[PATCH v6 1/8] sysctl: Add flags to support min/max range clamping

[Waiman Long]

When minimum/maximum values are specified for a sysctl parameter in
the ctl_table structure with proc_dointvec_minmax() handler, update
to that parameter will fail with error if the given value is outside
of the required range.

There are use cases where it may be better to clamp the value of
the sysctl parameter to the given range without failing the update,
especially if the users are not aware of the actual range limits.
Reading the value back after the update will now be a good practice
to see if the provided value exceeds the range limits.

To provide this less restrictive form of range checking, a new flags
field is added to the ctl_table structure. The new field is a 16-bit
value that just fits into the hole left by the 16-bit umode_t field
without increasing the size of the structure.

When either the CTL_FLAGS_CLAMP_SIGNED_RANGE or the
CTL_FLAGS_CLAMP_UNSIGNED_RANGE flag is set in the ctl_table entry, any
update from the userspace will be clamped to the given range without
error if either the proc_dointvec_minmax() or the proc_douintvec_minmax()
handlers is used respectively.

In the case of proc_doulongvec_minmax(), the out-of-range input value
is either ignored or clamped if the CTL_FLAGS_CLAMP_UNSIGNED_RANGE flag
is set.

The clamped value is either the maximum or minimum value that is
closest to the input value provided by the user.

This patch, by itself, does not require the use of separate signed
and unsigned flags.  However, the use of separate flags allows us to
perform more comprehensive checking in a later patch.

Extra braces are also used in this patch to make a latter patch easier
to read.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 include/linux/sysctl.h | 32 ++++++++++++++++++++++
 kernel/sysctl.c        | 74 ++++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 94 insertions(+), 12 deletions(-)

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index b769ecf..3a628cf 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -116,6 +116,7 @@ struct ctl_table
 	void *data;
 	int maxlen;
 	umode_t mode;
+	uint16_t flags;
 	struct ctl_table *child;	/* Deprecated */
 	proc_handler *proc_handler;	/* Callback for text formatting */
 	struct ctl_table_poll *poll;
@@ -123,6 +124,37 @@ struct ctl_table
 	void *extra2;
 } __randomize_layout;
 
+/**
+ * enum ctl_table_flags - flags for the ctl table (struct ctl_table.flags)
+ *
+ * @CTL_FLAGS_CLAMP_SIGNED_RANGE: Set to indicate that the entry holds a
+ *	signed value and should be flexibly clamped to the provided
+ *	min/max signed value in case the user provided a value outside
+ *	of the given range.  The clamped value is either the provided
+ *	minimum or maximum value that is closest to the input value.
+ *	No lower bound or upper bound checking will be done if the
+ *	corresponding minimum or maximum value isn't provided.
+ *
+ * @CTL_FLAGS_CLAMP_UNSIGNED_RANGE: Set to indicate that the entry holds
+ *	an unsigned value and should be flexibly clamped to the provided
+ *	min/max unsigned value in case the user provided a value outside
+ *	of the given range.  The clamped value is either the provided
+ *	minimum or maximum value that is closest to the input value.
+ *	No lower bound or upper bound checking will be done if the
+ *	corresponding minimum or maximum value isn't provided.
+ *
+ * At most 16 different flags are currently allowed.
+ */
+enum ctl_table_flags {
+	CTL_FLAGS_CLAMP_SIGNED_RANGE	= BIT(0),
+	CTL_FLAGS_CLAMP_UNSIGNED_RANGE	= BIT(1),
+	__CTL_FLAGS_MAX			= BIT(2),
+};
+
+#define CTL_FLAGS_CLAMP_RANGE	(CTL_FLAGS_CLAMP_SIGNED_RANGE|\
+				 CTL_FLAGS_CLAMP_UNSIGNED_RANGE)
+#define CTL_TABLE_FLAGS_ALL	(__CTL_FLAGS_MAX - 1)
+
 struct ctl_node {
 	struct rb_node node;
 	struct ctl_table_header *header;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6a78cf7..5b84c1d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2515,6 +2515,7 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
  * struct do_proc_dointvec_minmax_conv_param - proc_dointvec_minmax() range checking structure
  * @min: pointer to minimum allowable value
  * @max: pointer to maximum allowable value
+ * @flags: pointer to flags
  *
  * The do_proc_dointvec_minmax_conv_param structure provides the
  * minimum and maximum values for doing range checking for those sysctl
@@ -2523,6 +2524,7 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
 struct do_proc_dointvec_minmax_conv_param {
 	int *min;
 	int *max;
+	uint16_t *flags;
 };
 
 static int do_proc_dointvec_minmax_conv(bool *negp, unsigned long *lvalp,
@@ -2532,9 +2534,23 @@ static int do_proc_dointvec_minmax_conv(bool *negp, unsigned long *lvalp,
 	struct do_proc_dointvec_minmax_conv_param *param = data;
 	if (write) {
 		int val = *negp ? -*lvalp : *lvalp;
-		if ((param->min && *param->min > val) ||
-		    (param->max && *param->max < val))
-			return -EINVAL;
+		bool clamp = param->flags &&
+			   (*param->flags & CTL_FLAGS_CLAMP_SIGNED_RANGE);
+
+		if (param->min && *param->min > val) {
+			if (clamp) {
+				val = *param->min;
+			} else {
+				return -EINVAL;
+			}
+		}
+		if (param->max && *param->max < val) {
+			if (clamp) {
+				val = *param->max;
+			} else {
+				return -EINVAL;
+			}
+		}
 		*valp = val;
 	} else {
 		int val = *valp;
@@ -2563,7 +2579,8 @@ static int do_proc_dointvec_minmax_conv(bool *negp, unsigned long *lvalp,
  * This routine will ensure the values are within the range specified by
  * table->extra1 (min) and table->extra2 (max).
  *
- * Returns 0 on success or -EINVAL on write when the range check fails.
+ * Returns 0 on success or -EINVAL on write when the range check fails
+ * without the CTL_FLAGS_CLAMP_SIGNED_RANGE flag.
  */
 int proc_dointvec_minmax(struct ctl_table *table, int write,
 		  void __user *buffer, size_t *lenp, loff_t *ppos)
@@ -2571,6 +2588,7 @@ int proc_dointvec_minmax(struct ctl_table *table, int write,
 	struct do_proc_dointvec_minmax_conv_param param = {
 		.min = (int *) table->extra1,
 		.max = (int *) table->extra2,
+		.flags = &table->flags,
 	};
 	return do_proc_dointvec(table, write, buffer, lenp, ppos,
 				do_proc_dointvec_minmax_conv, &param);
@@ -2580,6 +2598,7 @@ int proc_dointvec_minmax(struct ctl_table *table, int write,
  * struct do_proc_douintvec_minmax_conv_param - proc_douintvec_minmax() range checking structure
  * @min: pointer to minimum allowable value
  * @max: pointer to maximum allowable value
+ * @flags: pointer to flags
  *
  * The do_proc_douintvec_minmax_conv_param structure provides the
  * minimum and maximum values for doing range checking for those sysctl
@@ -2588,6 +2607,7 @@ int proc_dointvec_minmax(struct ctl_table *table, int write,
 struct do_proc_douintvec_minmax_conv_param {
 	unsigned int *min;
 	unsigned int *max;
+	uint16_t *flags;
 };
 
 static int do_proc_douintvec_minmax_conv(unsigned long *lvalp,
@@ -2598,14 +2618,26 @@ static int do_proc_douintvec_minmax_conv(unsigned long *lvalp,
 
 	if (write) {
 		unsigned int val = *lvalp;
+		bool clamp = param->flags &&
+			   (*param->flags & CTL_FLAGS_CLAMP_UNSIGNED_RANGE);
 
 		if (*lvalp > UINT_MAX)
 			return -EINVAL;
 
-		if ((param->min && *param->min > val) ||
-		    (param->max && *param->max < val))
-			return -ERANGE;
-
+		if (param->min && *param->min > val) {
+			if (clamp) {
+				val = *param->min;
+			} else {
+				return -ERANGE;
+			}
+		}
+		if (param->max && *param->max < val) {
+			if (clamp) {
+				val = *param->max;
+			} else {
+				return -ERANGE;
+			}
+		}
 		*valp = val;
 	} else {
 		unsigned int val = *valp;
@@ -2632,7 +2664,8 @@ static int do_proc_douintvec_minmax_conv(unsigned long *lvalp,
  * check for UINT_MAX to avoid having to support wrap around uses from
  * userspace.
  *
- * Returns 0 on success or -ERANGE on write when the range check fails.
+ * Returns 0 on success or -ERANGE on write when the range check fails
+ * without the CTL_FLAGS_CLAMP_UNSIGNED_RANGE flag.
  */
 int proc_douintvec_minmax(struct ctl_table *table, int write,
 			  void __user *buffer, size_t *lenp, loff_t *ppos)
@@ -2640,6 +2673,7 @@ int proc_douintvec_minmax(struct ctl_table *table, int write,
 	struct do_proc_douintvec_minmax_conv_param param = {
 		.min = (unsigned int *) table->extra1,
 		.max = (unsigned int *) table->extra2,
+		.flags = &table->flags,
 	};
 	return do_proc_douintvec(table, write, buffer, lenp, ppos,
 				 do_proc_douintvec_minmax_conv, &param);
@@ -2716,6 +2750,7 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
 	int vleft, first = 1, err = 0;
 	size_t left;
 	char *kbuf = NULL, *p;
+	uint16_t flags;
 
 	if (!data || !table->maxlen || !*lenp || (*ppos && !write)) {
 		*lenp = 0;
@@ -2725,6 +2760,7 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
 	i = (unsigned long *) data;
 	min = (unsigned long *) table->extra1;
 	max = (unsigned long *) table->extra2;
+	flags = table->flags;
 	vleft = table->maxlen / sizeof(unsigned long);
 	left = *lenp;
 
@@ -2755,8 +2791,20 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
 			if (neg)
 				continue;
 			val = convmul * val / convdiv;
-			if ((min && val < *min) || (max && val > *max))
-				continue;
+			if (min && val < *min) {
+				if (flags & CTL_FLAGS_CLAMP_UNSIGNED_RANGE) {
+					val = *min;
+				} else {
+					continue;
+				}
+			}
+			if (max && val > *max) {
+				if (flags & CTL_FLAGS_CLAMP_UNSIGNED_RANGE) {
+					val = *max;
+				} else {
+					continue;
+				}
+			}
 			*i = val;
 		} else {
 			val = convdiv * (*i) / convmul;
@@ -2808,7 +2856,9 @@ static int do_proc_doulongvec_minmax(struct ctl_table *table, int write,
  * values from/to the user buffer, treated as an ASCII string.
  *
  * This routine will ensure the values are within the range specified by
- * table->extra1 (min) and table->extra2 (max).
+ * table->extra1 (min) and table->extra2 (max).  Values outside the range
+ * will be ignored or clamped to the given range if the
+ * CTL_FLAGS_CLAMP_UNSIGNED_RANGE flag is specified.
  *
  * Returns 0 on success.
  */
-- 
1.8.3.1