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RTLD-AUDIT(7) Linux Programmer's Manual RTLD-AUDIT(7)
rtld-audit - auditing API for the dynamic linker
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <link.h>
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.
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.
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).
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.
#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;
}
ldd(1), dlopen(3), ld.so(8), ldconfig(8)
This page is part of release 4.12 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)
Pages that refer to this page: dlopen(3), ld.so(8)