[RFC PATCH 11/20] hfs: Explicitly set hsb->nls_disk when hsb->nls_io is set

["Pali Rohár" <pali@kernel.org>]

It does not make any sense to set hsb->nls_io (NLS iocharset used between
VFS and hfs driver) when hsb->nls_disk (NLS codepage used between hfs
driver and disk) is not set.

Reverse engineering driver code shown what is doing in this special case:

    When codepage was not defined but iocharset was then
    hfs driver copied 8bit character from disk directly to
    16bit unicode wchar_t type. Which means it did conversion
    from Latin1 (ISO-8859-1) to Unicode because first 256
    Unicode code points matches 8bit ISO-8859-1 codepage table.
    So when iocharset was specified and codepage not, then
    codepage used implicit value "iso8859-1".

So when hsb->nls_disk is not set and hsb->nls_io is then explicitly set
hsb->nls_disk to "iso8859-1".

Such setup is obviously incompatible with Mac OS systems as they do not
support iso8859-1 encoding for hfs. So print warning into dmesg about this
fact.

After this change hsb->nls_disk is always set, so remove code paths for
case when hsb->nls_disk was not set as they are not needed anymore.

Signed-off-by: Pali Rohár <pali@kernel.org>
---
 fs/hfs/super.c | 31 +++++++++++++++++++++++++++++++
 fs/hfs/trans.c | 38 ++++++++++++++------------------------
 2 files changed, 45 insertions(+), 24 deletions(-)

diff --git a/fs/hfs/super.c b/fs/hfs/super.c
index 12d9bae39363..86bc46746c7f 100644
--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -351,6 +351,37 @@ static int parse_options(char *options, struct hfs_sb_info *hsb)
 		}
 	}
 
+	if (hsb->nls_io && !hsb->nls_disk) {
+		/*
+		 * Previous version of hfs driver did something unexpected:
+		 * When codepage was not defined but iocharset was then
+		 * hfs driver copied 8bit character from disk directly to
+		 * 16bit unicode wchar_t type. Which means it did conversion
+		 * from Latin1 (ISO-8859-1) to Unicode because first 256
+		 * Unicode code points matches 8bit ISO-8859-1 codepage table.
+		 * So when iocharset was specified and codepage not, then
+		 * codepage used implicit value "iso8859-1".
+		 *
+		 * To not change this previous default behavior as some users
+		 * may depend on it, we load iso8859-1 NLS table explicitly
+		 * to simplify code and make it more reable what happens.
+		 *
+		 * In context of hfs driver it is really strange to use
+		 * ISO-8859-1 codepage table for storing data to disk, but
+		 * nothing forbids it. Just it is highly incompatible with
+		 * Mac OS systems. So via pr_warn() inform user that this
+		 * is not probably what he wants.
+		 */
+		pr_warn("iocharset was specified but codepage not, "
+			"using default codepage=iso8859-1\n");
+		pr_warn("this default codepage=iso8859-1 is incompatible with "
+			"Mac OS systems and may be changed in the future");
+		hsb->nls_disk = load_nls("iso8859-1");
+		if (!hsb->nls_disk) {
+			pr_err("unable to load iso8859-1 codepage\n");
+			return 0;
+		}
+	}
 	if (hsb->nls_disk && !hsb->nls_io) {
 		hsb->nls_io = load_nls_default();
 		if (!hsb->nls_io) {
diff --git a/fs/hfs/trans.c b/fs/hfs/trans.c
index 39f5e343bf4d..c75682c61b06 100644
--- a/fs/hfs/trans.c
+++ b/fs/hfs/trans.c
@@ -48,18 +48,13 @@ int hfs_mac2asc(struct super_block *sb, char *out, const struct hfs_name *in)
 		wchar_t ch;
 
 		while (srclen > 0) {
-			if (nls_disk) {
-				size = nls_disk->char2uni(src, srclen, &ch);
-				if (size <= 0) {
-					ch = '?';
-					size = 1;
-				}
-				src += size;
-				srclen -= size;
-			} else {
-				ch = *src++;
-				srclen--;
+			size = nls_disk->char2uni(src, srclen, &ch);
+			if (size <= 0) {
+				ch = '?';
+				size = 1;
 			}
+			src += size;
+			srclen -= size;
 			if (ch == '/')
 				ch = ':';
 			size = nls_io->uni2char(ch, dst, dstlen);
@@ -119,20 +114,15 @@ void hfs_asc2mac(struct super_block *sb, struct hfs_name *out, const struct qstr
 			srclen -= size;
 			if (ch == ':')
 				ch = '/';
-			if (nls_disk) {
-				size = nls_disk->uni2char(ch, dst, dstlen);
-				if (size < 0) {
-					if (size == -ENAMETOOLONG)
-						goto out;
-					*dst = '?';
-					size = 1;
-				}
-				dst += size;
-				dstlen -= size;
-			} else {
-				*dst++ = ch > 0xff ? '?' : ch;
-				dstlen--;
+			size = nls_disk->uni2char(ch, dst, dstlen);
+			if (size < 0) {
+				if (size == -ENAMETOOLONG)
+					goto out;
+				*dst = '?';
+				size = 1;
 			}
+			dst += size;
+			dstlen -= size;
 		}
 	} else {
 		char ch;
-- 
2.20.1