.\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2009-01-12, mtk, Created
.\"
.TH RTLD-AUDIT 7 2024-06-15 "Linux man-pages (unreleased)"
.SH NAME
rtld\-audit \- auditing API for the dynamic linker
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <link.h>
.fi
.SH DESCRIPTION
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
.IR <link.h> .
.P
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.
.P
To employ the auditing interface, the environment variable
.B 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.
.SS la_version()
\&
.nf
.BI "unsigned int la_version(unsigned int " version );
.fi
.P
This is the only function that
.I 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
.IR version ,
the highest version of the auditing interface that the linker supports.
.P
A typical implementation of this function simply returns the constant
.BR LAV_CURRENT ,
which indicates the version of
.I <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.
.P
In order to enable backwards compatibility with older dynamic linkers,
an audit module can examine the
.I version
argument and return an earlier version than
.BR LAV_CURRENT ,
assuming the module can adjust its implementation to match the
requirements of the previous version of the audit interface.
The
.B la_version
function should not return the value of
.I version
without further checks because it could correspond to an interface
that does not match the
.I <link.h>
definitions used to build the audit module.
.SS la_objsearch()
\&
.nf
.BI "char *la_objsearch(const char *" name ", uintptr_t *" cookie ,
.BI "                   unsigned int " flag );
.fi
.P
The dynamic linker invokes this function to inform the auditing library
that it is about to search for a shared object.
The
.I name
argument is the filename or pathname that is to be searched for.
.I cookie
identifies the shared object that initiated the search.
.I flag
is set to one of the following values:
.TP 17
.B LA_SER_ORIG
This is the original name that is being searched for.
Typically, this name comes from an ELF
.B DT_NEEDED
entry, or is the
.I filename
argument given to
.BR dlopen (3).
.TP
.B LA_SER_LIBPATH
.I name
was created using a directory specified in
.BR LD_LIBRARY_PATH .
.TP
.B LA_SER_RUNPATH
.I name
was created using a directory specified in an ELF
.B DT_RPATH
or
.B DT_RUNPATH
list.
.TP
.B LA_SER_CONFIG
.I name
was found via the
.BR ldconfig (8)
cache
.RI ( /etc/ld.so.cache ).
.TP
.B LA_SER_DEFAULT
.I name
was found via a search of one of the default directories.
.TP
.B LA_SER_SECURE
.I name
is specific to a secure object (unused on Linux).
.P
As its function result,
.BR 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
.I name
should be returned.
.SS la_activity()
\&
.nf
.BI "void la_activity( uintptr_t *" cookie ", unsigned int "flag  );
.fi
.P
The dynamic linker calls this function to inform the auditing library
that link-map activity is occurring.
.I cookie
identifies the object at the head of the link map.
When the dynamic linker invokes this function,
.I flag
is set to one of the following values:
.TP 19
.B LA_ACT_ADD
New objects are being added to the link map.
.TP
.B LA_ACT_DELETE
Objects are being removed from the link map.
.TP
.B LA_ACT_CONSISTENT
Link-map activity has been completed: the map is once again consistent.
.SS la_objopen()
\&
.nf
.BI "unsigned int la_objopen(struct link_map *" map ", Lmid_t " lmid ,
.BI "                        uintptr_t *" cookie );
.fi
.P
The dynamic linker calls this function when a new shared object is loaded.
The
.I map
argument is a pointer to a link-map structure that describes the object.
The
.I lmid
field has one of the following values
.TP 17
.B LM_ID_BASE
Link map is part of the initial namespace.
.TP
.B LM_ID_NEWLM
Link map is part of a new namespace requested via
.BR dlmopen (3).
.P
.I 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.
.P
As its return value,
.BR 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
.BR la_symbind* ():
.TP 17
.B LA_FLG_BINDTO
Audit symbol bindings to this object.
.TP
.B LA_FLG_BINDFROM
Audit symbol bindings from this object.
.P
A return value of 0 from
.BR la_objopen ()
indicates that no symbol bindings should be audited for this object.
.SS la_objclose()
\&
.nf
.BI "unsigned int la_objclose(uintptr_t *" cookie );
.fi
.P
The dynamic linker invokes this function after any finalization
code for the object has been executed,
before the object is unloaded.
The
.I cookie
argument is the identifier obtained from a previous invocation of
.BR la_objopen ().
.P
In the current implementation, the value returned by
.BR la_objclose ()
is ignored.
.SS la_preinit()
\&
.nf
.BI "void la_preinit(uintptr_t *" cookie );
.fi
.P
The dynamic linker invokes this function after all shared objects
have been loaded, before control is passed to the application
(i.e., before calling
.IR main ()).
Note that
.IR main ()
may still later dynamically load objects using
.BR dlopen (3).
.SS la_symbind*()
\&
.nf
.BI "uintptr_t la_symbind32(Elf32_Sym *" sym ", unsigned int " ndx ,
.BI "                       uintptr_t *" refcook ", uintptr_t *" defcook ,
.BI "                       unsigned int *" flags ", const char *" symname );
.BI "uintptr_t la_symbind64(Elf64_Sym *" sym ", unsigned int " ndx ,
.BI "                       uintptr_t *" refcook ", uintptr_t *" defcook ,
.BI "                       unsigned int *" flags ", const char *" symname );
.fi
.P
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
.BR la_objopen ().
The
.BR la_symbind32 ()
function is employed on 32-bit platforms;
the
.BR la_symbind64 ()
function is employed on 64-bit platforms.
.P
The
.I sym
argument is a pointer to a structure
that provides information about the symbol being bound.
The structure definition is shown in
.IR <elf.h> .
Among the fields of this structure,
.I st_value
indicates the address to which the symbol is bound.
.P
The
.I ndx
argument gives the index of the symbol in the symbol table
of the bound shared object.
.P
The
.I refcook
argument identifies the shared object that is making the symbol reference;
this is the same identifier that is provided to the
.BR la_objopen ()
function that returned
.BR LA_FLG_BINDFROM .
The
.I defcook
argument identifies the shared object that defines the referenced symbol;
this is the same identifier that is provided to the
.BR la_objopen ()
function that returned
.BR LA_FLG_BINDTO .
.P
The
.I symname
argument points a string containing the name of the symbol.
.P
The
.I 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_STRUCTCALL appears to be unused
.TP 22
.B LA_SYMB_DLSYM
The binding resulted from a call to
.BR dlsym (3).
.TP
.B LA_SYMB_ALTVALUE
A previous
.BR la_symbind* ()
call returned an alternate value for this symbol.
.P
By default, if the auditing library implements
.BR la_pltenter ()
and
.BR la_pltexit ()
functions (see below), then these functions are invoked, after
.BR la_symbind (),
for PLT entries, each time the symbol is referenced.
.\" pltenter/pltexit are called for non-dynamically loaded libraries,
.\" but don't seem to be called for dynamically loaded libs?
.\" Is this the same on Solaris?
The following flags can be ORed into
.I *flags
to change this default behavior:
.TP 22
.B LA_SYMB_NOPLTENTER
Don't call
.BR la_pltenter ()
for this symbol.
.TP 22
.B LA_SYMB_NOPLTEXIT
Don't call
.BR la_pltexit ()
for this symbol.
.P
The return value of
.BR la_symbind32 ()
and
.BR 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
.IR sym\->st_value .
A different value may be returned if the library wishes to direct control
to an alternate location.
.SS la_pltenter()
The precise name and argument types for this function
depend on the hardware platform.
(The appropriate definition is supplied by
.IR <link.h> .)
Here is the definition for x86-32:
.P
.nf
.BI "Elf32_Addr la_i86_gnu_pltenter(Elf32_Sym *" sym ", unsigned int " ndx ,
.BI "                 uintptr_t *" refcook ", uintptr_t *" defcook ,
.BI "                 La_i86_regs *" regs ", unsigned int *" flags ,
.BI "                 const char *" symname ", long *" framesizep );
.fi
.P
This function is invoked just before a PLT entry is called,
between two shared objects that have been marked for binding notification.
.P
The
.IR sym ,
.IR ndx ,
.IR refcook ,
.IR defcook ,
and
.I symname
are as for
.BR la_symbind* ().
.P
The
.I regs
argument points to a structure (defined in
.IR <link.h> )
containing the values of registers to be used for
the call to this PLT entry.
.P
The
.I 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
.BR la_symbind* ().
.P
.\" FIXME . Is the following correct?
The
.I framesizep
argument points to a
.I long\~int
buffer that can be used to explicitly set the frame size
used for the call to this PLT entry.
If different
.BR la_pltenter ()
invocations for this symbol return different values,
then the maximum returned value is used.
The
.BR la_pltexit ()
function is called only if this buffer is
explicitly set to a suitable value.
.P
The return value of
.BR la_pltenter ()
is as for
.BR la_symbind* ().
.SS la_pltexit()
The precise name and argument types for this function
depend on the hardware platform.
(The appropriate definition is supplied by
.IR <link.h> .)
Here is the definition for x86-32:
.P
.nf
.BI "unsigned int la_i86_gnu_pltexit(Elf32_Sym *" sym ", unsigned int " ndx ,
.BI "                 uintptr_t *" refcook ", uintptr_t *" defcook ,
.BI "                 const La_i86_regs *" inregs ", La_i86_retval *" outregs ,
.BI "                 const char *" symname );
.fi
.P
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.
.P
The
.IR sym ,
.IR ndx ,
.IR refcook ,
.IR defcook ,
and
.I symname
are as for
.BR la_symbind* ().
.P
The
.I inregs
argument points to a structure (defined in
.IR <link.h> )
containing the values of registers used for the call to this PLT entry.
The
.I outregs
argument points to a structure (defined in
.IR <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.
.P
In the current GNU implementation, the return value of
.BR la_pltexit ()
is ignored.
.\" This differs from Solaris, where an audit library that monitors
.\" symbol binding should return the value of the 'retval' argument
.\" (not provided by GNU, but equivalent to returning outregs->lrv_eax
.\" on (say) x86-32).
.SH VERSIONS
This API is very similar to the Solaris API
described in the Solaris
.IR "Linker and Libraries Guide" ,
in the chapter
.IR "Runtime Linker Auditing Interface" .
.SH STANDARDS
None.
.SH NOTES
Note the following differences from the Solaris dynamic linker
auditing API:
.IP \[bu] 3
The Solaris
.BR la_objfilter ()
interface is not supported by the GNU implementation.
.IP \[bu]
The Solaris
.BR la_symbind32 ()
and
.BR la_pltexit ()
functions do not provide a
.I symname
argument.
.IP \[bu]
The Solaris
.BR la_pltexit ()
function does not provide
.I inregs
and
.I outregs
arguments (but does provide a
.I retval
argument with the function return value).
.SH BUGS
In glibc versions up to and include 2.9,
specifying more than one audit library in
.B LD_AUDIT
results in a run-time crash.
This is reportedly fixed in glibc 2.10.
.\" FIXME . Specifying multiple audit libraries doesn't work on GNU.
.\" My simple tests on Solaris work okay, but not on Linux -- mtk, Jan 2009
.\" glibc bug filed: http://sourceware.org/bugzilla/show_bug.cgi?id=9733
.\" Reportedly, this is fixed on 16 Mar 2009 (i.e., for glibc 2.10)
.SH EXAMPLES
.EX
#include <link.h>
#include <stdio.h>
\&
unsigned int
la_version(unsigned int version)
{
    printf("la_version(): version = %u; LAV_CURRENT = %u\[rs]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\[rs]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\[rs]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 \[rs]"%s\[rs]"; lmid = %s; cookie=%p\[rs]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\[rs]n", cookie);
\&
    return 0;
}
\&
void
la_preinit(uintptr_t *cookie)
{
    printf("la_preinit(): %p\[rs]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\[rs]n",
            symname, sym\->st_value);
    printf("        ndx = %u; flags = %#x", ndx, *flags);
    printf("; refcook = %p; defcook = %p\[rs]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\[rs]n",
            symname, sym\->st_value);
    printf("        ndx = %u; flags = %#x", ndx, *flags);
    printf("; refcook = %p; defcook = %p\[rs]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)\[rs]n", symname, sym\->st_value);
\&
    return sym\->st_value;
}
.EE
.SH SEE ALSO
.BR ldd (1),
.BR dlopen (3),
.BR ld.so (8),
.BR ldconfig (8)