = -Wl,--audit=tst-auditlogmod-1.so
>  $(objpfx)tst-auditlogmod-1.so: $(libsupport)
>  $(objpfx)tst-audit14.out: $(objpfx)tst-auditlogmod-1.so
> +LDFLAGS-tst-audit14a = -Wl,--audit=tst-auditlogmod-1.so,--enable-new-dtags
> +$(objpfx)tst-audit14a.out: $(objpfx)tst-auditlogmod-1.so

OK.

>  LDFLAGS-tst-audit15 = \
>    -Wl,--audit=tst-auditlogmod-1.so,--depaudit=tst-auditlogmod-2.so
>  $(objpfx)tst-auditlogmod-2.so: $(libsupport)
> @@ -1547,6 +1549,9 @@ tst-audit17-ENV = LD_AUDIT=$(objpfx)tst-auditmod17.so
>  $(objpfx)tst-audit14-cmp.out: tst-audit14.exp $(objpfx)tst-audit14.out
>  	cmp $^ > $@; \
>  	$(evaluate-test)
> +$(objpfx)tst-audit14a-cmp.out: tst-audit14.exp $(objpfx)tst-audit14a.out
> +	cmp $^ > $@; \
> +	$(evaluate-test)

OK.

>  $(objpfx)tst-audit15-cmp.out: tst-audit15.exp $(objpfx)tst-audit15.out
>  	cmp $^ > $@; \
>  	$(evaluate-test)
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index bf8957e73c..1f424ea7c8 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c

The preceding block is like this:

2146       /* When the object has the RUNPATH information we don't use any
2147          RPATHs.  */
2148       if (loader == NULL || loader->l_info[DT_RUNPATH] == NULL)
2149         {

The entire design of this block is to look into DT_RPATH specifically
under two conditions:

(a) We don't know anything about the loading module, it is NULL, and
    therefore we don't know if DT_RUNPATH was present, which if it
    was present, would prevent the use of DT_RPATH.

(b) We have a loader, and we know it doesn't have DT_RUNPATH and so
    we will look for DT_RPATH.

Your changes adds (c) to this case in which DT_RUNPATH is used, and
this seems to be logically distinct from the above two cases.

My suggestion is that the code you are adding should be in a distinct
block *after* the above block starting around line 2189.

We already have a block like this:
2189       /* Look at the RUNPATH information for this binary.  */
2190       if (fd == -1 && loader != NULL
2191           && cache_rpath (loader, &loader->l_runpath_dirs,
2192                           DT_RUNPATH, "RUNPATH"))
2193         fd = open_path (name, namelen, mode,
2194                         &loader->l_runpath_dirs, &realname, &fb, loader,
2195                         LA_SER_RUNPATH, &found_other_class);

I think it is logically more correct to add to this DT_RUNPATH handling
than ato add to the DT_RPATH handling above.

So after this block you might add:

/* When processing the lookup we may need to additionally try DT_RUNPATH
   in the executable for a glibc internal dlopen call when looking for
   audit modules.  */
if (__glibc_unlikely (mode & __RTLD_AUDIT)
    && fd == -1)
  {
    ...
  }

The hard part is that you may want to know a few things:
(a) main_map (unavailable, but computable via GL(dl_ns)[LM_ID_BASE]._ns_loaded)
(b) did_main_map (unavailable)
    * This seems like an optimization because if you already looked up via
      main_map and it failed, doing it twice won't help, so you can refactor
      did_main_map to function scope and retain the fact that you did this
      check already if you want to keep the optimization.

Otherwise everything else is there to implement the fix.

> @@ -2175,6 +2175,21 @@ _dl_map_object (struct link_map *loader, const char *name,
>  			    &main_map->l_rpath_dirs,
>  			    &realname, &fb, loader ?: main_map, LA_SER_RUNPATH,
>  			    &found_other_class);
> +
> +	  /* Also try DT_RUNPATH in the executable for glibc internal
> +	     dlopen call.  */
> +	  if (__glibc_unlikely (mode & __RTLD_AUDIT)
> +	      && fd == -1 && !did_main_map
> +	      && main_map != NULL && main_map->l_type != lt_loaded)
> +	    {
> +	      struct r_search_path_struct l_rpath_dirs;
> +	      l_rpath_dirs.dirs = NULL;
> +	      if (cache_rpath (main_map, &l_rpath_dirs,
> +			       DT_RUNPATH, "RUNPATH"))
> +		fd = open_path (name, namelen, mode, &l_rpath_dirs,
> +				&realname, &fb, loader ?: main_map,
> +				LA_SER_RUNPATH, &found_other_class);
> +	    }
>  	}
>  
>        /* Try the LD_LIBRARY_PATH environment variable.  */
> diff --git a/elf/tst-audit14a.c b/elf/tst-audit14a.c
> new file mode 100644
> index 0000000000..c6232eacf2
> --- /dev/null
> +++ b/elf/tst-audit14a.c
> @@ -0,0 +1 @@
> +#include "tst-audit14.c"
> 


-- 
Cheers,
Carlos.