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Linux 2.6 - man page for rtld-audit (linux section 7)

RTLD-AUDIT(7)			    Linux Programmer's Manual			    RTLD-AUDIT(7)

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 applica-
       tion 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 con-
       stants 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  imple-
       mentation 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  fur-
       ther  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 activ-
       ity  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  argu-
       ment  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 identi-
       fier 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  moni-
       tored 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  ref-
       erenced	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_pltenter() 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 val-
       ues  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 argu-
	  ment.

       *  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 = %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;
       }

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

COLOPHON
       This page is part of release 3.55 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/.

Linux					    2012-07-07				    RTLD-AUDIT(7)


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