diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/lguest/lguest.c | 134 |
1 files changed, 31 insertions, 103 deletions
diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c index 4950b03514e..32c2eaf94c4 100644 --- a/Documentation/lguest/lguest.c +++ b/Documentation/lguest/lguest.c @@ -178,19 +178,16 @@ static void *get_pages(unsigned int num) /* To find out where to start we look for the magic Guest string, which marks * the code we see in lguest_asm.S. This is a hack which we are currently * plotting to replace with the normal Linux entry point. */ -static unsigned long entry_point(const void *start, const void *end, - unsigned long page_offset) +static unsigned long entry_point(const void *start, const void *end) { const void *p; - /* The scan gives us the physical starting address. We want the - * virtual address in this case, and fortunately, we already figured - * out the physical-virtual difference and passed it here in - * "page_offset". */ + /* The scan gives us the physical starting address. We boot with + * pagetables set up with virtual and physical the same, so that's + * OK. */ for (p = start; p < end; p++) if (memcmp(p, "GenuineLguest", strlen("GenuineLguest")) == 0) - return to_guest_phys(p + strlen("GenuineLguest")) - + page_offset; + return to_guest_phys(p + strlen("GenuineLguest")); errx(1, "Is this image a genuine lguest?"); } @@ -224,14 +221,11 @@ static void map_at(int fd, void *addr, unsigned long offset, unsigned long len) * by all modern binaries on Linux including the kernel. * * The ELF headers give *two* addresses: a physical address, and a virtual - * address. The Guest kernel expects to be placed in memory at the physical - * address, and the page tables set up so it will correspond to that virtual - * address. We return the difference between the virtual and physical - * addresses in the "page_offset" pointer. + * address. We use the physical address; the Guest will map itself to the + * virtual address. * * We return the starting address. */ -static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr, - unsigned long *page_offset) +static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr) { void *start = (void *)-1, *end = NULL; Elf32_Phdr phdr[ehdr->e_phnum]; @@ -255,9 +249,6 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr, if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr)) err(1, "Reading program headers"); - /* We don't know page_offset yet. */ - *page_offset = 0; - /* Try all the headers: there are usually only three. A read-only one, * a read-write one, and a "note" section which isn't loadable. */ for (i = 0; i < ehdr->e_phnum; i++) { @@ -268,14 +259,6 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr, verbose("Section %i: size %i addr %p\n", i, phdr[i].p_memsz, (void *)phdr[i].p_paddr); - /* We expect a simple linear address space: every segment must - * have the same difference between virtual (p_vaddr) and - * physical (p_paddr) address. */ - if (!*page_offset) - *page_offset = phdr[i].p_vaddr - phdr[i].p_paddr; - else if (*page_offset != phdr[i].p_vaddr - phdr[i].p_paddr) - errx(1, "Page offset of section %i different", i); - /* We track the first and last address we mapped, so we can * tell entry_point() where to scan. */ if (from_guest_phys(phdr[i].p_paddr) < start) @@ -288,50 +271,13 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr, phdr[i].p_offset, phdr[i].p_filesz); } - return entry_point(start, end, *page_offset); -} - -/*L:170 Prepare to be SHOCKED and AMAZED. And possibly a trifle nauseated. - * - * We know that CONFIG_PAGE_OFFSET sets what virtual address the kernel expects - * to be. We don't know what that option was, but we can figure it out - * approximately by looking at the addresses in the code. I chose the common - * case of reading a memory location into the %eax register: - * - * movl <some-address>, %eax - * - * This gets encoded as five bytes: "0xA1 <4-byte-address>". For example, - * "0xA1 0x18 0x60 0x47 0xC0" reads the address 0xC0476018 into %eax. - * - * In this example can guess that the kernel was compiled with - * CONFIG_PAGE_OFFSET set to 0xC0000000 (it's always a round number). If the - * kernel were larger than 16MB, we might see 0xC1 addresses show up, but our - * kernel isn't that bloated yet. - * - * Unfortunately, x86 has variable-length instructions, so finding this - * particular instruction properly involves writing a disassembler. Instead, - * we rely on statistics. We look for "0xA1" and tally the different bytes - * which occur 4 bytes later (the "0xC0" in our example above). When one of - * those bytes appears three times, we can be reasonably confident that it - * forms the start of CONFIG_PAGE_OFFSET. - * - * This is amazingly reliable. */ -static unsigned long intuit_page_offset(unsigned char *img, unsigned long len) -{ - unsigned int i, possibilities[256] = { 0 }; - - for (i = 0; i + 4 < len; i++) { - /* mov 0xXXXXXXXX,%eax */ - if (img[i] == 0xA1 && ++possibilities[img[i+4]] > 3) - return (unsigned long)img[i+4] << 24; - } - errx(1, "could not determine page offset"); + return entry_point(start, end); } /*L:160 Unfortunately the entire ELF image isn't compressed: the segments * which need loading are extracted and compressed raw. This denies us the * information we need to make a fully-general loader. */ -static unsigned long unpack_bzimage(int fd, unsigned long *page_offset) +static unsigned long unpack_bzimage(int fd) { gzFile f; int ret, len = 0; @@ -352,12 +298,7 @@ static unsigned long unpack_bzimage(int fd, unsigned long *page_offset) verbose("Unpacked size %i addr %p\n", len, img); - /* Without the ELF header, we can't tell virtual-physical gap. This is - * CONFIG_PAGE_OFFSET, and people do actually change it. Fortunately, - * I have a clever way of figuring it out from the code itself. */ - *page_offset = intuit_page_offset(img, len); - - return entry_point(img, img + len, *page_offset); + return entry_point(img, img + len); } /*L:150 A bzImage, unlike an ELF file, is not meant to be loaded. You're @@ -368,7 +309,7 @@ static unsigned long unpack_bzimage(int fd, unsigned long *page_offset) * The bzImage is formed by putting the decompressing code in front of the * compressed kernel code. So we can simple scan through it looking for the * first "gzip" header, and start decompressing from there. */ -static unsigned long load_bzimage(int fd, unsigned long *page_offset) +static unsigned long load_bzimage(int fd) { unsigned char c; int state = 0; @@ -396,7 +337,7 @@ static unsigned long load_bzimage(int fd, unsigned long *page_offset) if (c != 0x03) state = -1; else - return unpack_bzimage(fd, page_offset); + return unpack_bzimage(fd); } } errx(1, "Could not find kernel in bzImage"); @@ -405,7 +346,7 @@ static unsigned long load_bzimage(int fd, unsigned long *page_offset) /*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels * come wrapped up in the self-decompressing "bzImage" format. With some funky * coding, we can load those, too. */ -static unsigned long load_kernel(int fd, unsigned long *page_offset) +static unsigned long load_kernel(int fd) { Elf32_Ehdr hdr; @@ -415,10 +356,10 @@ static unsigned long load_kernel(int fd, unsigned long *page_offset) /* If it's an ELF file, it starts with "\177ELF" */ if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0) - return map_elf(fd, &hdr, page_offset); + return map_elf(fd, &hdr); /* Otherwise we assume it's a bzImage, and try to unpack it */ - return load_bzimage(fd, page_offset); + return load_bzimage(fd); } /* This is a trivial little helper to align pages. Andi Kleen hated it because @@ -463,27 +404,20 @@ static unsigned long load_initrd(const char *name, unsigned long mem) return len; } -/* Once we know the address the Guest kernel expects, we can construct simple - * linear page tables for all of memory which will get the Guest far enough +/* Once we know how much memory we have, we can construct simple linear page + * tables which set virtual == physical which will get the Guest far enough * into the boot to create its own. * * We lay them out of the way, just below the initrd (which is why we need to * know its size). */ static unsigned long setup_pagetables(unsigned long mem, - unsigned long initrd_size, - unsigned long page_offset) + unsigned long initrd_size) { unsigned long *pgdir, *linear; unsigned int mapped_pages, i, linear_pages; unsigned int ptes_per_page = getpagesize()/sizeof(void *); - /* Ideally we map all physical memory starting at page_offset. - * However, if page_offset is 0xC0000000 we can only map 1G of physical - * (0xC0000000 + 1G overflows). */ - if (mem <= -page_offset) - mapped_pages = mem/getpagesize(); - else - mapped_pages = -page_offset/getpagesize(); + mapped_pages = mem/getpagesize(); /* Each PTE page can map ptes_per_page pages: how many do we need? */ linear_pages = (mapped_pages + ptes_per_page-1)/ptes_per_page; @@ -500,11 +434,9 @@ static unsigned long setup_pagetables(unsigned long mem, for (i = 0; i < mapped_pages; i++) linear[i] = ((i * getpagesize()) | PAGE_PRESENT); - /* The top level points to the linear page table pages above. The - * entry representing page_offset points to the first one, and they - * continue from there. */ + /* The top level points to the linear page table pages above. */ for (i = 0; i < mapped_pages; i += ptes_per_page) { - pgdir[(i + page_offset/getpagesize())/ptes_per_page] + pgdir[i/ptes_per_page] = ((to_guest_phys(linear) + i*sizeof(void *)) | PAGE_PRESENT); } @@ -535,15 +467,12 @@ static void concat(char *dst, char *args[]) /* This is where we actually tell the kernel to initialize the Guest. We saw * the arguments it expects when we looked at initialize() in lguest_user.c: * the base of guest "physical" memory, the top physical page to allow, the - * top level pagetable, the entry point and the page_offset constant for the - * Guest. */ -static int tell_kernel(unsigned long pgdir, unsigned long start, - unsigned long page_offset) + * top level pagetable and the entry point for the Guest. */ +static int tell_kernel(unsigned long pgdir, unsigned long start) { unsigned long args[] = { LHREQ_INITIALIZE, (unsigned long)guest_base, - guest_limit / getpagesize(), - pgdir, start, page_offset }; + guest_limit / getpagesize(), pgdir, start }; int fd; verbose("Guest: %p - %p (%#lx)\n", @@ -1424,9 +1353,9 @@ static void usage(void) /*L:105 The main routine is where the real work begins: */ int main(int argc, char *argv[]) { - /* Memory, top-level pagetable, code startpoint, PAGE_OFFSET and size - * of the (optional) initrd. */ - unsigned long mem = 0, pgdir, start, page_offset, initrd_size = 0; + /* Memory, top-level pagetable, code startpoint and size of the + * (optional) initrd. */ + unsigned long mem = 0, pgdir, start, initrd_size = 0; /* A temporary and the /dev/lguest file descriptor. */ int i, c, lguest_fd; /* The list of Guest devices, based on command line arguments. */ @@ -1500,8 +1429,7 @@ int main(int argc, char *argv[]) setup_console(&device_list); /* Now we load the kernel */ - start = load_kernel(open_or_die(argv[optind+1], O_RDONLY), - &page_offset); + start = load_kernel(open_or_die(argv[optind+1], O_RDONLY)); /* Boot information is stashed at physical address 0 */ boot = from_guest_phys(0); @@ -1518,7 +1446,7 @@ int main(int argc, char *argv[]) } /* Set up the initial linear pagetables, starting below the initrd. */ - pgdir = setup_pagetables(mem, initrd_size, page_offset); + pgdir = setup_pagetables(mem, initrd_size); /* The Linux boot header contains an "E820" memory map: ours is a * simple, single region. */ @@ -1535,7 +1463,7 @@ int main(int argc, char *argv[]) /* We tell the kernel to initialize the Guest: this returns the open * /dev/lguest file descriptor. */ - lguest_fd = tell_kernel(pgdir, start, page_offset); + lguest_fd = tell_kernel(pgdir, start); /* We fork off a child process, which wakes the Launcher whenever one * of the input file descriptors needs attention. Otherwise we would |