fsverity: explicitly check that there is no algorithm 0

Since libfsverity and some other code would break if 0 is ever allocated
as an FS_VERITY_HASH_ALG_* value, make fsverity_check_hash_algs()
explicitly check that there is no algorithm 0.

Link: https://lore.kernel.org/r/20230705211719.37713-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/verity/hash_algs.c b/fs/verity/hash_algs.c
index c598d20354763156b6dfaee044a69fca385697bc..6b08b1d9a7d7c49baf287a402ad1ac105fbb5614 100644 (file)
--- a/fs/verity/hash_algs.c
+++ b/fs/verity/hash_algs.c
@@ -226,6 +226,14 @@ void __init fsverity_check_hash_algs(void)
                if (!alg->name)
                        continue;
 
+               /*
+                * 0 must never be allocated as an FS_VERITY_HASH_ALG_* value,
+                * as it is reserved for users that use 0 to mean unspecified or
+                * a default value.  fs/verity/ itself doesn't care and doesn't
+                * have a default algorithm, but some users make use of this.
+                */
+               BUG_ON(i == 0);
+
                BUG_ON(alg->digest_size > FS_VERITY_MAX_DIGEST_SIZE);
 
                /*