On 10/9/2018 11:04 AM, Kamil Rytarowski wrote:
On 09.10.2018 16:12, Peter Maydell wrote:
On 9 October 2018 at 14:52, Kamil Rytarowski <n...@gmx.com> wrote:
On 07.10.2018 17:37, Brad Smith wrote:
Use MAP_STACK in qemu_alloc_stack() on OpenBSD.
Added to -current and will be in our soon to be 6.4 release.
MAP_STACK Indicate that the mapping is used as a stack. This
flag must be used in combination with MAP_ANON and
MAP_PRIVATE.
Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.
Signed-off-by: Brad Smith <b...@comstyle.com>
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index fbd0dc8c57..51e9a012c2 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -611,7 +611,11 @@ void *qemu_alloc_stack(size_t *sz)
*sz += pagesz;
ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE,
- MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+ MAP_PRIVATE | MAP_ANONYMOUS
+#ifdef MAP_STACK
+ | MAP_STACK
+#endif
+ , -1, 0);
if (ptr == MAP_FAILED) {
perror("failed to allocate memory for stack");
abort();
Can we handle it differently, storing MAP_* flags in a variable:
int flags = MAP_PRIVATE | MAP_ANONYMOUS;
#ifdef MAP_STACK
flags |= MAP_STACK;
#endif
ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE, flags, -1, 0);
This way it will look nicer as we won't ifdef the middle of a function call.
The other nice way to handle that is to have osdep.h do
#ifndef MAP_STACK
#define MAP_STACK 0
#endif
and then you can just unconditionally use MAP_STACK in your
expression for the mmap flags.
I assume that this is a cleaner solution.
I note that Linux also defines a MAP_STACK, about which the
manpage says:
MAP_STACK (since Linux 2.6.27)
Allocate the mapping at an address suitable for a
process or thread stack.
This flag is currently a no-op, but is used in the
glibc threading imple‐
mentation so that if some architectures require special
treatment for stack
allocations, support can later be transparently
implemented for glibc.
So this patch would be opting Linux QEMU builds into whatever that
potential future behaviour change is. That sounds I guess like
it's more likely to be the right thing than the wrong thing,
but it would be useful if some Linux expert could confirm...
thanks
-- PMM
There is a similar description on NetBSD:
MAP_STACK
Allocate a memory segment that can be used
either for a process or thread stack. This
currently has no effect, but its use is
reserved for architectures that might require
special treatment of that address space.
Unimplemented.
Apparently OpenBSD started to overload it for some hardening.
And from FreeBSD...
MAP_STACK MAP_STACK implies MAP_ANON, and /offset/ of 0. The/fd/
argument must be -1 and /prot/ must include at least
PROT_READ and PROT_WRITE.
This option creates a memory region that grows to at
most/len/ bytes in size, starting from the stack
top
and growing down. The stack top is the starting
address returned by the call, plus/len/ bytes. The
bottom of the stack at maximum growth is the starting
address returned by the call.
Stacks created with MAP_STACK automatically grow.
Guards prevent inadvertent use of the regions into
which those stacks can grow without requiring mapping
the whole stack in advance.
