Module Name: src
Committed By: maxv
Date: Sat Feb 15 10:41:25 UTC 2020
Modified Files:
src/sys/arch/amd64/stand/prekern: mm.c
Log Message:
Explain more.
To generate a diff of this commit:
cvs rdiff -u -r1.24 -r1.25 src/sys/arch/amd64/stand/prekern/mm.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/amd64/stand/prekern/mm.c
diff -u src/sys/arch/amd64/stand/prekern/mm.c:1.24 src/sys/arch/amd64/stand/prekern/mm.c:1.25
--- src/sys/arch/amd64/stand/prekern/mm.c:1.24 Sat Mar 9 08:42:25 2019
+++ src/sys/arch/amd64/stand/prekern/mm.c Sat Feb 15 10:41:25 2020
@@ -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 @@ mm_shift_segment(vaddr_t va, size_t page
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 @@ mm_map_head(void)
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 @@ mm_map_head(void)
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 @@ mm_map_segment(int segtype, paddr_t pa,
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 @@ mm_map_boot(void)
}
/*
- * 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.
*/
