[CRIU] [PATCH 1/3] rst: Don't allocate page for child stack

[Pavel Emelyanov]

On 09/12/2014 06:13 PM, Andrew Vagin wrote:
> On Fri, Sep 12, 2014 at 04:42:43PM +0400, Pavel Emelyanov wrote:
>> When clone-ing kids we can set their stack on current, as
>> it will anyway be COW-ed later.
> 
> glibc() will modify a new stack. Look at this:
> 
> [avagin at localhost ~]$ cat test.c 
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <sys/mman.h>
> #include <signal.h>
> #include <stdlib.h>
> 
> int child(void *args)
> {
> 	exit(0);
> }
> 
> int main()
> {
> 	void *stack;
> 
> 	stack = mmap(NULL, 4096, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

This is not equivalent to what I produce -- you give to child a single
screwed up (zeroed) page as a stack.

> 	clone(child, stack, SIGCHLD, 0);
> 
> 	return 0;
> }
> [avagin at localhost ~]$ strace ./a.out 
> execve("./a.out", ["./a.out"], [/* 60 vars */]) = 0
> ...
> mmap(NULL, 4096, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa331ab5000
> --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x7fa331ab4ff8} ---
> +++ killed by SIGSEGV (core dumped) +++
> Segmentation fault (core dumped)
> 
> 
>>
>> Signed-off-by: Pavel Emelyanov <xemul at parallels.com>
>> ---
>>  cr-restore.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/cr-restore.c b/cr-restore.c
>> index 4d5ccd5..ee15b6c 100644
>> --- a/cr-restore.c
>> +++ b/cr-restore.c
>> @@ -945,7 +945,6 @@ static int restore_one_task(int pid, CoreEntry *core)
>>  
>>  /* All arguments should be above stack, because it grows down */
>>  struct cr_clone_arg {
>> -	char stack[PAGE_SIZE] __attribute__((aligned (8)));
>>  	char stack_ptr[0];
>>  	struct pstree_item *item;
>>  	unsigned long clone_flags;
>> @@ -993,8 +992,8 @@ static void maybe_clone_parent(struct pstree_item *item,
>>  
>>  static inline int fork_with_pid(struct pstree_item *item)
>>  {
>> -	int ret = -1, fd;
>>  	struct cr_clone_arg ca;
>> +	int ret = -1, fd;
>>  	pid_t pid = item->pid.virt;
>>  
>>  	if (item->state != TASK_HELPER) {
>> @@ -1037,6 +1036,8 @@ static inline int fork_with_pid(struct pstree_item *item)
>>  	ca.item = item;
>>  	ca.clone_flags = item->rst->clone_flags;
>>  
>> +	BUG_ON(ca.clone_flags & CLONE_VM);
>> +
>>  	pr_info("Forking task with %d pid (flags 0x%lx)\n", pid, ca.clone_flags);
>>  
>>  	if (!(ca.clone_flags & CLONE_NEWPID)) {
>> -- 
>> 1.8.4.2
>>
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> .
>