rtld-audit(7) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | EXAMPLES | 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.

       A typical implementation of this function simply returns the
       constant LAV_CURRENT, which indicates 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 further checks
       because it could correspond to an interface that does not match
       the <link.h> definitions 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_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 *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.

EXAMPLES         top

       #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         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2020-11-01                  RTLD-AUDIT(7)

Pages that refer to this page: dlopen(3)ld.so(8)