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NAME

       rtld-audit - auditing API for the dynamic linker

SYNOPSIS

       #define _GNU_SOURCE

       #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.  If necessary, the auditing library can check that
       this version is sufficient for its requirements.

       As  its function result, this function should return the version of the
       auditing interface that this auditing library expects to use (returning
       version  is acceptable).  If the returned value is 0, or a version that
       is greater than that supported by the dynamic linker,  then  the  audit
       library is ignored.

   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_name.  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 int *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 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_pltenter()
       function  is only called 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.

CONFORMING TO

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

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).

EXAMPLE

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

       unsigned int
       la_version(unsigned int version)
       {
           printf("la_version(): %d\n", version);

           return version;
       }

       char *
       la_objsearch(const char *name, uintptr_t *cookie, unsigned int flag)
       {
           printf("la_objsearch(): name = %s; cookie = %x", 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 = %x; 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=%x\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(): %x\n", cookie);

           return 0;
       }

       void
       la_preinit(uintptr_t *cookie)
       {
           printf("la_preinit(): %x\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 = %d; flags = 0x%x", ndx, *flags);
           printf("; refcook = %x; defcook = %x\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 = %d; flags = 0x%x", ndx, *flags);
           printf("; refcook = %x; defcook = %x\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 int *framesizep)
       {
           printf("la_i86_gnu_pltenter(): %s (%p)\n", symname, sym->st_value);

           return sym->st_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.

SEE ALSO

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

COLOPHON

       This page is part of release 3.21 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.