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NAME

       rtld-audit - auditing API for the dynamic linker

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <link.h>

DESCRIPTION

       The  GNU  dynamic  linker  (run-time  linker)  provides  an  auditing  API  that allows an
       application to be notified when various dynamic linking events occur.  This  API  is  very
       similar  to the auditing interface provided by the Solaris run-time linker.  The necessary
       constants and prototypes are defined by including <link.h>.

       To use this interface, the programmer creates a shared library that implements a  standard
       set of function names.  Not all of the functions need to be implemented: in most cases, if
       the programmer is not interested  in  a  particular  class  of  auditing  event,  then  no
       implementation needs to be provided for the corresponding auditing function.

       To  employ  the  auditing  interface, the environment variable LD_AUDIT must be defined to
       contain a colon-separated list of shared libraries, each of which can implement (parts of)
       the  auditing  API.  When an auditable event occurs, the corresponding function is invoked
       in each library, in the order that the libraries are listed.

   la_version()

       unsigned int la_version(unsigned int version);

       This is the only function that must be defined by an auditing  library:  it  performs  the
       initial handshake between the dynamic linker and the auditing library.  When invoking this
       function, the dynamic linker passes, in version,  the  highest  version  of  the  auditing
       interface that the linker supports.

       A  typical  implementation of this function simply returns the constant LAV_CURRENT, which
       indicates the version of <link.h> that was used to build the audit module.  If the dynamic
       linker  does  not  support this version of the audit interface, it will refuse to activate
       this audit module.  If the function  returns  zero,  the  dynamic  linker  also  does  not
       activate this audit module.

       In order to enable backwards compatibility with older dynamic linkers, an audit module can
       examine the version argument and return an earlier version than LAV_CURRENT, assuming  the
       module  can adjust its implementation to match the requirements of the previous version of
       the audit interface.  The la_version function should  not  return  the  value  of  version
       without further checks because it could correspond to an interface that does not match the
       <link.h> definitions used to build the audit module.

   la_objsearch()

       char *la_objsearch(const char *name, uintptr_t *cookie,
                          unsigned int flag);

       The dynamic linker invokes this function to inform the auditing library that it  is  about
       to  search  for a shared object.  The name argument is the filename or pathname that is to
       be searched for.  cookie identifies the shared object that initiated the search.  flag  is
       set to one of the following values:

       LA_SER_ORIG      This  is  the  original name that is being searched for.  Typically, this
                        name comes from an ELF DT_NEEDED entry, or is the filename argument given
                        to dlopen(3).

       LA_SER_LIBPATH   name was created using a directory specified in LD_LIBRARY_PATH.

       LA_SER_RUNPATH   name  was  created  using  a  directory  specified  in an ELF DT_RPATH or
                        DT_RUNPATH list.

       LA_SER_CONFIG    name was found via the ldconfig(8) cache (/etc/ld.so.cache).

       LA_SER_DEFAULT   name was found via a search of one of the default directories.

       LA_SER_SECURE    name is specific to a secure object (unused on Linux).

       As its function result, la_objsearch() returns the pathname that the dynamic linker should
       use  for  further  processing.   If  NULL  is  returned, then this pathname is ignored for
       further processing.  If this audit library simply intends to monitor  search  paths,  then
       name should be returned.

   la_activity()

       void la_activity( uintptr_t *cookie, unsigned int flag);

       The  dynamic  linker  calls  this  function  to  inform the auditing library that link-map
       activity is occurring.  cookie identifies the object at the head of the  link  map.   When
       the dynamic linker invokes this function, flag is set to one of the following values:

       LA_ACT_ADD         New objects are being added to the link map.

       LA_ACT_DELETE      Objects are being removed from the link map.

       LA_ACT_CONSISTENT  Link-map activity has been completed: the map is once again consistent.

   la_objopen()

       unsigned int la_objopen(struct link_map *map, Lmid_t lmid,
                               uintptr_t *cookie);

       The  dynamic  linker  calls  this  function  when  a new shared object is loaded.  The map
       argument is a pointer to a link-map structure that describes the object.  The  lmid  field
       has one of the following values

       LM_ID_BASE       Link map is part of the initial namespace.

       LM_ID_NEWLM      Link map is part of a new namespace requested via dlmopen(3).

       cookie is a pointer to an identifier for this object.  The identifier is provided to later
       calls to functions in the auditing  library  in  order  to  identify  this  object.   This
       identifier  is initialized to point to object's link map, but the audit library can change
       the identifier to some other value that it may prefer to use to identify the object.

       As its return value, la_objopen() returns a bit mask created by ORing zero or more of  the
       following  constants,  which  allow  the  auditing  library  to  select  the objects to be
       monitored by la_symbind*():

       LA_FLG_BINDTO    Audit symbol bindings to this object.

       LA_FLG_BINDFROM  Audit symbol bindings from this object.

       A return value of 0 from la_objopen() indicates that no symbol bindings should be  audited
       for this object.

   la_objclose()

       unsigned int la_objclose(uintptr_t *cookie);

       The  dynamic  linker  invokes this function after any finalization code for the object has
       been executed, before the object is unloaded.   The  cookie  argument  is  the  identifier
       obtained from a previous invocation of la_objopen().

       In the current implementation, the value returned by la_objclose() is ignored.

   la_preinit()

       void la_preinit(uintptr_t *cookie);

       The dynamic linker invokes this function after all shared objects have been loaded, before
       control is passed to the application (i.e., before calling main()).  Note that main()  may
       still later dynamically load objects using dlopen(3).

   la_symbind*()

       uintptr_t la_symbind32(Elf32_Sym *sym, unsigned int ndx,
                              uintptr_t *refcook, uintptr_t *defcook,
                              unsigned int *flags, const char *symname);
       uintptr_t la_symbind64(Elf64_Sym *sym, unsigned int ndx,
                              uintptr_t *refcook, uintptr_t *defcook,
                              unsigned int *flags, const char *symname);

       The dynamic linker invokes one of these functions when a symbol binding occurs between two
       shared objects that have been marked  for  auditing  notification  by  la_objopen().   The
       la_symbind32()  function  is  employed on 32-bit platforms; the la_symbind64() function is
       employed on 64-bit platforms.

       The sym argument is a pointer to a structure that provides information  about  the  symbol
       being  bound.   The  structure  definition  is shown in <elf.h>.  Among the fields of this
       structure, st_value indicates the address to which the symbol is bound.

       The ndx argument gives the index of the symbol in the symbol table  of  the  bound  shared
       object.

       The  refcook  argument  identifies  the shared object that is making the symbol reference;
       this is the same identifier that is provided to the la_objopen()  function  that  returned
       LA_FLG_BINDFROM.   The  defcook  argument  identifies  the  shared object that defines the
       referenced symbol; this is the same  identifier  that  is  provided  to  the  la_objopen()
       function that returned LA_FLG_BINDTO.

       The symname argument points a string containing the name of the symbol.

       The  flags  argument is a bit mask that both provides information about the symbol and can
       be used to modify further auditing of this  PLT  (Procedure  Linkage  Table)  entry.   The
       dynamic linker may supply the following bit values in this argument:

       LA_SYMB_DLSYM         The binding resulted from a call to dlsym(3).

       LA_SYMB_ALTVALUE      A  previous  la_symbind*() call returned an alternate value for this
                             symbol.

       By default, if the auditing library implements la_pltenter()  and  la_pltexit()  functions
       (see  below),  then these functions are invoked, after la_symbind(), for PLT entries, each
       time the symbol is referenced.  The following flags can be ORed into *flags to change this
       default behavior:

       LA_SYMB_NOPLTENTER    Don't call la_pltenter() for this symbol.

       LA_SYMB_NOPLTEXIT     Don't call la_pltexit() for this symbol.

       The  return  value  of  la_symbind32()  and la_symbind64() is the address to which control
       should be passed after the function returns.  If the auditing library is simply monitoring
       symbol  bindings,  then it should return sym->st_value.  A different value may be returned
       if the library wishes to direct control to an alternate location.

   la_pltenter()
       The precise name and argument types for this function depend  on  the  hardware  platform.
       (The appropriate definition is supplied by <link.h>.)  Here is the definition for x86-32:

       Elf32_Addr la_i86_gnu_pltenter(Elf32_Sym *sym, unsigned int ndx,
                        uintptr_t *refcook, uintptr_t *defcook,
                        La_i86_regs *regs, unsigned int *flags,
                        const char *symname, long *framesizep);

       This  function  is  invoked  just before a PLT entry is called, between two shared objects
       that have been marked for binding notification.

       The sym, ndx, refcook, defcook, and symname are as for la_symbind*().

       The regs argument points to a structure (defined in <link.h>)  containing  the  values  of
       registers to be used for the call to this PLT entry.

       The flags argument points to a bit mask that conveys information about, and can be used to
       modify subsequent auditing of, this PLT entry, as for la_symbind*().

       The framesizep argument points to a long int buffer that can be used to explicitly set the
       frame  size  used  for the call to this PLT entry.  If different la_pltenter() invocations
       for this symbol return different values, then the maximum returned  value  is  used.   The
       la_pltexit() function is called only if this buffer is explicitly set to a suitable value.

       The return value of la_pltenter() is as for la_symbind*().

   la_pltexit()
       The  precise  name  and  argument types for this function depend on the hardware platform.
       (The appropriate definition is supplied by <link.h>.)  Here is the definition for x86-32:

       unsigned int la_i86_gnu_pltexit(Elf32_Sym *sym, unsigned int ndx,
                        uintptr_t *refcook, uintptr_t *defcook,
                        const La_i86_regs *inregs, La_i86_retval *outregs,
                        const char *symname);

       This function is called when a PLT entry, made between two shared objects that  have  been
       marked  for  binding  notification,  returns.   The function is called just before control
       returns to the caller of the PLT entry.

       The sym, ndx, refcook, defcook, and symname are as for la_symbind*().

       The inregs argument points to a structure (defined in <link.h>) containing the  values  of
       registers used for the call to this PLT entry.  The outregs argument points to a structure
       (defined in <link.h>) containing return values for the call  to  this  PLT  entry.   These
       values can be modified by the caller, and the changes will be visible to the caller of the
       PLT entry.

       In the current GNU implementation, the return value of la_pltexit() is ignored.

VERSIONS

       This API is very similar to the Solaris API described in the Solaris Linker and  Libraries
       Guide, in the chapter Runtime Linker Auditing Interface.

STANDARDS

       None.

NOTES

       Note the following differences from the Solaris dynamic linker auditing API:

       •  The Solaris la_objfilter() interface is not supported by the GNU implementation.

       •  The  Solaris  la_symbind32()  and  la_pltexit()  functions  do  not  provide  a symname
          argument.

       •  The Solaris la_pltexit() function does not provide inregs and  outregs  arguments  (but
          does provide a retval argument with the function return value).

BUGS

       In  glibc  versions  up  to  and  include  2.9,  specifying more than one audit library in
       LD_AUDIT results in a run-time crash.  This is reportedly fixed in glibc 2.10.

EXAMPLES

       #include <link.h>
       #include <stdio.h>

       unsigned int
       la_version(unsigned int version)
       {
           printf("la_version(): version = %u; LAV_CURRENT = %u\n",
                   version, LAV_CURRENT);

           return LAV_CURRENT;
       }

       char *
       la_objsearch(const char *name, uintptr_t *cookie, unsigned int flag)
       {
           printf("la_objsearch(): name = %s; cookie = %p", name, cookie);
           printf("; flag = %s\n",
                   (flag == LA_SER_ORIG) ?    "LA_SER_ORIG" :
                   (flag == LA_SER_LIBPATH) ? "LA_SER_LIBPATH" :
                   (flag == LA_SER_RUNPATH) ? "LA_SER_RUNPATH" :
                   (flag == LA_SER_DEFAULT) ? "LA_SER_DEFAULT" :
                   (flag == LA_SER_CONFIG) ?  "LA_SER_CONFIG" :
                   (flag == LA_SER_SECURE) ?  "LA_SER_SECURE" :
                   "???");

           return name;
       }

       void
       la_activity (uintptr_t *cookie, unsigned int flag)
       {
           printf("la_activity(): cookie = %p; flag = %s\n", cookie,
                   (flag == LA_ACT_CONSISTENT) ? "LA_ACT_CONSISTENT" :
                   (flag == LA_ACT_ADD) ?        "LA_ACT_ADD" :
                   (flag == LA_ACT_DELETE) ?     "LA_ACT_DELETE" :
                   "???");
       }

       unsigned int
       la_objopen(struct link_map *map, Lmid_t lmid, uintptr_t *cookie)
       {
           printf("la_objopen(): loading \"%s\"; lmid = %s; cookie=%p\n",
                   map->l_name,
                   (lmid == LM_ID_BASE) ?  "LM_ID_BASE" :
                   (lmid == LM_ID_NEWLM) ? "LM_ID_NEWLM" :
                   "???",
                   cookie);

           return LA_FLG_BINDTO | LA_FLG_BINDFROM;
       }

       unsigned int
       la_objclose (uintptr_t *cookie)
       {
           printf("la_objclose(): %p\n", cookie);

           return 0;
       }

       void
       la_preinit(uintptr_t *cookie)
       {
           printf("la_preinit(): %p\n", cookie);
       }

       uintptr_t
       la_symbind32(Elf32_Sym *sym, unsigned int ndx, uintptr_t *refcook,
               uintptr_t *defcook, unsigned int *flags, const char *symname)
       {
           printf("la_symbind32(): symname = %s; sym->st_value = %p\n",
                   symname, sym->st_value);
           printf("        ndx = %u; flags = %#x", ndx, *flags);
           printf("; refcook = %p; defcook = %p\n", refcook, defcook);

           return sym->st_value;
       }

       uintptr_t
       la_symbind64(Elf64_Sym *sym, unsigned int ndx, uintptr_t *refcook,
               uintptr_t *defcook, unsigned int *flags, const char *symname)
       {
           printf("la_symbind64(): symname = %s; sym->st_value = %p\n",
                   symname, sym->st_value);
           printf("        ndx = %u; flags = %#x", ndx, *flags);
           printf("; refcook = %p; defcook = %p\n", refcook, defcook);

           return sym->st_value;
       }

       Elf32_Addr
       la_i86_gnu_pltenter(Elf32_Sym *sym, unsigned int ndx,
               uintptr_t *refcook, uintptr_t *defcook, La_i86_regs *regs,
               unsigned int *flags, const char *symname, long *framesizep)
       {
           printf("la_i86_gnu_pltenter(): %s (%p)\n", symname, sym->st_value);

           return sym->st_value;
       }

SEE ALSO

       ldd(1), dlopen(3), ld.so(8), ldconfig(8)