1 files changed, 29 insertions, 64 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 004c5c6aba6a..3949620e42fa 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -326,74 +326,39 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
         return ehdr->e_entry;
 }
 
-/*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)
-{
-        gzFile f;
-        int ret, len = 0;
-        /* A bzImage always gets loaded at physical address 1M.  This is
-         * actually configurable as CONFIG_PHYSICAL_START, but as the comment
-         * there says, "Don't change this unless you know what you are doing".
-         * Indeed. */
-        void *img = from_guest_phys(0x100000);
-
-        /* gzdopen takes our file descriptor (carefully placed at the start of
-         * the GZIP header we found) and returns a gzFile. */
-        f = gzdopen(fd, "rb");
-        /* We read it into memory in 64k chunks until we hit the end. */
-        while ((ret = gzread(f, img + len, 65536)) > 0)
-                len += ret;
-        if (ret < 0)
-                err(1, "reading image from bzImage");
-
-        verbose("Unpacked size %i addr %p\n", len, img);
-
-        /* The entry point for a bzImage is always the first byte */
-        return (unsigned long)img;
-}
-
 /*L:150 A bzImage, unlike an ELF file, is not meant to be loaded.  You're
- * supposed to jump into it and it will unpack itself.  We can't do that
- * because the Guest can't run the unpacking code, and adding features to
- * lguest kills puppies, so we don't want to.
+ * supposed to jump into it and it will unpack itself.  We used to have to
+ * perform some hairy magic because the unpacking code scared me.
  *
- * 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. */
+ * Fortunately, Jeremy Fitzhardinge convinced me it wasn't that hard and wrote
+ * a small patch to jump over the tricky bits in the Guest, so now we just read
+ * the funky header so we know where in the file to load, and away we go! */
 static unsigned long load_bzimage(int fd)
 {
-        unsigned char c;
-        int state = 0;
-
-        /* GZIP header is 0x1F 0x8B <method> <flags>... <compressed-by>. */
-        while (read(fd, &c, 1) == 1) {
-                switch (state) {
-                case 0:
-                        if (c == 0x1F)
-                                state++;
-                        break;
-                case 1:
-                        if (c == 0x8B)
-                                state++;
-                        else
-                                state = 0;
-                        break;
-                case 2 ... 8:
-                        state++;
-                        break;
-                case 9:
-                        /* Seek back to the start of the gzip header. */
-                        lseek(fd, -10, SEEK_CUR);
-                        /* One final check: "compressed under UNIX". */
-                        if (c != 0x03)
-                                state = -1;
-                        else
-                                return unpack_bzimage(fd);
-                }
-        }
-        errx(1, "Could not find kernel in bzImage");
+        u8 hdr[1024];
+        int r;
+        /* Modern bzImages get loaded at 1M. */
+        void *p = from_guest_phys(0x100000);
+
+        /* Go back to the start of the file and read the header.  It should be
+         * a Linux boot header (see Documentation/i386/boot.txt) */
+        lseek(fd, 0, SEEK_SET);
+        read(fd, hdr, sizeof(hdr));
+
+        /* At offset 0x202, we expect the magic "HdrS" */
+        if (memcmp(hdr + 0x202, "HdrS", 4) != 0)
+                errx(1, "This doesn't look like a bzImage to me");
+
+        /* The byte at 0x1F1 tells us how many extra sectors of
+         * header: skip over them all. */
+        lseek(fd, (unsigned long)(hdr[0x1F1]+1) * 512, SEEK_SET);
+
+        /* Now read everything into memory. in nice big chunks. */
+        while ((r = read(fd, p, 65536)) > 0)
+                p += r;
+
+        /* Finally, 0x214 tells us where to start the kernel. */
+        return *(unsigned long *)&hdr[0x214];
 }
 
 /*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels