The auxiliary vector (a.k.a. auxv) is a list of relevant kernel information (e.g. program fd, RUID, EUID, …) inserted by the loader at the bottom of the stack.
Auxiliary Vector Entry Struct:
typedef struct
{
uint32_t a_type; /* Entry type */
union
{
uint32_t a_val; /* Integer value */
/* We use to have pointer elements added here. We cannot do that,
though, since it does not work when using 32-bit definitions
on 64-bit platforms and vice versa. */
} a_un;
} Elf32_auxv_t;where a_type may be one of:
// snippet in arch/ia64/include/uapi/asm/auxvec.h
/*
* Architecture-neutral AT_ values are in the range 0-17. Leave some room for more of
* them, start the architecture-specific ones at 32.
*/
#define AT_SYSINFO 32
#define AT_SYSINFO_EHDR 33
#define AT_VECTOR_SIZE_ARCH 2 /* entries in ARCH_DLINFO */
// snippet in include/uapi/linux/auxvec.h
/* Symbolic values for the entries in the auxiliary table
put on the initial stack */
#define AT_NULL 0 /* end of vector */
#define AT_IGNORE 1 /* entry should be ignored */
#define AT_EXECFD 2 /* file descriptor of program */
#define AT_PHDR 3 /* program headers for program */
#define AT_PHENT 4 /* size of program header entry */
#define AT_PHNUM 5 /* number of program headers */
#define AT_PAGESZ 6 /* system page size */
#define AT_BASE 7 /* base address of interpreter */
#define AT_FLAGS 8 /* flags */
#define AT_ENTRY 9 /* entry point of program */
#define AT_NOTELF 10 /* program is not ELF */
#define AT_UID 11 /* real uid */
#define AT_EUID 12 /* effective uid */
#define AT_GID 13 /* real gid */
#define AT_EGID 14 /* effective gid */
#define AT_PLATFORM 15 /* string identifying CPU for optimizations */
#define AT_HWCAP 16 /* arch dependent hints at CPU capabilities */
#define AT_CLKTCK 17 /* frequency at which times() increments */
/* AT_* values 18 through 22 are reserved */
#define AT_SECURE 23 /* secure mode boolean */
#define AT_BASE_PLATFORM 24 /* string identifying real platform, may
* differ from AT_PLATFORM. */
#define AT_RANDOM 25 /* address of 16 random bytes */
#define AT_HWCAP2 26 /* extension of AT_HWCAP */
#define AT_EXECFN 31 /* filename of program */
#ifndef AT_MINSIGSTKSZ
#define AT_MINSIGSTKSZ 51 /* minimal stack size for signal delivery */
#endif