Accept elf files that are valid but have sections that can not be mmap'ed for some reason.

Plan9 kernel binaries don't neatly align their ELF sections to our
page boundaries.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

@@ -15,6 +15,7 @@
 #include <stdlib.h>
 #include <elf.h>
 #include <sys/mman.h>
+#include <sys/param.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
@@ -162,6 +163,30 @@ static unsigned long entry_point(void *start, void *end,
 	errx(1, "Is this image a genuine lguest?");
 }
 
+/* This routine is used to load the kernel or initrd.  It tries mmap, but if
+ * that fails (Plan 9's kernel file isn't nicely aligned on page boundaries),
+ * it falls back to reading the memory in. */
+static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
+{
+	ssize_t r;
+
+	/* We map writable even though for some segments are marked read-only.
+	 * The kernel really wants to be writable: it patches its own
+	 * instructions.
+	 *
+	 * MAP_PRIVATE means that the page won't be copied until a write is
+	 * done to it.  This allows us to share untouched memory between
+	 * Guests. */
+	if (mmap(addr, len, PROT_READ|PROT_WRITE|PROT_EXEC,
+		 MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
+		return;
+
+	/* pread does a seek and a read in one shot: saves a few lines. */
+	r = pread(fd, addr, len, offset);
+	if (r != len)
+		err(1, "Reading offset %lu len %lu gave %zi", offset, len, r);
+}
+
 /* This routine takes an open vmlinux image, which is in ELF, and maps it into
  * the Guest memory.  ELF = Embedded Linking Format, which is the format used
  * by all modern binaries on Linux including the kernel.
@@ -176,7 +201,6 @@ static unsigned long entry_point(void *start, void *end,
 static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
 			     unsigned long *page_offset)
 {
-	void *addr;
 	Elf32_Phdr phdr[ehdr->e_phnum];
 	unsigned int i;
 	unsigned long start = -1UL, end = 0;
@@ -227,23 +251,9 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
 		if (phdr[i].p_paddr + phdr[i].p_filesz > end)
 			end = phdr[i].p_paddr + phdr[i].p_filesz;
 
-		/* We map this section of the file at its physical address.  We
-		 * map it read & write even if the header says this segment is
-		 * read-only.  The kernel really wants to be writable: it
-		 * patches its own instructions which would normally be
-		 * read-only.
-		 *
-		 * MAP_PRIVATE means that the page won't be copied until a
-		 * write is done to it.  This allows us to share much of the
-		 * kernel memory between Guests. */
-		addr = mmap((void *)phdr[i].p_paddr,
-			    phdr[i].p_filesz,
-			    PROT_READ|PROT_WRITE|PROT_EXEC,
-			    MAP_FIXED|MAP_PRIVATE,
-			    elf_fd, phdr[i].p_offset);
-		if (addr != (void *)phdr[i].p_paddr)
-			err(1, "Mmaping vmlinux seg %i gave %p not %p",
-			    i, addr, (void *)phdr[i].p_paddr);
+		/* We map this section of the file at its physical address. */
+		map_at(elf_fd, (void *)phdr[i].p_paddr,
+		       phdr[i].p_offset, phdr[i].p_filesz);
 	}
 
 	return entry_point((void *)start, (void *)end, *page_offset);
@@ -402,27 +412,20 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
 	int ifd;
 	struct stat st;
 	unsigned long len;
-	void *iaddr;
 
 	ifd = open_or_die(name, O_RDONLY);
 	/* fstat() is needed to get the file size. */
 	if (fstat(ifd, &st) < 0)
 		err(1, "fstat() on initrd '%s'", name);
 
-	/* The length needs to be rounded up to a page size: mmap needs the
-	 * address to be page aligned. */
+	/* We map the initrd at the top of memory, but mmap wants it to be
+	 * page-aligned, so we round the size up for that. */
 	len = page_align(st.st_size);
-	/* We map the initrd at the top of memory. */
-	iaddr = mmap((void *)mem - len, st.st_size,
-		     PROT_READ|PROT_EXEC|PROT_WRITE,
-		     MAP_FIXED|MAP_PRIVATE, ifd, 0);
-	if (iaddr != (void *)mem - len)
-		err(1, "Mmaping initrd '%s' returned %p not %p",
-		    name, iaddr, (void *)mem - len);
+	map_at(ifd, (void *)mem - len, 0, st.st_size);
 	/* Once a file is mapped, you can close the file descriptor.  It's a
 	 * little odd, but quite useful. */
 	close(ifd);
-	verbose("mapped initrd %s size=%lu @ %p\n", name, st.st_size, iaddr);
+	verbose("mapped initrd %s size=%lu @ %p\n", name, len, (void*)mem-len);
 
 	/* We return the initrd size. */
 	return len;