.\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk .\" .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" 2009-01-12, mtk, Created .\" .TH RTLD-AUDIT 7 2015-12-05 "Linux" "Linux Programmer's Manual" .SH NAME rtld-audit \- auditing API for the dynamic linker .SH SYNOPSIS .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .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 . 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 .BR 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 .PP 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. 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 .I 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. .SS la_objsearch() \& .nf .BI "char *la_objsearch(const char *" name ", uintptr_t *" cookie , .BI " unsigned int " flag ); .fi .PP 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). .PP 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 .PP 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 .PP 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). .PP .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. .PP 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. .PP 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 .PP 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 (). 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 .PP 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 .PP 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. 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 . Among the fields of this structure, .I st_value indicates the address to which the symbol is bound. The .I ndx argument gives the index of the symbol in the symbol table of the bound shared object. 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 . The .I symname argument points a string containing the name of the symbol. 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. .PP 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 .IR *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. .PP 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 .) Here is the definition for x86-32: .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 int *" framesizep ); .fi This function is invoked just before a PLT entry is called, between two shared objects that have been marked for binding notification. The .IR sym , .IR ndx , .IR refcook , .IR defcook , and .IR symname are as for .BR la_symbind* (). The .I regs argument points to a structure (defined in .IR ) containing the values of registers to be used for the call to this PLT entry. 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* (). .\" FIXME . Is the following correct? The .IR framesizep argument points to a .IR "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. 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 .) Here is the definition for x86-32: .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 .PP 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. .PP The .IR sym , .IR ndx , .IR refcook , .IR defcook , and .IR symname are as for .BR la_symbind* (). The .I inregs argument points to a structure (defined in .IR ) containing the values of registers used for the call to this PLT entry. The .I outregs argument points to a structure (defined in .IR ) 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 .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 CONFORMING TO This API is nonstandard, but very similar to the Solaris API, described in the Solaris .IR "Linker and Libraries Guide" , in the chapter .IR "Runtime Linker Auditing Interface" . .SH NOTES Note the following differences from the Solaris dynamic linker auditing API: .IP * 3 The Solaris .BR la_objfilter () interface is not supported by the GNU implementation. .IP * The Solaris .BR la_symbind32 () and .BR la_pltexit () functions do not provide a .I symname argument. .IP * The Solaris .BR la_pltexit () function does not provide .I inregs and .I outregs arguments (but does provide a .IR 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 EXAMPLE .nf #include #include 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; } .fi .SH SEE ALSO .BR ldd (1), .BR dlopen (3), .BR ld.so (8), .BR ldconfig (8) .SH COLOPHON This page is part of release 4.10 of the Linux .I 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/.