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.
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.
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.
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.
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.
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.
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).
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.
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*().
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(): 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;
}
ldd(1),
dlopen(3),
ld.so(8),
ldconfig(8)