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From: "Eugene L. Vorokov" <vel@bugz.infotecs.ru>
Message-Id: <200107221700.f6MH0tZ00313@bugz.infotecs.ru>
Subject: using syscalls in a module (stack problem ?)
To: freebsd-hackers@freebsd.org
Date: Sun, 22 Jul 2001 21:00:54 +0400 (MSD)
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Hello,

using my ugly hack to do file i/o from a module, I discovered some
problem calling mmap() from a function with a lot of local buffers
defined. I have:

char * pizda_malloc(struct proc *p, int size)
{
 struct mmap_args mem; int res; register_t save; char *buf;

 save = p->p_retval[0];
 mem.addr = NULL;
 mem.len = size;
 mem.prot = PROT_READ | PROT_WRITE;
 mem.flags = MAP_ANON;
 mem.fd = -1;
 mem.pad = 0;
 mem.pos = 0;
 res = mmap(p, &mem);
 if (res)
  {
   p->p_retval[0] = save;
   return NULL;
  }
 buf = (char *)p->p_retval[0];
 p->p_retval[0] = save;
 subyte(buf, 0);
 return buf;
}

I call this function with (curproc, PATH_MAX+1), and everything is fine
when I have just a few local variables defined in the caller (it all
works on MOD_LOAD only). However, if I have 2 buffers, 4096 bytes each,
as local variables and then try to allocate userspace memory the same
way, kernel crashes - sometimes inside mmap(), sometimes a bit later.

Why could this happen ? Is it related to possible stack overflow ?
(Yes, I know I can use MALLOC instead of static buffers, but I love
to understand what happens ...)

Regards,
Eugene


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