runtime: Fix GC bug caused by Entersyscall modifying reg.

This patch fixes a rare but serious bug.  The Go garbage
collector only examines Go stacks.  When Go code calls a
function that is not written in Go, it first calls
syscall.Entersyscall.  Entersyscall records the position of
the Go stack pointer and saves a copy of all the registers.
If the garbage collector runs while the thread is executing
the non-Go code, the garbage collector fetches the stack
pointer and registers from the saved location.

Entersyscall saves the registers using the getcontext
function.  Unfortunately I didn't consider the possibility
that Entersyscall might itself change a register before
calling getcontext.  This only matters for callee-saved
registers, as caller-saved registers would be visible on the
saved stack.  And it only matters if Entersyscall is compiled
to save and modify a callee-saved register before it calls
getcontext.  And it only matters if a garbage collection
occurs while the non-Go code is executing.  And it only
matters if the only copy of a valid Go pointer happens to be
in the callee-saved register when Entersyscall is called.
When all those conditions are true, the Go pointer might get
collected incorrectly, leading to memory corruption.

This patch tries to avoid the problem by splitting
Entersyscall into two functions.  The first is a simple
function that just calls getcontext and then calls the rest of
Entersyscall.  This should fix the problem, provided the
simple Entersyscall function does not itself modify any
callee-saved registers before calling getcontext.  That seems
to be true on the systems I checked.  But since the argument
to getcontext is an offset from a TLS variable, it won't be
true on a system which needs to save callee-saved registers in
order to get the address of a TLS variable.  I don't know why
any system would work that way, but I don't know how to rule
it out.  I think that on any such system this will have to be
implemented in assembler.  I can't put the ucontext_t
structure on the stack, because this function can not split
stacks, and the ucontext_t structure is large enough that it
could cause a stack overflow.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@208390 138bc75d-0d04-0410-961f-82ee72b054a4
	* sort.c: New file.
	* stest.c: New file.
	* internal.h (backtrace_qsort): Declare.
	* dwarf.c (read_abbrevs): Call backtrace_qsort instead of qsort.
	(read_line_info, read_function_entry): Likewise.
	(read_function_info, build_dwarf_data): Likewise.
	* elf.c (elf_initialize_syminfo): Likewise.
	* Makefile.am (libbacktrace_la_SOURCES): Add sort.c.
	(stest_SOURCES, stest_LDADD): Define.
	(check_PROGRAMS): Add stest.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@208392 138bc75d-0d04-0410-961f-82ee72b054a4