man > rtld-audit(7)

RTLD-AUDIT(7)                     Miscellaneous Information 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 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 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 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 con‐
       tain 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  ini‐
       tial handshake between the dynamic linker and the auditing library.  When invoking this func‐
       tion,  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  in‐
       dicates  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 fur‐
       ther checks because it could correspond to an interface that does not match the <link.h> def‐
       initions 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_RUN‐
                        PATH 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_sym‐
       bind32() 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 ob‐
       ject.

       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  refer‐
       enced  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 sym‐
                             bol.

       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 be‐
       havior:

       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  li‐
       brary 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 regis‐
       ters 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_pl‐
       texit() 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 reg‐
       isters  used for the call to this PLT entry.  The outregs argument points to a structure (de‐
       fined 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)

Linux man-pages 6.7                          2023-10-31                                RTLD-AUDIT(7)