JFFS2: Add parameter to reserve disk space for root

Add a new rp_size= parameter which creates a "reserved pool" of disk
space which can only be used by root. Other users are not permitted
to write to disk when the available space is less than the pool size.

Based on original code by Artem Bityutskiy in
https://dev.laptop.org/ticket/5317

[dwmw2: use capable(CAP_SYS_RESOURCE) not uid/gid check, fix debug prints]
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

fs/jffs2/jffs2_fs_sb.h

@@ -32,6 +32,13 @@ struct jffs2_inodirty;
 struct jffs2_mount_opts {
 	bool override_compr;
 	unsigned int compr;
+
+	/* The size of the reserved pool. The reserved pool is the JFFS2 flash
+	 * space which may only be used by root cannot be used by the other
+	 * users. This is implemented simply by means of not allowing the
+	 * latter users to write to the file system if the amount if the
+	 * available space is less then 'rp_size'. */
+	unsigned int rp_size;
 };
 
 /* A struct for the overall file system control.  Pointers to

fs/jffs2/nodemgmt.c

@@ -18,6 +18,37 @@
 #include "nodelist.h"
 #include "debug.h"
 
+/*
+ * Check whether the user is allowed to write.
+ */
+static int jffs2_rp_can_write(struct jffs2_sb_info *c)
+{
+	uint32_t avail;
+	struct jffs2_mount_opts *opts = &c->mount_opts;
+
+	avail = c->dirty_size + c->free_size + c->unchecked_size +
+		c->erasing_size - c->resv_blocks_write * c->sector_size
+		- c->nospc_dirty_size;
+
+	if (avail < 2 * opts->rp_size)
+		jffs2_dbg(1, "rpsize %u, dirty_size %u, free_size %u, "
+			  "erasing_size %u, unchecked_size %u, "
+			  "nr_erasing_blocks %u, avail %u, resrv %u\n",
+			  opts->rp_size, c->dirty_size, c->free_size,
+			  c->erasing_size, c->unchecked_size,
+			  c->nr_erasing_blocks, avail, c->nospc_dirty_size);
+
+	if (avail > opts->rp_size)
+		return 1;
+
+	/* Always allow root */
+	if (capable(CAP_SYS_RESOURCE))
+		return 1;
+
+	jffs2_dbg(1, "forbid writing\n");
+	return 0;
+}
+
 /**
  *	jffs2_reserve_space - request physical space to write nodes to flash
  *	@c: superblock info
@@ -55,6 +86,15 @@ int jffs2_reserve_space(struct jffs2_sb_info *c, uint32_t minsize,
 
 	spin_lock(&c->erase_completion_lock);
 
+	/*
+	 * Check if the free space is greater then size of the reserved pool.
+	 * If not, only allow root to proceed with writing.
+	 */
+	if (prio != ALLOC_DELETION && !jffs2_rp_can_write(c)) {
+		ret = -ENOSPC;
+		goto out;
+	}
+
 	/* this needs a little more thought (true <tglx> :)) */
 	while(ret == -EAGAIN) {
 		while(c->nr_free_blocks + c->nr_erasing_blocks < blocksneeded) {
@@ -158,6 +198,8 @@ int jffs2_reserve_space(struct jffs2_sb_info *c, uint32_t minsize,
 			jffs2_dbg(1, "%s(): ret is %d\n", __func__, ret);
 		}
 	}
+
+out:
 	spin_unlock(&c->erase_completion_lock);
 	if (!ret)
 		ret = jffs2_prealloc_raw_node_refs(c, c->nextblock, 1);

fs/jffs2/super.c

@@ -105,6 +105,8 @@ static int jffs2_show_options(struct seq_file *s, struct dentry *root)
 
 	if (opts->override_compr)
 		seq_printf(s, ",compr=%s", jffs2_compr_name(opts->compr));
+	if (opts->rp_size)
+		seq_printf(s, ",rp_size=%u", opts->rp_size / 1024);
 
 	return 0;
 }
@@ -171,15 +173,18 @@ static const struct export_operations jffs2_export_ops = {
  * JFFS2 mount options.
  *
  * Opt_override_compr: override default compressor
+ * Opt_rp_size: size of reserved pool in KiB
  * Opt_err: just end of array marker
  */
 enum {
 	Opt_override_compr,
+	Opt_rp_size,
 	Opt_err,
 };
 
 static const match_table_t tokens = {
 	{Opt_override_compr, "compr=%s"},
+	{Opt_rp_size, "rp_size=%u"},
 	{Opt_err, NULL},
 };
 
@@ -187,6 +192,7 @@ static int jffs2_parse_options(struct jffs2_sb_info *c, char *data)
 {
 	substring_t args[MAX_OPT_ARGS];
 	char *p, *name;
+	unsigned int opt;
 
 	if (!data)
 		return 0;
@@ -224,6 +230,17 @@ static int jffs2_parse_options(struct jffs2_sb_info *c, char *data)
 			kfree(name);
 			c->mount_opts.override_compr = true;
 			break;
+		case Opt_rp_size:
+			if (match_int(&args[0], &opt))
+				return -EINVAL;
+			opt *= 1024;
+			if (opt > c->mtd->size) {
+				pr_warn("Too large reserve pool specified, max "
+					"is %llu KB\n", c->mtd->size / 1024);
+				return -EINVAL;
+			}
+			c->mount_opts.rp_size = opt;
+			break;
 		default:
 			pr_err("Error: unrecognized mount option '%s' or missing value\n",
 			       p);