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Re: [Qemu-devel] [PATCH V8 1/6] oslib-posix: add helpers for stack alloc

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH V8 1/6] oslib-posix: add helpers for stack alloc and free
Date: Mon, 26 Sep 2016 16:43:27 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

Am 26.09.2016 um 15:44 schrieb Kevin Wolf:
Am 26.09.2016 um 13:44 hat Peter Lieven geschrieben:
the allocated stack will be adjusted to the minimum supported stack size
by the OS and rounded up to be a multiple of the system pagesize.
Additionally an architecture dependent guard page is added to the stack
to catch stack overflows.

Signed-off-by: Peter Lieven <address@hidden>
  include/sysemu/os-posix.h | 27 +++++++++++++++++++++++++++
  util/oslib-posix.c        | 43 +++++++++++++++++++++++++++++++++++++++++++
  2 files changed, 70 insertions(+)

diff --git a/include/sysemu/os-posix.h b/include/sysemu/os-posix.h
index 9c7dfdf..4a0f493 100644
--- a/include/sysemu/os-posix.h
+++ b/include/sysemu/os-posix.h
@@ -60,4 +60,31 @@ int qemu_utimens(const char *path, const qemu_timespec 
bool is_daemonized(void); +/**
+ * qemu_alloc_stack:
+ * @sz: pointer to a size_t holding the requested stack size
+ *
+ * Allocate memory that can be used as a stack, for instance for
+ * coroutines. If the memory cannot be allocated, this function
+ * will abort (like g_malloc()). This function also inserts an
+ * additional guard page to catch a potential stack overflow.
+ * Note that the useable stack memory can be greater than the
+ * requested stack size due to alignment and minimal stack size
+ * restrictions. In this case the value of sz is adjusted.
+ *
+ * The allocated stack must be freed with qemu_free_stack().
+ *
+ * Returns: pointer to (the lowest address of) the stack memory.
Not quite. It's the pointer to the lowest address of the guard page,
while the returned stack size doesn't include the guard page. This is an
awkward interface, and consequently patch 3 fails to use it correctly.

So you end up with something like:

      **** **** ****

     G = guard page
     . = allocated stack page
     * = stack as used for makecontext()

That is, the guard page is included in the stack used to create the
coroutine context, and the last page stays unused. On systems where we
only allocate a single page for the stack, this obviously means that the
tests still fail.

you are right. so I should adjust the size to allocsz instead?

the other option would be to keep version 7 of this series and
adjust the COROUTINE_SIZE to MAX(2*pagesize(), 1 << 16) to
avoid the problem?


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