Userspace stack space seems to be rather fragile

[penberg]

I tried allocating 4K on userspace stack and noticed that the kernel crashes hard. You can see the issue if you update the new user/utils/pwd.c to look like this:

penberg@jaguar:~/src/tinyos$ git diff
diff --git a/user/utils/pwd.c b/user/utils/pwd.c
index 53aee22..3c46b49 100644
--- a/user/utils/pwd.c
+++ b/user/utils/pwd.c
@@ -1,10 +1,10 @@
#include <ulib.h>

#define BUF_SIZE 4096
-static char buf[BUF_SIZE];

int main(int argc, char **argv)
{

•   char buf[BUF_SIZE];
    char *cwd;
  
    cwd = getcwd(buf, BUF_SIZE);
  

[chobits]

Hi, penberg
It only allocs one page(just 4KB) as userspace stack in
sys_execute()->task_execute()->execute_argument()
and the page is mapped on userspace address USER_STACK(3GB - 4KB - 4KB),
so if we alloc 4KB or larger on userspace stack, it will trigger pagefault.
Here is userspace memory map:

---

• kernel space
  ---------------------- 0xc000 0000

• 4KB hole
  ---------------------- 0xbfff f000

• user stack
  ---------------------- 0xbfff e000

• other parts
  ---------------------- 0x000 0000

  I think there are two solutions:

• alloc enough user space stack in sys_execute()
  I am now busy at college final exam, so I may not submit new patch in time :"(

• just use global defined data: 'static char buf[..]' or 'char buf[..]'
  Because tinyos doesnt provide brk/sbrk and malloc() now,
  we need manage the memory manually.)
  When I use global defined 'static char buf[4MB]', it triggers a pagefault panic.
  The new patch 'Fix bug: map wrong userspace virtual address in map_userspace()' for
  this panic has been commited.

  Thanks,
  Xiaochen