Created student weenix repository

1 files changed, 58 insertions, 0 deletions

diff --git a/kernel/vm/brk.c b/kernel/vm/brk.c
new file mode 100644
index 0000000..5169a42
--- /dev/null
+++ b/kernel/vm/brk.c
@@ -0,0 +1,58 @@
+#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: ***none***");
+    return 0;
+}