fs: follow symlinks like Linux; Linux mknod; /proc/partitions

Symlinks are followed in namex_from() now, in the middle of a path as
well as at the end, and a relative target resolves from the directory
that holds the link rather than from the caller's cwd. `ln -s f d1/s;
cat d1/s` said "Symbolic link loop" because it looked for ./f. A loop is
ELOOP after 40 links, a dangling link ENOENT, and open(O_CREAT) through
a link to a file opens the file. divesymlink() is gone.

mknod from a Linux program passes (mode, dev) where the native call
passes (major, minor), and was read as the native one: `mknod n c 1 2`
made node 432,258, and mkfifo made a character device. Both are decoded
now; a FIFO is EPERM, and an existing name is EEXIST.

/proc/mounts listed devtmpfs twice: once from a fixed line and once
from the mount table. The fixed line is gone, and mount_add() refuses
a filesystem mounted on its own root, which is where a second devtmpfs
mount lands. A read-only FAT mount says ro.

/proc/partitions exists, in Linux's format, and /proc/filesystems lists
vfat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/dev/blkdev.c

@@ -372,23 +372,35 @@ putu(char *buf, int cap, int off, uint64 v)
   return off;
 }
 
+// putu(), right-aligned in `w` columns, the way printf("%*llu") would.
+static int
+putw(char *buf, int cap, int off, uint64 v, int w)
+{
+  int n = 1;
+  for (uint64 t = v; t >= 10; t /= 10)
+    n++;
+  for (; w > n; w--)
+    off = put(buf, cap, off, " ");
+  return putu(buf, cap, off, v);
+}
+
+// /proc/partitions, in Linux's format to the column, because that is what
+// reads it: lsblk's fallback, fdisk -l, every installer and rescue script.
+// Sizes are in 1 KB blocks. The major is 8 for all of them, the one devtmpfs
+// gives every disk here, and the minor is the device number, as in /dev.
 int
 blkdev_info(char *buf, int cap)
 {
-  int off = put(buf, cap, 0, "dev  name          MB  type\n");
+  int off = put(buf, cap, 0, "major minor  #blocks  name\n\n");
   for (int s = 0; s < NBLKDEV; s++) {
     struct blkdev *d = &devs[s];
     if (!d->used)
       continue;
-    off = putu(buf, cap, off, BLKDEV_FIRST + s);
-    off = put(buf, cap, off, "    ");
+    off = putw(buf, cap, off, 8, 4);
+    off = putw(buf, cap, off, BLKDEV_FIRST + s, 8);
+    off = putw(buf, cap, off, (d->nsectors * d->secsize) >> 10, 11);
+    off = put(buf, cap, off, " ");
     off = put(buf, cap, off, d->name);
-    for (int pad = 12 - strlen(d->name); pad > 0; pad--)
-      off = put(buf, cap, off, " ");
-    off = putu(buf, cap, off, (d->nsectors * d->secsize) >> 20);
-    off = put(buf, cap, off, d->parent < 0 ? "  disk" :
-                             d->schfs > 0 ? "  schfs" :
-                             d->schfs < 0 ? "  fat" : "  part");
     off = put(buf, cap, off, "\n");
   }
   return off;

kernel/dev/procfs.c

@@ -38,6 +38,8 @@
 #include "sbuf.h"
 #include "kmem.h"
 #include "dbg.h"
+#include "blkdev.h"
+#include "fat.h"
 
 #define PROC_BUFSZ 2048
 
@@ -241,9 +243,9 @@ gen_mounts(struct sbuf *sb)
   char path[MAXPATH], source[MAXPATH], type[16];
   uint fsdev;
 
-  sb_puts(sb, "devtmpfs /dev devtmpfs rw 0 0\n");
-
-  for (int i = 0; i < 8; i++) {
+  // Every mount is in the table, /dev included. A fixed devtmpfs line here
+  // listed it twice once init mounted it the ordinary way.
+  for (int i = 0; i < 16; i++) {
     if (mount_get(i, path, source, type, &fsdev) != 0)
       continue;
     // "/" is emitted last, below, so skip it if it somehow appears here.
@@ -256,7 +258,8 @@ gen_mounts(struct sbuf *sb)
     sb_puts(sb, type[0] ? type : "schfs");
     // A mounted snapshot is genuinely read-only, and saying so is what stops
     // anything trying to write to one and reporting a confusing EROFS later.
-    sb_puts(sb, SCHFS_IS_SNAP_DEV(fsdev) ? " ro 0 0\n" : " rw 0 0\n");
+    int ro = SCHFS_IS_SNAP_DEV(fsdev) || (FAT_IS_DEV(fsdev) && fat_readonly(fsdev));
+    sb_puts(sb, ro ? " ro 0 0\n" : " rw 0 0\n");
   }
 
   // Root must be last: df keeps the last mount point that prefixes the path
@@ -323,6 +326,7 @@ gen_filesystems(struct sbuf *sb)
   sb_puts(sb, "\tschfs\n");
   sb_puts(sb, "nodev\tproc\n");
   sb_puts(sb, "nodev\tdevtmpfs\n");
+  sb_puts(sb, "\tvfat\n");
 }
 
 // --- dispatch --------------------------------------------------------------
@@ -347,6 +351,7 @@ gen_filesystems(struct sbuf *sb)
 #define PROC_BOOTMODE     19
 #define PROC_BOOTDEBUG    30
 #define PROC_SLAB         20
+#define PROC_PARTITIONS   21
 
 // Render global file `id` (a PROC_* constant) into `buf`. Returns its length.
 //
@@ -401,6 +406,8 @@ proc_gen_global(int id, char *buf, int cap)
     sb_puts(&sb, boot_debug ? "on\n" : "off\n");
     break;
   case PROC_DEVICES:     gen_devices(&sb);     break;
+  // The block layer renders its own table, like PCI and SchFS above.
+  case PROC_PARTITIONS:  sb.len = blkdev_info(buf, PROC_BUFSZ); break;
   default:               return 0;
   }
   return sb.len;