1RTLD-AUDIT(7)              Linux Programmer's Manual             RTLD-AUDIT(7)
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NAME

6       rtld-audit - auditing API for the dynamic linker
7

SYNOPSIS

9       #define _GNU_SOURCE             /* See feature_test_macros(7) */
10       #include <link.h>
11

DESCRIPTION

13       The  GNU dynamic linker (run-time linker) provides an auditing API that
14       allows an application to  be  notified  when  various  dynamic  linking
15       events  occur.  This API is very similar to the auditing interface pro‐
16       vided by the Solaris run-time linker.  The necessary constants and pro‐
17       totypes are defined by including <link.h>.
18
19       To  use  this  interface,  the programmer creates a shared library that
20       implements a standard set of function names.  Not all of the  functions
21       need  to be implemented: in most cases, if the programmer is not inter‐
22       ested in a particular class of auditing event, then  no  implementation
23       needs to be provided for the corresponding auditing function.
24
25       To  employ  the  auditing  interface, the environment variable LD_AUDIT
26       must be defined to contain a colon-separated list of shared  libraries,
27       each  of  which  can  implement  (parts  of) the auditing API.  When an
28       auditable event occurs, the corresponding function is invoked  in  each
29       library, in the order that the libraries are listed.
30
31   la_version()
32
33       unsigned int la_version(unsigned int version);
34
35       This  is the only function that must be defined by an auditing library:
36       it performs the initial handshake between the dynamic  linker  and  the
37       auditing  library.   When  invoking  this  function, the dynamic linker
38       passes, in version, the highest version of the auditing interface  that
39       the linker supports.  If necessary, the auditing library can check that
40       this version is sufficient for its requirements.
41
42       As its function result, this function should return the version of  the
43       auditing interface that this auditing library expects to use (returning
44       version is acceptable).  If the returned value is 0, or a version  that
45       is  greater  than  that supported by the dynamic linker, then the audit
46       library is ignored.
47
48   la_objsearch()
49
50       char *la_objsearch(const char *name, uintptr_t *cookie,
51                          unsigned int flag);
52
53       The dynamic linker invokes this function to inform the auditing library
54       that  it  is about to search for a shared object.  The name argument is
55       the filename or pathname that is to be searched for.  cookie identifies
56       the shared object that initiated the search.  flag is set to one of the
57       following values:
58
59       LA_SER_ORIG      This is the original name that is being searched  for.
60                        Typically,  this  name  comes  from  an  ELF DT_NEEDED
61                        entry, or is the filename argument given to dlopen(3).
62
63       LA_SER_LIBPATH   name  was  created  using  a  directory  specified  in
64                        LD_LIBRARY_PATH.
65
66       LA_SER_RUNPATH   name was created using a directory specified in an ELF
67                        DT_RPATH or DT_RUNPATH list.
68
69       LA_SER_CONFIG    name   was   found   via   the    ldconfig(8)    cache
70                        (/etc/ld.so.cache).
71
72       LA_SER_DEFAULT   name  was  found  via  a  search of one of the default
73                        directories.
74
75       LA_SER_SECURE    name is specific to a secure object (unused on Linux).
76
77       As its function result, la_objsearch() returns the  pathname  that  the
78       dynamic linker should use for further processing.  If NULL is returned,
79       then this pathname is ignored for further processing.   If  this  audit
80       library  simply  intends  to  monitor search paths, then name should be
81       returned.
82
83   la_activity()
84
85       void la_activity( uintptr_t *cookie, unsigned int flag);
86
87       The dynamic linker calls this function to inform the  auditing  library
88       that  link-map  activity is occurring.  cookie identifies the object at
89       the head of the link map.  When the dynamic linker invokes  this  func‐
90       tion, flag is set to one of the following values:
91
92       LA_ACT_ADD         New objects are being added to the link map.
93
94       LA_ACT_DELETE      Objects are being removed from the link map.
95
96       LA_ACT_CONSISTENT  Link-map  activity  has  been  completed: the map is
97                          once again consistent.
98
99   la_objopen()
100
101       unsigned int la_objopen(struct link_map *map, Lmid_t lmid,
102                               uintptr_t *cookie);
103
104       The dynamic linker calls this function when  a  new  shared  object  is
105       loaded.   The  map  argument  is a pointer to a link-map structure that
106       describes the object.  The lmid field has one of the following values
107
108       LM_ID_BASE       Link map is part of the initial namespace.
109
110       LM_ID_NEWLM      Link map is part of  a  new  namespace  requested  via
111                        dlmopen(3).
112
113       cookie  is  a pointer to an identifier for this object.  The identifier
114       is provided to later calls to functions  in  the  auditing  library  in
115       order to identify this object.  This identifier is initialized to point
116       to object's link map, but the audit library can change  the  identifier
117       to some other value that it may prefer to use to identify the object.
118
119       As  its  return value, la_objopen() returns a bit mask created by ORing
120       zero or more of the  following  constants,  which  allow  the  auditing
121       library to select the objects to be monitored by la_symbind*():
122
123       LA_FLG_BINDTO    Audit symbol bindings to this object.
124
125       LA_FLG_BINDFROM  Audit symbol bindings from this object.
126
127       A return value of 0 from la_objopen() indicates that no symbol bindings
128       should be audited for this object.
129
130   la_objclose()
131
132       unsigned int la_objclose(uintptr_t *cookie);
133
134       The dynamic linker invokes this function after  any  finalization  code
135       for  the  object has been executed, before the object is unloaded.  The
136       cookie argument is the identifier obtained from a  previous  invocation
137       of la_objopen().
138
139       In  the  current implementation, the value returned by la_objclose() is
140       ignored.
141
142   la_preinit()
143
144       void la_preinit(uintptr_t *cookie);
145
146       The dynamic linker invokes this function after all shared objects  have
147       been  loaded, before control is passed to the application (i.e., before
148       calling main()).  Note that main() may  still  later  dynamically  load
149       objects using dlopen(3).
150
151   la_symbind*()
152
153       uintptr_t la_symbind32(Elf32_Sym *sym, unsigned int ndx,
154                              uintptr_t *refcook, uintptr_t *defcook,
155                              unsigned int *flags, const char *symname);
156       uintptr_t la_symbind64(Elf64_Sym *sym, unsigned int ndx,
157                              uintptr_t *refcook, uintptr_t *defcook,
158                              unsigned int *flags, const char *symname);
159
160       The dynamic linker invokes one of these functions when a symbol binding
161       occurs between two shared objects that have been  marked  for  auditing
162       notification  by la_objopen().  The la_symbind32() function is employed
163       on 32-bit platforms; the la_symbind64() function is employed on  64-bit
164       platforms.
165
166       The  sym argument is a pointer to a structure that provides information
167       about the symbol being bound.  The structure  definition  is  shown  in
168       <elf.h>.   Among  the  fields of this structure, st_value indicates the
169       address to which the symbol is bound.
170
171       The ndx argument gives the index of the symbol in the symbol  table  of
172       the bound shared object.
173
174       The  refcook  argument  identifies the shared object that is making the
175       symbol reference; this is the same identifier that is provided  to  the
176       la_objopen() function that returned LA_FLG_BINDFROM.  The defcook argu‐
177       ment identifies the shared object that defines the  referenced  symbol;
178       this  is the same identifier that is provided to the la_objopen() func‐
179       tion that returned LA_FLG_BINDTO.
180
181       The symname argument points a string containing the name of the symbol.
182
183       The flags argument is a bit mask that both provides  information  about
184       the symbol and can be used to modify further auditing of this PLT (Pro‐
185       cedure Linkage Table) entry.  The dynamic linker may supply the follow‐
186       ing bit values in this argument:
187
188       LA_SYMB_DLSYM         The binding resulted from a call to dlsym(3).
189
190       LA_SYMB_ALTVALUE      A  previous la_symbind*() call returned an alter‐
191                             nate value for this symbol.
192
193       By default,  if  the  auditing  library  implements  la_pltenter()  and
194       la_pltexit()  functions  (see below), then these functions are invoked,
195       after la_symbind(), for PLT entries, each time  the  symbol  is  refer‐
196       enced.   The  following  flags  can  be ORed into *flags to change this
197       default behavior:
198
199       LA_SYMB_NOPLTENTER    Don't call la_pltenter() for this symbol.
200
201       LA_SYMB_NOPLTEXIT     Don't call la_pltexit() for this symbol.
202
203       The return value of la_symbind32() and la_symbind64() is the address to
204       which  control  should  be  passed  after the function returns.  If the
205       auditing library is simply monitoring symbol bindings, then  it  should
206       return sym->st_value.  A different value may be returned if the library
207       wishes to direct control to an alternate location.
208
209   la_pltenter()
210       The precise name and argument types for this  function  depend  on  the
211       hardware   platform.    (The  appropriate  definition  is  supplied  by
212       <link.h>.)  Here is the definition for x86-32:
213
214       Elf32_Addr la_i86_gnu_pltenter(Elf32_Sym *sym, unsigned int ndx,
215                        uintptr_t *refcook, uintptr_t *defcook,
216                        La_i86_regs *regs, unsigned int *flags,
217                        const char *symname, long int *framesizep);
218
219       This function is invoked just before a PLT entry is called, between two
220       shared objects that have been marked for binding notification.
221
222       The sym, ndx, refcook, defcook, and symname are as for la_symbind*().
223
224       The  regs argument points to a structure (defined in <link.h>) contain‐
225       ing the values of registers to be used for the call to this PLT entry.
226
227       The flags argument points to a bit mask that conveys information about,
228       and  can  be  used to modify subsequent auditing of, this PLT entry, as
229       for la_symbind*().
230
231       The framesizep argument points to a long int buffer that can be used to
232       explicitly  set the frame size used for the call to this PLT entry.  If
233       different la_pltenter() invocations for this  symbol  return  different
234       values,  then  the  maximum  returned  value is used.  The la_pltexit()
235       function is called only if this buffer is explicitly set to a  suitable
236       value.
237
238       The return value of la_pltenter() is as for la_symbind*().
239
240   la_pltexit()
241       The  precise  name  and  argument types for this function depend on the
242       hardware  platform.   (The  appropriate  definition  is   supplied   by
243       <link.h>.)  Here is the definition for x86-32:
244
245       unsigned int la_i86_gnu_pltexit(Elf32_Sym *sym, unsigned int ndx,
246                        uintptr_t *refcook, uintptr_t *defcook,
247                        const La_i86_regs *inregs, La_i86_retval *outregs,
248                        const char *symname);
249
250       This  function  is  called  when  a  PLT entry, made between two shared
251       objects that have been marked for binding notification,  returns.   The
252       function is called just before control returns to the caller of the PLT
253       entry.
254
255       The sym, ndx, refcook, defcook, and symname are as for la_symbind*().
256
257       The inregs argument points to a structure (defined  in  <link.h>)  con‐
258       taining  the  values  of registers used for the call to this PLT entry.
259       The outregs argument points to a structure (defined in  <link.h>)  con‐
260       taining return values for the call to this PLT entry.  These values can
261       be modified by the caller, and the changes will be visible to the call‐
262       er of the PLT entry.
263
264       In  the current GNU implementation, the return value of la_pltexit() is
265       ignored.
266

CONFORMING TO

268       This API is nonstandard, but very similar to the Solaris API, described
269       in  the  Solaris  Linker  and  Libraries  Guide, in the chapter Runtime
270       Linker Auditing Interface.
271

NOTES

273       Note the following differences from the Solaris dynamic linker auditing
274       API:
275
276       *  The  Solaris  la_objfilter()  interface  is not supported by the GNU
277          implementation.
278
279       *  The Solaris la_symbind32() and la_pltexit() functions do not provide
280          a symname argument.
281
282       *  The  Solaris  la_pltexit() function does not provide inregs and out‐
283          regs arguments (but does provide a retval argument with the function
284          return value).
285

BUGS

287       In glibc versions up to and include 2.9, specifying more than one audit
288       library in LD_AUDIT results in a run-time crash.   This  is  reportedly
289       fixed in glibc 2.10.
290

EXAMPLE

292       #include <link.h>
293       #include <stdio.h>
294
295       unsigned int
296       la_version(unsigned int version)
297       {
298           printf("la_version(): %d\n", version);
299
300           return version;
301       }
302
303       char *
304       la_objsearch(const char *name, uintptr_t *cookie, unsigned int flag)
305       {
306           printf("la_objsearch(): name = %s; cookie = %p", name, cookie);
307           printf("; flag = %s\n",
308                   (flag == LA_SER_ORIG) ?    "LA_SER_ORIG" :
309                   (flag == LA_SER_LIBPATH) ? "LA_SER_LIBPATH" :
310                   (flag == LA_SER_RUNPATH) ? "LA_SER_RUNPATH" :
311                   (flag == LA_SER_DEFAULT) ? "LA_SER_DEFAULT" :
312                   (flag == LA_SER_CONFIG) ?  "LA_SER_CONFIG" :
313                   (flag == LA_SER_SECURE) ?  "LA_SER_SECURE" :
314                   "???");
315
316           return name;
317       }
318
319       void
320       la_activity (uintptr_t *cookie, unsigned int flag)
321       {
322           printf("la_activity(): cookie = %p; flag = %s\n", cookie,
323                   (flag == LA_ACT_CONSISTENT) ? "LA_ACT_CONSISTENT" :
324                   (flag == LA_ACT_ADD) ?        "LA_ACT_ADD" :
325                   (flag == LA_ACT_DELETE) ?     "LA_ACT_DELETE" :
326                   "???");
327       }
328
329       unsigned int
330       la_objopen(struct link_map *map, Lmid_t lmid, uintptr_t *cookie)
331       {
332           printf("la_objopen(): loading \"%s\"; lmid = %s; cookie=%p\n",
333                   map->l_name,
334                   (lmid == LM_ID_BASE) ?  "LM_ID_BASE" :
335                   (lmid == LM_ID_NEWLM) ? "LM_ID_NEWLM" :
336                   "???",
337                   cookie);
338
339           return LA_FLG_BINDTO | LA_FLG_BINDFROM;
340       }
341
342       unsigned int
343       la_objclose (uintptr_t *cookie)
344       {
345           printf("la_objclose(): %p\n", cookie);
346
347           return 0;
348       }
349
350       void
351       la_preinit(uintptr_t *cookie)
352       {
353           printf("la_preinit(): %p\n", cookie);
354       }
355
356       uintptr_t
357       la_symbind32(Elf32_Sym *sym, unsigned int ndx, uintptr_t *refcook,
358               uintptr_t *defcook, unsigned int *flags, const char *symname)
359       {
360           printf("la_symbind32(): symname = %s; sym->st_value = %p\n",
361                   symname, sym->st_value);
362           printf("        ndx = %d; flags = 0x%x", ndx, *flags);
363           printf("; refcook = %p; defcook = %p\n", refcook, defcook);
364
365           return sym->st_value;
366       }
367
368       uintptr_t
369       la_symbind64(Elf64_Sym *sym, unsigned int ndx, uintptr_t *refcook,
370               uintptr_t *defcook, unsigned int *flags, const char *symname)
371       {
372           printf("la_symbind64(): symname = %s; sym->st_value = %p\n",
373                   symname, sym->st_value);
374           printf("        ndx = %d; flags = 0x%x", ndx, *flags);
375           printf("; refcook = %p; defcook = %p\n", refcook, defcook);
376
377           return sym->st_value;
378       }
379
380       Elf32_Addr
381       la_i86_gnu_pltenter(Elf32_Sym *sym, unsigned int ndx,
382               uintptr_t *refcook, uintptr_t *defcook, La_i86_regs *regs,
383               unsigned int *flags, const char *symname, long int *framesizep)
384       {
385           printf("la_i86_gnu_pltenter(): %s (%p)\n", symname, sym->st_value);
386
387           return sym->st_value;
388       }
389

SEE ALSO

391       ldd(1), dlopen(3), ld.so(8), ldconfig(8)
392

COLOPHON

394       This  page  is  part of release 4.15 of the Linux man-pages project.  A
395       description of the project, information about reporting bugs,  and  the
396       latest     version     of     this    page,    can    be    found    at
397       https://www.kernel.org/doc/man-pages/.
398
399
400
401Linux                             2017-09-15                     RTLD-AUDIT(7)
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