NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       rtld-audit - auditing API for the dynamic linker

SYNOPSIS         top

       #define _GNU_SOURCE             /* See feature_test_macros(7) */

       #include <link.h>

DESCRIPTION         top

       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         top

       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         top

       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         top

       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         top

       #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         top

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

COLOPHON         top

       This page is part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2015-12-05                    RTLD-AUDIT(7)