[PATCH v5 5/6] fscrypt: replace local base64url helpers with lib/base64

Guan-Chun Wu
2025-11-14T06:02:27Z

Replace the base64url encoding and decoding functions in fscrypt with the
generic base64_encode() and base64_decode() helpers from lib/base64.

This removes the custom implementation in fscrypt, reduces code
duplication, and relies on the shared Base64 implementation in lib.
The helpers preserve RFC 4648-compliant URL-safe Base64 encoding without
padding, so there are no functional changes.

This change also improves performance: encoding is about 2.7x faster and
decoding achieves 43-52x speedups compared to the previous implementation.

Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Guan-Chun Wu <409411716@gms.tku.edu.tw>
---
fs/crypto/fname.c | 89 ++++-------------------------------------------
1 file changed, 6 insertions(+), 83 deletions(-)

diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
index 8e4c213d418b..a9a4432d12ba 100644
--- a/fs/crypto/fname.c
+++ b/fs/crypto/fname.c
@@ -16,6 +16,7 @@
#include <linux/export.h>
#include <linux/namei.h>
#include <linux/scatterlist.h>
+#include <linux/base64.h>

#include "fscrypt_private.h"

@@ -71,7 +72,7 @@ struct fscrypt_nokey_name {

/* Encoded size of max-size no-key name */
#define FSCRYPT_NOKEY_NAME_MAX_ENCODED \
- FSCRYPT_BASE64URL_CHARS(FSCRYPT_NOKEY_NAME_MAX)
+ BASE64_CHARS(FSCRYPT_NOKEY_NAME_MAX)

static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
{
@@ -162,84 +163,6 @@ static int fname_decrypt(const struct inode *inode,
return 0;
}

-static const char base64url_table[65] =
- "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
-
-#define FSCRYPT_BASE64URL_CHARS(nbytes) DIV_ROUND_UP((nbytes) * 4, 3)
-
-/**
- * fscrypt_base64url_encode() - base64url-encode some binary data
- * @src: the binary data to encode
- * @srclen: the length of @src in bytes
- * @dst: (output) the base64url-encoded string. Not NUL-terminated.
- *
- * Encodes data using base64url encoding, i.e. the "Base 64 Encoding with URL
- * and Filename Safe Alphabet" specified by RFC 4648. '='-padding isn't used,
- * as it's unneeded and not required by the RFC. base64url is used instead of
- * base64 to avoid the '/' character, which isn't allowed in filenames.
- *
- * Return: the length of the resulting base64url-encoded string in bytes.
- * This will be equal to FSCRYPT_BASE64URL_CHARS(srclen).
- */
-static int fscrypt_base64url_encode(const u8 *src, int srclen, char *dst)
-{
- u32 ac = 0;
- int bits = 0;
- int i;
- char *cp = dst;
-
- for (i = 0; i < srclen; i++) {
- ac = (ac << 8) | src[i];
- bits += 8;
- do {
- bits -= 6;
- *cp++ = base64url_table[(ac >> bits) & 0x3f];
- } while (bits >= 6);
- }
- if (bits)
- *cp++ = base64url_table[(ac << (6 - bits)) & 0x3f];
- return cp - dst;
-}
-
-/**
- * fscrypt_base64url_decode() - base64url-decode a string
- * @src: the string to decode. Doesn't need to be NUL-terminated.
- * @srclen: the length of @src in bytes
- * @dst: (output) the decoded binary data
- *
- * Decodes a string using base64url encoding, i.e. the "Base 64 Encoding with
- * URL and Filename Safe Alphabet" specified by RFC 4648. '='-padding isn't
- * accepted, nor are non-encoding characters such as whitespace.
- *
- * This implementation hasn't been optimized for performance.
- *
- * Return: the length of the resulting decoded binary data in bytes,
- * or -1 if the string isn't a valid base64url string.
- */
-static int fscrypt_base64url_decode(const char *src, int srclen, u8 *dst)
-{
- u32 ac = 0;
- int bits = 0;
- int i;
- u8 *bp = dst;
-
- for (i = 0; i < srclen; i++) {
- const char *p = strchr(base64url_table, src[i]);
-
- if (p == NULL || src[i] == 0)
- return -1;
- ac = (ac << 6) | (p - base64url_table);
- bits += 6;
- if (bits >= 8) {
- bits -= 8;
- *bp++ = (u8)(ac >> bits);
- }
- }
- if (ac & ((1 << bits) - 1))
- return -1;
- return bp - dst;
-}
-
bool __fscrypt_fname_encrypted_size(const union fscrypt_policy *policy,
u32 orig_len, u32 max_len,
u32 *encrypted_len_ret)
@@ -387,8 +310,8 @@ int fscrypt_fname_disk_to_usr(const struct inode *inode,
nokey_name.sha256);
size = FSCRYPT_NOKEY_NAME_MAX;
}
- oname->len = fscrypt_base64url_encode((const u8 *)&nokey_name, size,
- oname->name);
+ oname->len = base64_encode((const u8 *)&nokey_name, size,
+ oname->name, false, BASE64_URLSAFE);
return 0;
}
EXPORT_SYMBOL(fscrypt_fname_disk_to_usr);
@@ -467,8 +390,8 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
if (fname->crypto_buf.name == NULL)
return -ENOMEM;

- ret = fscrypt_base64url_decode(iname->name, iname->len,
- fname->crypto_buf.name);
+ ret = base64_decode(iname->name, iname->len,
+ fname->crypto_buf.name, false, BASE64_URLSAFE);
if (ret < (int)offsetof(struct fscrypt_nokey_name, bytes[1]) ||
(ret > offsetof(struct fscrypt_nokey_name, sha256) &&
ret != FSCRYPT_NOKEY_NAME_MAX)) {
--
2.34.1