base之fstab

系统启动阶段会调用到脚本"mountall.sh"，其中有一段和"fstab"相关的代码:

# # Mount local filesystems in /etc/fstab. For some reason, people # might want to mount "proc" several times, and mount -v complains # about this. So we mount "proc" filesystems without -v. # test "$VERBOSE" != no && echo "Mounting local filesystems..." mount -at nonfs,nosmbfs,noncpfs 2>/dev/null

这本该是一个很普通的操作，无非是读取"/etc/fstab"文件中的每一项做相应的分析和处理(mount文件系统)。但poky的"fstab"中有一项却有点让人感到不解:

rootfs / auto defaults 1 1

难道此处是要在根节点上再mount一个文件系统(且无法正常访问的)?跟踪调试代码到内核部分，发现最终这一项的mount操作会因为找不到"rootfs"这个文件或设备而失败。不过因为该项对文件系统类型的设置为"auto"，busybox实现的mount操作会依次尝试"/etc/filesystems"和"/proc/filesystems"中所列举的类型(Not a nodev filesystem)，直至最终全部失败。

难道这一项就是为了屡战屡败而设?作者也太无聊了吧!事实并非如此，这里需要关注到另一个启动脚本"checkroot.sh"，相关代码如下:

# # Read /etc/fstab. # exec 9>&0 </etc/fstab rootmode=rw rootopts=rw rootcheck=no swap_on_md=no devfs= while read fs mnt type opts dump pass junk do case "$fs" in ""|/#*) continue; ;; /dev/md*) # Swap on md device. test "$type" = swap && swap_on_md=yes ;; /dev/*) ;; *) # Might be a swapfile. test "$type" = swap && swap_on_md=yes ;; esac test "$type" = devfs && devfs="$fs" test "$mnt" != / && continue rootopts="$opts" test "$pass" = 0 -o "$pass" = "" && rootcheck=no case "$opts" in ro|ro,*|*,ro|*,ro,*) rootmode=ro ;; esac done exec 0>&9 9>&-

筛查到"fstab"文件中"dir"字段为"/"的项，然后根据"pass"字段的值，设置"rootcheck"值，以决定是否需要对根文件系统做"fsck"。

 

另外，这里还需要提到一点:如果我们使用"util-linux"版本的mount(非busybox版本)，则对"mount -a"的实现有所不同，参考代码如下:

/* Mount all filesystems of the specified types except swap and root. */ /* With the --fork option: fork and let different incarnations of mount handle different filesystems. However, try to avoid several simultaneous mounts on the same physical disk, since that is very slow. */ #define DISKMAJOR(m) (((int) m) & ~0xf) static int do_mount_all (char *types, char *options, char *test_opts) { struct mntentchn *mc, *mc0, *mtmp; int status = 0; struct stat statbuf; struct child { pid_t pid; char *group; struct mntentchn *mec; struct mntentchn *meclast; struct child *nxt; } childhead, *childtail, *cp; char major[22]; char *g, *colon; /* build a chain of what we have to do, or maybe several chains, one for each major or NFS host */ childhead.nxt = 0; childtail = &childhead; mc0 = fstab_head(); for (mc = mc0->nxt; mc && mc != mc0; mc = mc->nxt) { if (has_noauto (mc->m.mnt_opts)) continue; if (matching_type (mc->m.mnt_type, types) && matching_opts (mc->m.mnt_opts, test_opts) && !streq (mc->m.mnt_dir, "/") && !streq (mc->m.mnt_dir, "root")) { if (mounted (mc->m.mnt_fsname, mc->m.mnt_dir)) { if (verbose) printf(_("mount: %s already mounted " "on %s/n"), mc->m.mnt_fsname, mc->m.mnt_dir); continue; } mtmp = (struct mntentchn *) xmalloc(sizeof(*mtmp)); *mtmp = *mc; mtmp->nxt = 0; g = NULL; if (optfork) { if (stat(mc->m.mnt_fsname, &statbuf) == 0 && S_ISBLK(statbuf.st_mode)) { sprintf(major, "#%x", DISKMAJOR(statbuf.st_rdev)); g = major; } #ifdef HAVE_NFS if (strcmp(mc->m.mnt_type, "nfs") == 0) { g = xstrdup(mc->m.mnt_fsname); colon = strchr(g, ':'); if (colon) *colon = '/0'; } #endif } if (g) { for (cp = childhead.nxt; cp; cp = cp->nxt) if (cp->group && strcmp(cp->group, g) == 0) { cp->meclast->nxt = mtmp; cp->meclast = mtmp; goto fnd; } } cp = (struct child *) xmalloc(sizeof *cp); cp->nxt = 0; cp->mec = cp->meclast = mtmp; cp->group = xstrdup(g); cp->pid = 0; childtail->nxt = cp; childtail = cp; fnd:; } } /* now do everything */ for (cp = childhead.nxt; cp; cp = cp->nxt) { pid_t p = -1; if (optfork) { p = fork(); if (p == -1) { int errsv = errno; error(_("mount: cannot fork: %s"), strerror (errsv)); } else if (p != 0) cp->pid = p; } /* if child, or not forked, do the mounting */ if (p == 0 || p == -1) { for (mc = cp->mec; mc; mc = mc->nxt) { status |= mount_one (mc->m.mnt_fsname, mc->m.mnt_dir, mc->m.mnt_type, mc->m.mnt_opts, options, 0, 0); } if (mountcount) status |= EX_SOMEOK; if (p == 0) exit(status); } } /* wait for children, if any */ while ((cp = childhead.nxt) != NULL) { childhead.nxt = cp->nxt; if (cp->pid) { int ret; keep_waiting: if(waitpid(cp->pid, &ret, 0) == -1) { if (errno == EINTR) goto keep_waiting; perror("waitpid"); } else if (WIFEXITED(ret)) status |= WEXITSTATUS(ret); else status |= EX_SYSERR; } } if (mountcount) status |= EX_SOMEOK; return status; }

这里对"fstab"文件中"dir"字段为"/"和"root"的项均略过，不做进一步的处理。目前来看，这样的做法似乎更合理一些。