linux/spl: base proc_dohostid() on proc_dostring()

This fixes /proc/sys/kernel/spl/hostid on kernels with mainline commit
32927393dc1ccd60fb2bdc05b9e8e88753761469 ("sysctl: pass kernel pointers
to ->proc_handler") ‒ 5.7-rc1 and up

The access_ok() check in copy_to_user() in proc_copyout_string() would
always fail, so all userspace reads and writes would fail with EINVAL

This strictens write parsing by disallowing writes with extraneous
whitespace at the end, i.e. "012345678\n" is still fine,
but "012345678   \n" isn't anymore

This also alters what happens when an invalid value is written ‒
previously it'd get set to what-ever simple_strtoul() returned
(probably 0, thereby resetting it to default), now it does nothing

Signed-off-by: Ahelenia Ziemiańska <[email protected]>
Closes #11878

Author: nabijaczleweli

module/os/linux/spl/spl-proc.c

@@ -53,60 +53,6 @@ static struct proc_dir_entry *proc_spl_taskq_all = NULL;
 static struct proc_dir_entry *proc_spl_taskq = NULL;
 struct proc_dir_entry *proc_spl_kstat = NULL;
 
-static int
-proc_copyin_string(char *kbuffer, int kbuffer_size, const char *ubuffer,
-    int ubuffer_size)
-{
-	int size;
-
-	if (ubuffer_size > kbuffer_size)
-		return (-EOVERFLOW);
-
-	if (copy_from_user((void *)kbuffer, (void *)ubuffer, ubuffer_size))
-		return (-EFAULT);
-
-	/* strip trailing whitespace */
-	size = strnlen(kbuffer, ubuffer_size);
-	while (size-- >= 0)
-		if (!isspace(kbuffer[size]))
-			break;
-
-	/* empty string */
-	if (size < 0)
-		return (-EINVAL);
-
-	/* no space to terminate */
-	if (size == kbuffer_size)
-		return (-EOVERFLOW);
-
-	kbuffer[size + 1] = 0;
-	return (0);
-}
-
-static int
-proc_copyout_string(char *ubuffer, int ubuffer_size, const char *kbuffer,
-    char *append)
-{
-	/*
-	 * NB if 'append' != NULL, it's a single character to append to the
-	 * copied out string - usually "\n", for /proc entries and
-	 * (i.e. a terminating zero byte) for sysctl entries
-	 */
-	int size = MIN(strlen(kbuffer), ubuffer_size);
-
-	if (copy_to_user(ubuffer, kbuffer, size))
-		return (-EFAULT);
-
-	if (append != NULL && size < ubuffer_size) {
-		if (copy_to_user(ubuffer + size, append, 1))
-			return (-EFAULT);
-
-		size++;
-	}
-
-	return (size);
-}
-
 #ifdef DEBUG_KMEM
 static int
 proc_domemused(struct ctl_table *table, int write,
@@ -187,39 +133,35 @@ static int
 proc_dohostid(struct ctl_table *table, int write,
     void __user *buffer, size_t *lenp, loff_t *ppos)
 {
-	int len, rc = 0;
 	char *end, str[32];
+	unsigned long hid;
+	spl_ctl_table dummy = *table;
+
+	dummy.data = str;
+	dummy.maxlen = sizeof (str) - 1;
+
+	if (!write)
+		snprintf(str, sizeof (str), "%lx",
+		    (unsigned long) zone_get_hostid(NULL));
+
+	/* always returns 0 */
+	proc_dostring(&dummy, write, buffer, lenp, ppos);
 
 	if (write) {
 		/*
 		 * We can't use proc_doulongvec_minmax() in the write
-		 * case here because hostid while a hex value has no
-		 * leading 0x which confuses the helper function.
+		 * case here because hostid, while a hex value, has no
+		 * leading 0x, which confuses the helper function.
 		 */
-		rc = proc_copyin_string(str, sizeof (str), buffer, *lenp);
-		if (rc < 0)
-			return (rc);
 
-		spl_hostid = simple_strtoul(str, &end, 16);
+		/* proc_dostring() already stripped the final newline */
+		hid = simple_strtoul(str, &end, 16);
 		if (str == end)
 			return (-EINVAL);
-
-	} else {
-		len = snprintf(str, sizeof (str), "%lx",
-		    (unsigned long) zone_get_hostid(NULL));
-		if (*ppos >= len)
-			rc = 0;
-		else
-			rc = proc_copyout_string(buffer,
-			    *lenp, str + *ppos, "\n");
-
-		if (rc >= 0) {
-			*lenp = rc;
-			*ppos += rc;
-		}
+		spl_hostid = hid;
 	}
 
-	return (rc);
+	return (0);
 }
 
 static void