UXP uses the current stack frame address and the stack size
as a sort of heuristic for various things in the JavaScript
engine. The js::GetNativeStackBaseImpl() function is used to
get the base stack address (i.e. the address from which the stack
grows, so this can be either the first or last memory address of
the stack memory space depending on the CPU architecture).
On Linux, this function is implemented using the pthreads APIs.
For non-main threads, the queried thread info is stored in
memory. The main thread does not have this information on hand,
so it gets the stack memory range via the /proc/self/maps file
(see glibc's pthread_get_attr_np.c).
Fortunately (per discussions with the firefox devs in #jsapi)
the base address only needs to be approximate. In reality,
environment variables, args, and other things are stored in
stack space between the end/beginning of the mapped stack
memory and the 'top' of the stack space used by stack frames.
When using glibc, we can get the top of this usable stack from
__libc_stack_end, which is a void* set by glibc during program
initialization, avoiding the need to access /proc.
Non-main threads still get their stack-base through the usual
pthreads APIs.
Other libc implementations like musl will fall back to the
standard UNIX-like implementation which calls pthread's
pthread_attr_getstack() also from the main thread, which
may imply /proc access and not work in restricted
environments.
