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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:


uint_t la_version(uint_t version);
la_version()

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

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 returns a version of zero, or a value greater than the rtld-audit interface the runtime linker supports, the audit library will not be used.


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

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

lmp provides the link-map structure describing the new object. lmid identifies the link-map list to which the object has been added (see "Establishing a Name-space"). cookie provides a pointer to an identifer. This identifier is initialized to the objects lmp, but can be modified by the audit library to better identify the object to other rtld-audit interface routines

This function returns a value indicating the symbol bindings of interest for this object, which will result in later calls to la_symbind32(). 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 la_symbind() 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.


void la_preinit(uintptr_t * cookie);
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.

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


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);
la_symbind32(), la_symbind64()

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

sym is a constructed symbol structure (see /usr/include/sys/elf.h), whose sym->st_value indicates the address of the symbol definition being bound. la_symbind32() has the sym->st_name adjusted to point to the actual symbol name, while la_symbind64() does not adjust sym->st_name and it is an index into the 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 as passed to the la_objopen() 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 be used to modify the continued auditing of procedure linkage table symbol entries. This value is a mask of the following flags defined in /usr/include/link.h:

sym_name point to the string name for the symbol being bound (for la_symbind64 only).

  • LA_SYMB_NOPLTENTER -- The la_*_pltenter() function will not be called for this symbol.

  • LA_SYMB_NOPLTEXIT -- The la_pltexit() function will not be called for this symbol.

  • LA_SYMB_DLSYM -- The symbol binding occurred as a result of calling dlsym(3X).

    • LA_SYMB_ALTVALUE (LAV_VERSION2) -- An alternate value was returned for the symbol value by a previous call to la_symbind32/la_symbind64.

By default, if the la_*_pltenter() or la_pltexit() functions exist within the audit library, these will be called (after la_symbind32()) for procedure linkage table symbols 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.

The sym_name parameter to la_symbind64() contains the name of the symbol being processed, which is available in the sym->st_name field from the 32-bit interfaces.


uint_t la_sparcv8_pltenter(Elf32_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook,
        La_sparcv8_regs * regs, uint_t * flags);
 
uint_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);
 
uint_t la_i86_pltenter(Elf32_Sym * sym, uint_t ndx,
        uintptr_t * refcook, uintptr_t * defcook,
        La_i86_regs * regs, uint_t * flags);
la_sparcv8_pltenter(), la_i86_pltenter(), la_sparcv9_pltenter()

These functions are called on a SPARC and x86 system respectively, when a procedure linkage symbol entry between two objects that have been tagged for binding notification is called (see la_objopen() and la_symbind32()).

sym, ndx, refcook, defcook and sym_name provide the same information as passed to la_symbind32()/la_symbind64().

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 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_symbind32(). This value is a mask of the following flags defined in /usr/include/link.h:

  • LA_SYMB_NOPLTENTER -- The la_sparcv8_pltenter() or la_i86_pltenter() function will not be called subsequently for this symbol.

  • LA_SYMB_NOPLTEXIT -- The la_pltexit() function will not be called for this symbol.

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.

The sym_name parameter to la_sparcv9_pltenter() contains the name of the symbol being processed, which is available in the sym->st_name field from the 32-bit interfaces.


uint_t la_pltexit(Elf32_Sym * sym, uint_t ndx, uintptr_t * refcook,
        uintptr_t * defcook, uintptr_t retval);
uint_t la_pltexit64(Elf64_Sym * sym, uint_t ndx, uintptr_t * refcook,
        uintptr_t * defcook, uintptr_t retval, const char * sym_name);
la_pltexit()

This function is called when a procedure linkage symbol entry between two objects that have been tagged for binding notification (see la_objopen() and la_symbind32()) returns, but before control reaches the caller.

sym, ndx, refcook and defcook provide the same information as passed to la_symbind32(). retval is the return code from the bound function. The sym_name parameter to la_pltexit64() contains the name of the symbol being processed, and is available from the sym->st_name field in the 32-bit implementation.

An audit library that simply monitors symbol binding should return retval. An audit library can intentionally return a different value.

Note -

This interface function is experimental; see "Audit Interface Limitations".


uint_t la_objclose(uintptr_t * cookie);
la_objclose()

This function is called after any termination code for an object has been executed (see "Initialization and Termination Routines") and prior to the object being unloaded.

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