Re: request for help with aux

[Rich Felker <dalias@libc.org>]

On Mon, Jun 27, 2016 at 01:07:59PM -0700, Daniel Wilkerson wrote:
> The musl crt0 code seems to expect that when the process starts that
> the stack pointer points to a data page having the following four data
> structures immediately contiguous: argc, argv, env, aux.

It's not necessarily a page. It's just an array of sorts that's
aligned to the (pointer) wordsize.

> I'm writing a loader for musl-riscv and I need to know how to
> initialize this data.  The only one I wonder about is the aux array.
> It seems that aux is a collection of name/value pairs which are used
> as follows in musl-riscv/src/env/__libc_start_main.c:

There are various places it's used; try something like:

	grep -r aux.*AT_ src ldso

For static linking only, at least AT_PHDR/AT_PHENT/AT_PHNUM need to be
correct in order for the executable's TLS image to be located at
runtime. AT_RANDOM should be valid (and point to a buffer of at least
16 random bytes IIRC) in order for stack protector to work correctly.
Other fields that are used if present but probably don't matter for
bare-metal setups you're looking at are AT_HWCAP, AT_SYSINFO_EHDR, and
AT_SECURE and the related uid/gid fields.

Rich