dm cache metadata: replace deprecated strncpy with strscpy

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

It seems `cmd->policy_name` is intended to be NUL-terminated based on a
now changed line of code from Commit (c6b4fcbad044e6ff "dm: add cache
target"):
|       if (strcmp(cmd->policy_name, policy_name)) { // ...

However, now a length-bounded strncmp is used:
|       if (strncmp(cmd->policy_name, policy_name, sizeof(cmd->policy_name)))
... which means NUL-terminated may not strictly be required. However, I
believe the intent of the code is clear and we should maintain
NUL-termination of policy_names.

Moreover, __begin_transaction_flags() zero-allocates `cmd` before
calling read_superblock_fields():
|       cmd = kzalloc(sizeof(*cmd), GFP_KERNEL);

Also, `disk_super->policy_name` is zero-initialized
|       memset(disk_super->policy_name, 0, sizeof(disk_super->policy_name));
... therefore any NUL-padding is redundant.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
Link: https://github.com/KSPP/linux/issues/90
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

diff --git a/drivers/md/dm-cache-metadata.c b/drivers/md/dm-cache-metadata.c
index acffed750e3edc8d248a409626e49ba57172e5c6..5a18b80d3666e75fe6a4702de9aab7003a55ff18 100644 (file)
--- a/drivers/md/dm-cache-metadata.c
+++ b/drivers/md/dm-cache-metadata.c
@@ -597,7 +597,7 @@ static void read_superblock_fields(struct dm_cache_metadata *cmd,
        cmd->discard_nr_blocks = to_dblock(le64_to_cpu(disk_super->discard_nr_blocks));
        cmd->data_block_size = le32_to_cpu(disk_super->data_block_size);
        cmd->cache_blocks = to_cblock(le32_to_cpu(disk_super->cache_blocks));
-       strncpy(cmd->policy_name, disk_super->policy_name, sizeof(cmd->policy_name));
+       strscpy(cmd->policy_name, disk_super->policy_name, sizeof(cmd->policy_name));
        cmd->policy_version[0] = le32_to_cpu(disk_super->policy_version[0]);
        cmd->policy_version[1] = le32_to_cpu(disk_super->policy_version[1]);
        cmd->policy_version[2] = le32_to_cpu(disk_super->policy_version[2]);
@@ -707,7 +707,7 @@ static int __commit_transaction(struct dm_cache_metadata *cmd,
        disk_super->discard_block_size = cpu_to_le64(cmd->discard_block_size);
        disk_super->discard_nr_blocks = cpu_to_le64(from_dblock(cmd->discard_nr_blocks));
        disk_super->cache_blocks = cpu_to_le32(from_cblock(cmd->cache_blocks));
-       strncpy(disk_super->policy_name, cmd->policy_name, sizeof(disk_super->policy_name));
+       strscpy(disk_super->policy_name, cmd->policy_name, sizeof(disk_super->policy_name));
        disk_super->policy_version[0] = cpu_to_le32(cmd->policy_version[0]);
        disk_super->policy_version[1] = cpu_to_le32(cmd->policy_version[1]);
        disk_super->policy_version[2] = cpu_to_le32(cmd->policy_version[2]);
@@ -1726,7 +1726,7 @@ static int write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *po
            (strlen(policy_name) > sizeof(cmd->policy_name) - 1))
                return -EINVAL;
 
-       strncpy(cmd->policy_name, policy_name, sizeof(cmd->policy_name));
+       strscpy(cmd->policy_name, policy_name, sizeof(cmd->policy_name));
        memcpy(cmd->policy_version, policy_version, sizeof(cmd->policy_version));
 
        hint_size = dm_cache_policy_get_hint_size(policy);