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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.  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_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 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 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.

CONFORMING TO

       This  API  is  nonstandard,  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).

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.

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 = %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 = %d; flags = 0x%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 = %d; flags = 0x%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 int *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)

COLOPHON

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       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.