[PATCH 0/4] Add support for SafeStack

[Daniele Buono <dbuono@linux.vnet.ibm.com>]

LLVM supports SafeStack instrumentation to protect against stack buffer
overflows, since version 3.7

From https://clang.llvm.org/docs/SafeStack.html:
"It works by separating the program stack into two distinct regions: the
safe stack and the unsafe stack. The safe stack stores return addresses,
register spills, and local variables that are always accessed in a safe
way, while the unsafe stack stores everything else. This separation
ensures that buffer overflows on the unsafe stack cannot be used to
overwrite anything on the safe stack."

Unfortunately, the use of two stack regions does not cope well with
QEMU's coroutines. The second stack region is not properly set up with
both ucontext and sigaltstack, so multiple coroutines end up sharing the
same memory area for the unsafe stack, causing undefined behaviors at
runtime (and most iochecks to fail).

This patch series fixes the implementation of the ucontext backend and
make sure that sigaltstack is never used if the compiler is applying
the SafeStack instrumentation. It also adds a configure flag to enable
SafeStack, and enables iotests when SafeStack is used.

This is an RFC mainly because of the low-level use of the SafeStack
runtime.
When running swapcontext(), we have to manually set the unsafe stack
pointer to the new area allocated for the coroutine. LLVM does not allow
this by using builtin, so we have to use implementation details that may
change in the future.
This patch has been tested briefly ( make check on an x86 system ) with
clang v3.9, v4.0, v5.0, v6.0
Heavier testing, with make check-acceptance has been performed with
clang v7.0

Daniele Buono (4):
  coroutine: support SafeStack in ucontext backend
  coroutine: Add check for SafeStack in sigalstack
  configure: add flag to enable SafeStack
  check-block: Enable iotests with SafeStack

 configure                    | 29 +++++++++++++++++++++++++++++
 include/qemu/coroutine_int.h |  6 ++++++
 tests/check-block.sh         | 12 +++++++++++-
 util/coroutine-sigaltstack.c |  4 ++++
 util/coroutine-ucontext.c    | 25 +++++++++++++++++++++++++
 5 files changed, 75 insertions(+), 1 deletion(-)

-- 
2.26.2