Audit Interface Functions (Linker and Libraries Guide)

Linker and Libraries Guide

Audit Interface Functions

The following functions are provided by the rtld-audit interface and are described in their expected order of use.

Note –

References to architecture, or object class specific interfaces are reduced to their generic name to simplify the discussions. For example, a reference to la_symbind32() and la_symbind64() is specified as la_symbind().

la_version()

This function provides the initial handshake between the runtime linker and the audit library. This interface must be provided for the audit library to be loaded.

uint_t la_version(uint_t version);

The runtime linker calls this interface with the highest version of the rtld-audit interface it is capable of supporting. The audit library can verify that this version is sufficient for its use, and return the version it expects to use. This version is normally LAV_CURRENT, which is defined in /usr/include/link.h.

If the audit library return is zero, or a version greater than the rtld-audit interface the runtime linker supports, the audit library is discarded.

la_activity()

This function informs an auditor that link-map activity is occurring.

void la_activity(uintptr_t * cookie, uint_t flags);

cookie identifies the object heading the link-map. flags indicates the type of activity as defined in /usr/include/link.h:

LA_ACT_ADD – Objects are being added to the link-map list.
LA_ACT_DELETE – Objects are being deleted from the link-map list.
LA_ACT_CONSISTENT – Object activity has been completed.

la_objsearch()

This function informs an auditor that an object is about to be searched for.

char * la_objsearch(const char * name, uintptr_t * cookie, uint_t flags);

name indicates the file or path name being searched for. cookie identifies the object initiating the search. flags identifies the origin and creation of name as defined in /usr/include/link.h:

LA_SER_ORIG – This is the initial search name. Typically, this indicates the file name that is recorded as a DT_NEEDED entry, or the argument supplied to dlmopen(3DL).
LA_SER_LIBPATH – The path name has been created from a LD_LIBRARY_PATH component.
LA_SER_RUNPATH – The path name has been created from a runpath component.
LA_SER_DEFAULT – The path name has been created from a default search path component.
LA_SER_CONFIG – The path component originated from a configuration file. See the crle(1) man page.
LA_SER_SECURE – The path component is specific to secure objects.

The return value indicates the search path name that the runtime linker should continue to process. A value of zero indicates that this path should be ignored. An audit library that simply monitors search paths should return name.

la_objopen()

This function is called each time a new object is loaded by the runtime linker.

uint_t la_objopen(Link_map * lmp, Lmid_t lmid, uintptr_t * cookie);

lmp provides the link-map structure that describes the new object. lmid identifies the link-map list to which the object has been added. cookie provides a pointer to an identifier. This identifier is initialized to the objects lmp. This identifier can be modified by the audit library to better identify the object to other rtld-audit interface routines

The la_objopen() function returns a value that indicates the symbol bindings of interest for this object. These values can result in later calls to la_symbind(). The return value is a mask of the following values defined in/usr/include/link.h:

LA_FLG_BINDTO – Audit symbol bindings to this object.
LA_FLG_BINDFROM – Audit symbol bindings from this object.

See the la_symbind() function for more details on the use of these two flags.

A return value of zero indicates that binding information is of no interest for this object.

la_preinit()

This function is called once after all objects have been loaded for the application, but before transfer of control to the application occurs.

void la_preinit(uintptr_t * cookie);

cookie identifies the primary object that started the process, normally the dynamic executable.

la_symbind()

This function is called when a binding occurs between two objects that have been tagged for binding notification from la_objopen().

uintptr_t la_symbind32(Elf32_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook, uint_t * flags);
 
uintptr_t la_symbind64(Elf64_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook, uint_t * flags,
        const char * sym_name);

sym is a constructed symbol structure, whose sym->st_value indicates the address of the symbol definition being bound. See /usr/include/sys/elf.h. la_symbind32() adjusts the sym->st_name to point to the actual symbol name. la_symbind64() leaves sym->st_name to be the index into the bound objects string table.

ndx indicates the symbol index within the bound object's dynamic symbol table. refcook describes the object making reference to this symbol. This identifier is the same identifier as passed to the la_objopen() function that returned LA_FLG_BINDFROM. defcook describes the object defining this symbol. This identifier is the same as passed to the la_objopen() that returned LA_FLG_BINDTO.

flags points to a data item that can convey information regarding the binding. This data item can be used to modify the continued auditing of procedure linkage table entries. This value is a mask of the following flags defined in /usr/include/link.h:

LA_SYMB_NOPLTENTER – The la_pltenter() function is not called for this symbol.
LA_SYMB_NOPLTEXIT – The la_pltexit() function is not called for this symbol.
LA_SYMB_DLSYM – The symbol binding occurred as a result of calling dlsym(3DL).
LA_SYMB_ALTVALUE (LAV_VERSION2) – An alternate value was returned for the symbol value by a previous call to la_symbind().

By default, if la_pltenter() or la_pltexit() functions exist, they are called after la_symbind() for procedure linkage table entries each time the symbol is referenced. See also Audit Interface Limitations.

The return value indicates the address to which control should be passed following this call. An audit library that simply monitors symbol binding should return the value of sym->st_value so that control is passed to the bound symbol definition. An audit library can intentionally redirect a symbol binding by returning a different value.

sym_name, which is applicable for la_symbind64() only, contains the name of the symbol being processed. This name is available in the sym->st_name field for the 32–bit interface.

la_pltenter()

These functions are called on a SPARC and x86 system respectively. These functions are called when a procedure linkage table entry, between two objects that have been tagged for binding notification, is called.

uintptr_t la_sparcv8_pltenter(Elf32_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook,
        La_sparcv8_regs * regs, uint_t * flags);
 
uintptr_t la_sparcv9_pltenter(Elf64_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook,
        La_sparcv9_regs * regs, uint_t * flags,
        const char * sym_name);
 
uintptr_t la_i86_pltenter(Elf32_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook,
        La_i86_regs * regs, uint_t * flags);

sym, ndx, refcook, defcook and sym_name provide the same information as passed to la_symbind().

regs points to the out registers on a SPARC system, and the stack and frame registers on a x86 system, as defined in /usr/include/link.h.

flags points to a data item that can convey information regarding the binding. This data can be used to modify the continuing auditing of this procedure linkage table entry. This data item is the same as pointed to by the flags from la_symbind(). This value is a mask of the following flags defined in /usr/include/link.h:

LA_SYMB_NOPLTENTER – la_pltenter() is not be called again for this symbol.
LA_SYMB_NOPLTEXIT – la_pltexit() is not be called for this symbol.

la_pltexit()

This function is called when a procedure linkage table entry, between two objects that have been tagged for binding notification, returns. This function is called before control reaches the caller.

uintptr_t la_pltexit(Elf32_Sym * sym, uint_t ndx, uintptr_t * refcook,
        uintptr_t * defcook, uintptr_t retval);

uintptr_t la_pltexit64(Elf64_Sym * sym, uint_t ndx, uintptr_t * refcook,
        uintptr_t * defcook, uintptr_t retval, const char * sym_name);

sym, ndx, refcook, defcook and sym_name provide the same information as passed to la_symbind(). retval is the return code from the bound function. An audit library that simply monitors symbol binding should return retval. An audit library can intentionally return a different value.

Note –

The la_pltexit() interface is experimental. See Audit Interface Limitations.

la_objclose()

This function is called after any termination code for an object has been executed and prior to the object being unloaded.

uint_t la_objclose(uintptr_t * cookie);

cookie identifies the object, and was obtained from a previous la_objopen(). Any return value is presently ignored.