From c30cbcb0a646b1f13a22c645616dce624465b883 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Wed, 10 Jun 2015 02:27:40 +0000
Subject: implement arch-generic version of __unmapself
this can be used to put off writing an asm version of __unmapself for
new archs, or as a permanent solution on archs where it's not
practical or even possible to run momentarily with no stack.
the concept here is simple: the caller takes a lock on a global shared
stack and uses it to make the munmap and exit syscalls. the only trick
is unlocking, which must be done after the thread exits, and this is
achieved by using the set_tid_address syscall to have the kernel zero
and futex-wake the lock word as part of the exit syscall.
---
src/thread/__unmapself.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)
(limited to 'src/thread')
diff --git a/src/thread/__unmapself.c b/src/thread/__unmapself.c
index e69de29b..1d3bee1d 100644
--- a/src/thread/__unmapself.c
+++ b/src/thread/__unmapself.c
@@ -0,0 +1,29 @@
+#include "pthread_impl.h"
+#include "atomic.h"
+#include "syscall.h"
+/* cheat and reuse CRTJMP macro from dynlink code */
+#include "dynlink.h"
+
+static volatile int lock;
+static void *unmap_base;
+static size_t unmap_size;
+static char shared_stack[256];
+
+static void do_unmap()
+{
+ __syscall(SYS_munmap, unmap_base, unmap_size);
+ __syscall(SYS_exit);
+}
+
+void __unmapself(void *base, size_t size)
+{
+ int tid=__pthread_self()->tid;
+ char *stack = shared_stack + sizeof shared_stack;
+ stack -= (uintptr_t)stack % 16;
+ while (lock || a_cas(&lock, 0, tid))
+ a_spin();
+ __syscall(SYS_set_tid_address, &lock);
+ unmap_base = base;
+ unmap_size = size;
+ CRTJMP(do_unmap, stack);
+}
--
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