Crate auxv

Just what is the auxiliary vector?

The auxiliary vector (aka auxv) is some memory near the start of a running ELF program’s stack. Specifically, it’s a sequence of pairs of either 64 bit or 32 bit unsigned ints. The two components of the pair form a key and a value. This data is mostly there to help things like runtime linkers, but sometimes it’s useful for other reasons. It is ELF-specific; it does not exist in, say, Mach-O.

On most Unixy systems, you can have the linker print out the contents of the aux vector by setting an environment variable when running a command like LD_SHOW_AUXV=1 cat /dev/null.

The keys used in the aux vector are defined in various header files and typically prefixed with AT_. Some of the data there is not available from any other source, like AT_HWCAP and AT_HWCAP2. These expose bit vectors of architecture-dependent hardware capability information. On ARM, for instance, the bit 1 << 12 in the value for AT_HWCAP will be set if the CPU supports NEON, and 1 << 3 will be set in the value for AT_HWCAP2 if the CPU supports SHA-256 acceleration. Handy, if you’re doing that sort of thing.

Other keys are typically not used directly by programs, like AT_UID: the real user id is great and all, but you’d pobably call getuid(2) in C or libc::getuid from Rust instead.

For most people, probably the most interesting data in auxv is for AT_HWCAP or AT_HWCAP2 so those have constants defined in auxv, but you can of course use any other key as well; you’ll just have to look up the appropriate number.

More info on the auxiliary vector:

• http://articles.manugarg.com/aboutelfauxiliaryvectors.html
• http://phrack.org/issues/58/5.html
• See include/uapi/linux/auxvec.h in the Linux source (or getauxval(3)) for defined keys, as well as other header files for architecture-specific types.
• See fs/binfmt_elf.c in the Linux source for how the vector is generated.
• Searching for AT_ in your OS of choice is likely to yield some good leads on the available constants and how it’s generated.

Reading the auxiliary vector

Unfortunately, there is no one best option for how to access the aux vector.

• getauxval(3) is available in glibc 2.16+, Android libc (Bionic) since Android 4,3, and musl 1.1.0+. Since it is a non-standard extension, if you’re not using those libc implementations (e.g. you’re using uclibc, etc), this will not be available. Also, if you’re on glibc older than 2.19, or Bionic before March 2015, getauxval is unable to express the concept of “not found” and will instead “find” the value 0.
• /proc/self/auxv exposes the contents of the aux vector, but it only exists on Linux. Furthermore, the OS may be configured to not allow access to it (see proc(5)).
• Navigating the ELF stack layout manually is also (sometimes) possible. There isn’t a standardized way of jumping directly to auxv in the stack, but we can start at the environ pointer (which is specified in POSIX) and navigate from there. This will work on any ELF OS, but it is unsafe and only is possible if the environment has not been modified since the process started.

This library lets you use all of these options, so chances are pretty good that at least one of them will work in any given host. See each submodule for details on how and when to use it.

For most users, it would be best practice to try the getauxval way first, and then try the procfs way if getauxval is not available at runtime. You should only try the stack crawling way if you are sure that it is safe; see its docs for details.

See the examples dir for examples of each way of accessing auxv.

Auxv type width

AuxvType is selected at compile time to be either u32 or u64 depending on the pointer width of the system. This type is used for the key and value.

Modules

getauxval

Read auxv entries one at a time via getauxval.

procfs

Read auxv entries via Linux procfs.

stack

Read auxv entries by chasing pointers in the ELF stack layout.

Structs

AuxvPair

An auxv key-value pair.

Constants

AT_HWCAP

AT_HWCAP2

Type Aliases

AuxvType

The type used in auxv keys and values.