1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
|
#include "errno.h"
#include "globals.h"
#include "mm/mm.h"
#include "util/debug.h"
#include "mm/mman.h"
/*
* This function implements the brk(2) system call.
*
* This routine manages the calling process's "break" -- the ending address
* of the process's dynamic region (heap)
*
* Some important details on the range of values 'p_brk' can take:
* 1) 'p_brk' should not be set to a value lower than 'p_start_brk', since this
* could overrite data in another memory region. But, 'p_brk' can be equal to
* 'p_start_brk', which would mean that there is no heap yet/is empty.
* 2) Growth of the 'p_brk' cannot overlap with/expand into an existing
* mapping. Use vmmap_is_range_empty() to help with this.
* 3) 'p_brk' cannot go beyond the region of the address space allocated for use by
* userland (USER_MEM_HIGH)
*
* Before setting 'p_brk' to 'addr', you must account for all scenarios by comparing
* the page numbers of addr, 'p_brk' and 'p_start_brk' as the vmarea that represents the heap
* has page granularity. Think about the following sub-cases (note that the heap
* should always be represented by at most one vmarea):
* 1) The heap needs to be created. What permissions and attributes does a process
* expect the heap to have?
* 2) The heap already exists, so you need to modify its end appropriately.
* 3) The heap needs to shrink.
*
* Beware of page alignment!:
* 1) The starting break is not necessarily page aligned. Since the loader sets
* 'p_start_brk' to be the end of the bss section, 'p_start_brk' should always be
* aligned up to start the dynamic region at the first page after bss_end.
* 2) vmareas only have page granularity, so you will need to take this
* into account when deciding how to set the mappings if p_brk or p_start_brk
* is not page aligned. The caller of do_brk() would be very disappointed if
* you give them less than they asked for!
*
* Some additional details:
* 1) You are guaranteed that the process data/bss region is non-empty.
* That is, if the starting brk is not page-aligned, its page has
* read/write permissions.
* 2) If 'addr' is NULL, you should return the current break. We use this to
* implement sbrk(0) without writing a separate syscall. Look in
* user/libc/syscall.c if you're curious.
* 3) Return 0 on success, -errno on failure. The 'ret' argument should be used to
* return the updated 'p_brk' on success.
*
* Error cases do_brk is responsible for generating:
* - ENOMEM: attempting to set p_brk beyond its valid range
*/
long do_brk(void *addr, void **ret)
{
NOT_YET_IMPLEMENTED("VM: do_brk");
return 0;
}
|
