[src/trunk]: src/sys/arch/amd64/stand/prekern Explain more.

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/941532c84fe1
branches:  trunk
changeset: 744877:941532c84fe1
user:      maxv <maxv%NetBSD.org@localhost>
date:      Sat Feb 15 10:41:25 2020 +0000

description:
Explain more.

diffstat:

 sys/arch/amd64/stand/prekern/mm.c |  53 ++++++++++++++++++++++++++++++++++----
 1 files changed, 47 insertions(+), 6 deletions(-)

diffs (115 lines):

diff -r e09812b6852e -r 941532c84fe1 sys/arch/amd64/stand/prekern/mm.c
--- a/sys/arch/amd64/stand/prekern/mm.c Sat Feb 15 09:57:30 2020 +0000
+++ b/sys/arch/amd64/stand/prekern/mm.c Sat Feb 15 10:41:25 2020 +0000
@@ -1,7 +1,7 @@
-/*     $NetBSD: mm.c,v 1.24 2019/03/09 08:42:25 maxv Exp $     */
+/*     $NetBSD: mm.c,v 1.25 2020/02/15 10:41:25 maxv Exp $     */
 
 /*
- * Copyright (c) 2017 The NetBSD Foundation, Inc. All rights reserved.
+ * Copyright (c) 2017-2020 The NetBSD Foundation, Inc. All rights reserved.
  *
  * This code is derived from software contributed to The NetBSD Foundation
  * by Maxime Villard.
@@ -289,6 +289,12 @@
        size_t shiftsize, offset;
        uint64_t rnd;
 
+       /*
+        * If possible, shift the segment in memory using a random offset. Once
+        * shifted the segment remains in the same page, of size pagesz. Make
+        * sure to respect the ELF alignment constraint.
+        */
+
        if (elfalign == 0) {
                elfalign = ELFROUND;
        }
@@ -317,6 +323,13 @@
        vaddr_t randva;
 
        /*
+        * The HEAD window is 1GB below the main KASLR window. This is to
+        * ensure that head always comes first in virtual memory. The reason
+        * for that is that we use (headva + sh_offset), and sh_offset is
+        * unsigned.
+        */
+
+       /*
         * To get the size of the head, we give a look at the read-only
         * mapping of the kernel we created in locore. We're identity mapped,
         * so kernpa = kernva.
@@ -324,6 +337,10 @@
        size = elf_get_head_size((vaddr_t)kernpa_start);
        npages = size / PAGE_SIZE;
 
+       /*
+        * Choose a random range of VAs in the HEAD window, and create the page
+        * tree for it.
+        */
        prng_get_rand(&rnd, sizeof(rnd));
        randva = rounddown(HEAD_WINDOW_BASE + rnd % (HEAD_WINDOW_SIZE - size),
            PAGE_SIZE);
@@ -355,22 +372,27 @@
                pagesz = NBPD_L2;
        }
 
+       /* Create the page tree */
        size = roundup(elfsz, pagesz);
        randva = mm_randva_kregion(size, pagesz);
 
+       /* Enter the segment */
        npages = size / PAGE_SIZE;
        for (i = 0; i < npages; i++) {
                mm_enter_pa(pa + i * PAGE_SIZE,
                    randva + i * PAGE_SIZE, MM_PROT_READ|MM_PROT_WRITE);
        }
 
+       /* Shift the segment in memory */
        offset = mm_shift_segment(randva, pagesz, elfsz, elfalign);
        ASSERT(offset + elfsz <= size);
 
+       /* Fill the paddings */
        pad = pads[segtype];
        memset((void *)randva, pad, offset);
        memset((void *)(randva + offset + elfsz), pad, size - elfsz - offset);
 
+       /* Register the bootspace information */
        bootspace_addseg(segtype, randva, pa, size);
 
        return (randva + offset);
@@ -425,12 +447,31 @@
 }
 
 /*
- * There is a variable number of independent regions: one head, several kernel
- * segments, one boot. They are all mapped at random VAs.
+ * The bootloader has set up the following layout of physical memory:
+ * +------------+-----------------+---------------+------------------+-------+
+ * | ELF HEADER | SECTION HEADERS | KERN SECTIONS | SYM+REL SECTIONS | EXTRA |
+ * +------------+-----------------+---------------+------------------+-------+
+ * Which we abstract into several "regions":
+ * +------------------------------+---------------+--------------------------+
+ * |         Head region          | Several segs  |       Boot region        |
+ * +------------------------------+---------------+--------------------------+
+ * See loadfile_elf32.c:loadfile_dynamic() for the details.
+ *
+ * There is a variable number of independent regions we create: one head,
+ * several kernel segments, one boot. They are all mapped at random VAs.
  *
  * Head contains the ELF Header and ELF Section Headers, and we use them to
- * map the rest of the regions. Head must be placed in memory *before* the
- * other regions.
+ * map the rest of the regions. Head must be placed in both virtual memory
+ * and physical memory *before* the rest.
+ *
+ * The Kernel Sections are mapped at random VAs using individual segments
+ * in bootspace.
+ *
+ * Boot contains various information, including the ELF Sym+Rel sections,
+ * plus extra memory the prekern has used so far; it is a region that the
+ * kernel will eventually use for module_map. Boot is placed *after* the
+ * other regions in physical memory. In virtual memory however there is no
+ * constraint, so its VA is randomly selected in the main KASLR window.
  *
  * At the end of this function, the bootspace structure is fully constructed.
  */