fscrypt: avoid data race on fscrypt_mode::logged_impl_name

The access to logged_impl_name is technically a data race, which tools
like KCSAN could complain about in the future.  See:
https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE

Fix by using xchg(), which also ensures that only one thread does the
logging.

This also required switching from bool to int, to avoid a build error on
the RISC-V architecture which doesn't implement xchg on bytes.

Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index dacf8fc..d9a3e86 100644 (file)
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -435,7 +435,7 @@ struct fscrypt_mode {
        const char *cipher_str;
        int keysize;
        int ivsize;
-       bool logged_impl_name;
+       int logged_impl_name;
 };
 
 static inline bool

diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index b03b336..28bc2da 100644 (file)
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -81,15 +81,13 @@ struct crypto_skcipher *fscrypt_allocate_skcipher(struct fscrypt_mode *mode,
                            mode->cipher_str, PTR_ERR(tfm));
                return tfm;
        }
-       if (unlikely(!mode->logged_impl_name)) {
+       if (!xchg(&mode->logged_impl_name, 1)) {
                /*
                 * fscrypt performance can vary greatly depending on which
                 * crypto algorithm implementation is used.  Help people debug
                 * performance problems by logging the ->cra_driver_name the
-                * first time a mode is used.  Note that multiple threads can
-                * race here, but it doesn't really matter.
+                * first time a mode is used.
                 */
-               mode->logged_impl_name = true;
                pr_info("fscrypt: %s using implementation \"%s\"\n",
                        mode->friendly_name,
                        crypto_skcipher_alg(tfm)->base.cra_driver_name);