rtld-audit


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 pro-
       vided by the Solaris run-time linker.  The necessary constants and pro-
       totypes 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 inter-
       ested 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.
       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  func-
       tion, 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.

       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 argu-
       ment identifies the shared object that defines the  referenced  symbol;
       this  is the same identifier that is provided to the la_objopen() func-
       tion 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 (Pro-
       cedure Linkage Table) entry.  The dynamic linker may supply the follow-
       ing 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 alter-
                             nate value for this symbol.

       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>) contain-
       ing 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_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>)  con-
       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 out-
          regs 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_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.
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