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NAME | SYNOPSIS | DESCRIPTION | VERSIONS | STANDARDS | NOTES | BUGS | EXAMPLES | SEE ALSO |
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RTLD-AUDIT(7) Miscellaneous Information 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.
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.
This API is very similar to the Solaris API described in the
Solaris Linker and Libraries Guide, in the chapter Runtime Linker
Auditing Interface.
None.
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)
Linux man-pages (unreleased) (date) RTLD-AUDIT(7)
Pages that refer to this page: dlopen(3), ld.so(8)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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