[settings] allow value to have smaller than expected length (#4499)

Newer versions may add additional fields to the end of the settings
structure. However, there may be situations where an older version may
nead to read the structure.

This commit allows the stored settings value size to be smaller than
expected.

src/core/common/settings.cpp

@@ -119,11 +119,12 @@ otError Settings::SaveOperationalDataset(bool aIsActive, const MeshCoP::Dataset
 
 otError Settings::ReadOperationalDataset(bool aIsActive, MeshCoP::Dataset &aDataset) const
 {
-    otError  error = OT_ERROR_NONE;
-    uint16_t length;
+    otError  error  = OT_ERROR_NONE;
+    uint16_t length = MeshCoP::Dataset::kMaxSize;
+
+    SuccessOrExit(error = Read(aIsActive ? kKeyActiveDataset : kKeyPendingDataset, aDataset.GetBytes(), length));
+    VerifyOrExit(length <= MeshCoP::Dataset::kMaxSize, error = OT_ERROR_NOT_FOUND);
 
-    SuccessOrExit(error = Read(aIsActive ? kKeyActiveDataset : kKeyPendingDataset, aDataset.GetBytes(),
-                               MeshCoP::Dataset::kMaxSize, length));
     aDataset.SetSize(length);
 
 exit:
@@ -141,10 +142,11 @@ otError Settings::DeleteOperationalDataset(bool aIsActive)
 
 otError Settings::ReadNetworkInfo(NetworkInfo &aNetworkInfo) const
 {
-    otError error;
+    otError  error;
+    uint16_t length = sizeof(NetworkInfo);
 
     aNetworkInfo.Init();
-    SuccessOrExit(error = ReadFixedSize(kKeyNetworkInfo, &aNetworkInfo, sizeof(NetworkInfo)));
+    SuccessOrExit(error = Read(kKeyNetworkInfo, &aNetworkInfo, length));
     LogNetworkInfo("Read", aNetworkInfo);
 
 exit:
@@ -155,8 +157,9 @@ otError Settings::SaveNetworkInfo(const NetworkInfo &aNetworkInfo)
 {
     otError     error = OT_ERROR_NONE;
     NetworkInfo prevNetworkInfo;
+    uint16_t    length = sizeof(prevNetworkInfo);
 
-    if ((ReadFixedSize(kKeyNetworkInfo, &prevNetworkInfo, sizeof(NetworkInfo)) == OT_ERROR_NONE) &&
+    if ((Read(kKeyNetworkInfo, &prevNetworkInfo, length) == OT_ERROR_NONE) && (length == sizeof(NetworkInfo)) &&
         (memcmp(&prevNetworkInfo, &aNetworkInfo, sizeof(NetworkInfo)) == 0))
     {
         LogNetworkInfo("Re-saved", aNetworkInfo);
@@ -185,10 +188,11 @@ exit:
 
 otError Settings::ReadParentInfo(ParentInfo &aParentInfo) const
 {
-    otError error;
+    otError  error;
+    uint16_t length = sizeof(ParentInfo);
 
     aParentInfo.Init();
-    SuccessOrExit(error = ReadFixedSize(kKeyParentInfo, &aParentInfo, sizeof(ParentInfo)));
+    SuccessOrExit(error = Read(kKeyParentInfo, &aParentInfo, length));
     LogParentInfo("Read", aParentInfo);
 
 exit:
@@ -199,8 +203,9 @@ otError Settings::SaveParentInfo(const ParentInfo &aParentInfo)
 {
     otError    error = OT_ERROR_NONE;
     ParentInfo prevParentInfo;
+    uint16_t   length = sizeof(ParentInfo);
 
-    if ((ReadFixedSize(kKeyParentInfo, &prevParentInfo, sizeof(ParentInfo)) == OT_ERROR_NONE) &&
+    if ((Read(kKeyParentInfo, &prevParentInfo, length) == OT_ERROR_NONE) && (length == sizeof(ParentInfo)) &&
         (memcmp(&prevParentInfo, &aParentInfo, sizeof(ParentInfo)) == 0))
     {
         LogParentInfo("Re-saved", aParentInfo);
@@ -290,38 +295,23 @@ exit:
 
 void Settings::ChildInfoIterator::Read(void)
 {
-    uint16_t size = sizeof(ChildInfo);
+    uint16_t length = sizeof(ChildInfo);
     otError  error;
 
     mChildInfo.Init();
     SuccessOrExit(error = otPlatSettingsGet(&GetInstance(), kKeyChildInfo, mIndex,
-                                            reinterpret_cast<uint8_t *>(&mChildInfo), &size));
-    VerifyOrExit(size >= sizeof(ChildInfo), error = OT_ERROR_NOT_FOUND);
+                                            reinterpret_cast<uint8_t *>(&mChildInfo), &length));
     LogChildInfo("Read", mChildInfo);
 
 exit:
     mIsDone = (error != OT_ERROR_NONE);
 }
 
-otError Settings::ReadFixedSize(Key aKey, void *aBuffer, uint16_t aExpectedSize) const
+otError Settings::Read(Key aKey, void *aBuffer, uint16_t &aSize) const
 {
-    uint16_t size = aExpectedSize;
-    otError  error;
+    otError error;
 
-    SuccessOrExit(error = otPlatSettingsGet(&GetInstance(), aKey, 0, reinterpret_cast<uint8_t *>(aBuffer), &size));
-    VerifyOrExit(size >= aExpectedSize, error = OT_ERROR_NOT_FOUND);
-
-exit:
-    return error;
-}
-
-otError Settings::Read(Key aKey, void *aBuffer, uint16_t aMaxBufferSize, uint16_t &aReadSize) const
-{
-    uint16_t size = aMaxBufferSize;
-    otError  error;
-
-    SuccessOrExit(error = otPlatSettingsGet(&GetInstance(), aKey, 0, reinterpret_cast<uint8_t *>(aBuffer), &size));
-    aReadSize = (size <= aMaxBufferSize) ? size : aMaxBufferSize;
+    SuccessOrExit(error = otPlatSettingsGet(&GetInstance(), aKey, 0, reinterpret_cast<uint8_t *>(aBuffer), &aSize));
 
 exit:
     return error;

src/core/common/settings.hpp

@@ -584,7 +584,9 @@ public:
      */
     otError ReadSlaacIidSecretKey(Utils::Slaac::IidSecretKey &aKey)
     {
-        return ReadFixedSize(kKeySlaacIidSecretKey, &aKey, sizeof(Utils::Slaac::IidSecretKey));
+        uint16_t length = sizeof(aKey);
+
+        return Read(kKeySlaacIidSecretKey, &aKey, length);
     }
 
     /**
@@ -703,8 +705,7 @@ public:
     };
 
 private:
-    otError ReadFixedSize(Key aKey, void *aBuffer, uint16_t aExpectedSize) const;
-    otError Read(Key aKey, void *aBuffer, uint16_t aMaxBufferSize, uint16_t &aReadSize) const;
+    otError Read(Key aKey, void *aBuffer, uint16_t &aSize) const;
     otError Save(Key aKey, const void *aValue, uint16_t aSize);
     otError Add(Key aKey, const void *aValue, uint16_t aSize);
     otError Delete(Key aKey);

src/core/meshcop/dataset.hpp

@@ -55,7 +55,7 @@ public:
     enum
     {
         kMaxSize      = 256, ///< Maximum size of MeshCoP Dataset (bytes)
-        kMaxValueSize = 16,  /// < Maximum size of each Dataset TLV value (bytes)
+        kMaxValueSize = 16,  ///< Maximum size of each Dataset TLV value (bytes)
     };
 
     /**