Extra memory mapping seen on freebsd-12 which was not seen in freebsd-11

[Paul Floyd]

> On 16 Oct 2020, at 07:13, karnajit wangkhem <karnajitw at gmail.com> wrote:
> 
> Thanks for the reply. It helped in my understanding.
> 
> Below is a sample code
> 
> #include <stdio.h>
> #include <string.h>
> #include <errno.h>
> #include <sys/mman.h>
> 
> int main()
> {
>  char *str = NULL;
>  str = (char *)mmap((void *)0x7fffdfffe000UL, 0x2000, PROT_READ |
> PROT_WRITE, MAP_FIXED | MAP_ANON, -1, 0);
>  if ((void *)str == (void *)MAP_FAILED) {
>    int err = errno;
>    printf("mmap failed. err (%s)\n", strerror(err));
>  } else {
>    memcpy(str, "Hello World", 12);
>    printf("str = %s\n", str);
>  }
> 
>  return 0;
> }
> 
> Now, the below code under valgrind will give
> - mmap failed. err (Invalid argument)
> 
> But, if we give control of this segment to the client program
> with VG_(am_change_ownership_v_to_c), then valgrind allows the client to do
> the following mmap.
> - str = Hello World
> 
> And, the resultant procstat result looks like this:
> 2382        0x7fbfff000        0x7fc001000 rwx    2    2   1   0 ----- df
> 2382     0x7fffdfffe000     0x7fffe0000000 rw-    0    0   0   0 ----- --
>   <<< Client mmap call
> 2382     0x7fffe0000000     0x7ffffffdf000 ---    0    0   0   0 ----- --
>     <<< 0x1000 bytes is taken away from the MAP_GUARD area
> 2382     0x7ffffffdf000     0x7ffffffff000 rw-    1    1   1   0 ---D- df
> 2382     0x7ffffffff000     0x800000000000 r-x    1    1 104   0 ----- ph
> 
> So, is it right for the application with or without valgrind to cross the
> above boundary, If that memory which the application reserved is just for
> normal application specific use?

Hi

Obviously threre are some restrictions for the guest application running under Valgrind. 
Valgrind needs its own stack and heap, so the guest can’t mmap these regions.

Why do you need to mmap into this region?

A+
Paul