[RFC,v8,03/20] elf/dl-object.c: Implement a helper function to proxy link_map entries

  Provides the minimal functionality needed to take an existing
link_map entry and create a proxy for it in the specified namespace.
---
 elf/dl-object.c            | 84 ++++++++++++++++++++++++++++++++++++++
 sysdeps/generic/ldsodefs.h |  5 +++
 2 files changed, 89 insertions(+)
  

On 09/02/2021 14:18, Vivek Das Mohapatra via Libc-alpha wrote:
> Provides the minimal functionality needed to take an existing
> link_map entry and create a proxy for it in the specified namespace.

This patch also should be combined with the 4th of the set, even though
it would incur a larger patch.

> ---
>  elf/dl-object.c            | 84 ++++++++++++++++++++++++++++++++++++++
>  sysdeps/generic/ldsodefs.h |  5 +++
>  2 files changed, 89 insertions(+)
> 
> diff --git a/elf/dl-object.c b/elf/dl-object.c
> index 1875599eb2..1a9d35c96b 100644
> --- a/elf/dl-object.c
> +++ b/elf/dl-object.c
> @@ -21,6 +21,7 @@
>  #include <stdlib.h>
>  #include <unistd.h>
>  #include <ldsodefs.h>
> +#include <libintl.h>
>  
>  #include <assert.h>
>  
> @@ -50,6 +51,89 @@ _dl_add_to_namespace_list (struct link_map *new, Lmid_t nsid)
>    __rtld_lock_unlock_recursive (GL(dl_load_write_lock));
>  }
>  
> +/* Proxy an existing link map entry into a new link map:
> +   This is based on _dl_new_object, skipping the steps we know we won't need
> +   because this is mostly just a shell for the l_real pointer holding the real
> +   link map entry (normally l == l->l_real, but not for ld.so in non-main
> +   link maps or RTLD_SHARED proxies).
> +   It also flags the proxy by setting l_proxy, and sets the the no-delete
> +   flag in the original if it is an lt_loaded.  */
> +struct link_map *
> +_dl_new_proxy (struct link_map *old, int mode, Lmid_t nsid)
> +{
> +  const char *name;
> +  struct link_map *new;
> +  struct libname_list *newname;
> +#ifdef SHARED
> +  unsigned int na = GLRO(dl_naudit);
> +
> +  if ((mode & __RTLD_OPENEXEC) != 0)
> +    na = DL_NNS;
> +
> +  size_t audit_space = na * sizeof (struct auditstate);
> +#else
> +# define audit_space 0
> +#endif
> +
> +  name = old->l_name;
> +
> +  /* Find the original link map entry if `old' is itself a proxy. */
> +  while (old && old->l_proxy)
> +    old = old->l_real;

No implicit chech (even though there are copies from older implementations):

  while (old != NULL && old->l_proxy)

I am not sure though about bit-fields checks, I think implicit check as
booleans should be fine.

> +
> +  if (old == NULL)
> +    _dl_signal_error (EINVAL, name, NULL, N_("cannot proxy NULL link_map"));
> +
> +  /* Object already exists in the target namespace. This should get handled

Double space after period.

> +     by dl_open_worker but just in case we get this far, handle it:  */
> +  if (__glibc_unlikely (old->l_ns == nsid))
> +    {
> +      /* Not actually possible, given the sanity checks above.  */
> +      if (old->l_proxy)
> +        return old;

Not sure if this check make sense, from the 'while' above 'old' will be
either null (which will be bail out with the _dl_signal_error above)
or old->l_proxy would not be true.

> +
> +      _dl_signal_error (EEXIST, name, NULL,
> +                        N_("existing object cannot be demoted to a proxy"));
> +    }
> +
> +  /* Now duplicate as little of _dl_new_object as possible to get a
> +     working proxied object in the target link map.  */
> +  new = (struct link_map *) calloc (sizeof (*new) + audit_space
> +                                    + sizeof (struct link_map *)
> +                                    + sizeof (*newname) + PATH_MAX, 1);
> +
> +  if (new == NULL)
> +      _dl_signal_error (ENOMEM, name, NULL,
> +                        N_("cannot create shared object descriptor"));

Indentation seems of here.

> +
> +  /* Specific to the proxy.  */
> +  new->l_real = old;
> +  new->l_proxy = 1;
> +  new->l_ns = nsid;
> +
> +  /* Copied from the origin.  */
> +  new->l_libname = old->l_libname;
> +  new->l_name = old->l_name;
> +  /* Proxies are considered lt_loaded if the real entry type is lt_library.  */
> +  new->l_type = (old->l_type == lt_library) ? lt_loaded : old->l_type;
> +
> +  if (__glibc_unlikely (mode & RTLD_NODELETE))
> +    new->l_flags_1 |= DF_1_NODELETE;
> +
> +  /* Specific to the origin. Ideally we'd do some accounting here but

Double space after period.

> +     for now it's easier to pin the original so the proxy remains valid.  */
> +  if (old->l_type == lt_loaded)
> +      old->l_flags_1 |= DF_1_NODELETE;

Indentation seems of here.

> +
> +  /* Fix up the searchlist so that relocations work.  */
> +  _dl_map_object_deps (new, NULL, 0, 0,
> +		       mode & (__RTLD_DLOPEN | RTLD_DEEPBIND | __RTLD_AUDIT));
> +
> +  /* And finally put the proxy in the target namespace.  */
> +  _dl_add_to_namespace_list (new, nsid);
> +
> +  return new;
> +}
>  
>  /* Allocate a `struct link_map' for a new object being loaded,
>     and enter it into the _dl_loaded list.  */
> diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
> index 9720a4e446..f58b4ac49f 100644
> --- a/sysdeps/generic/ldsodefs.h
> +++ b/sysdeps/generic/ldsodefs.h
> @@ -974,6 +974,11 @@ extern lookup_t _dl_lookup_symbol_x (const char *undef,
>  				     struct link_map *skip_map)
>       attribute_hidden;
>  
> +/* Proxy an existing link map entry into a new link map */
> +extern struct link_map *_dl_new_proxy (struct link_map *old,
> +				       int mode,
> +				       Lmid_t nsid)
> +     attribute_hidden;
>  
>  /* Restricted version of _dl_lookup_symbol_x.  Searches MAP (and only
>     MAP) for the symbol UNDEF_NAME, with GNU hash NEW_HASH (computed
>