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xtensa: reset windowbase/windowstart when cloning the VM

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When we copy a user thread with CLONE_VM, we also have to reset
windowbase and windowstart to start a pristine stack frame. Otherwise,
overflows can happen using the address 0 as the stack pointer.
Also add a special case for vfork, which continues on the
parent stack until it calls execve. Because this could be a call8, we
need to spill the stack pointer of the previus frame (if still 'live' in
the register file).

Signed-off-by: Chris Zankel <chris@zankel.net>
This commit is contained in:
Chris Zankel 2012-10-24 13:15:21 -07:00
parent cddfcbcd05
commit 6ebe7da25b
1 changed files with 25 additions and 1 deletions
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26
arch/xtensa/kernel/process.c
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@ -220,8 +220,32 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
if (user_mode(regs)) {
childregs->areg[1] = usp;
/* When sharing memory with the parent thread, the child
usually starts on a pristine stack, so we have to reset
windowbase, windowstart and wmask.
(Note that such a new thread is required to always create
an initial call4 frame)
The exception is vfork, where the new thread continues to
run on the parent's stack until it calls execve. This could
be a call8 or call12, which requires a legal stack frame
of the previous caller for the overflow handlers to work.
(Note that it's always legal to overflow live registers).
In this case, ensure to spill at least the stack pointer
of that frame. */
if (clone_flags & CLONE_VM) {
childregs->wmask = 1; /* can't share live windows */
/* check that caller window is live and same stack */
int len = childregs->wmask & ~0xf;
if (regs->areg[1] == usp && len != 0) {
int callinc = (regs->areg[0] >> 30) & 3;
int caller_ars = XCHAL_NUM_AREGS - callinc * 4;
put_user(regs->areg[caller_ars+1],
(unsigned __user*)(usp - 12));
}
childregs->wmask = 1;
childregs->windowstart = 1;
childregs->windowbase = 0;
} else {
int len = childregs->wmask & ~0xf;
memcpy(&childregs->areg[XCHAL_NUM_AREGS - len/4],