kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 188 insertions, 0 deletions

diff --git a/fs/verity/hash_algs.c b/fs/verity/hash_algs.c
new file mode 100644
index 000000000..de53e14c8
--- /dev/null
+++ b/fs/verity/hash_algs.c
@@ -0,0 +1,188 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fs-verity hash algorithms
+ *
+ * Copyright 2019 Google LLC
+ */
+
+#include "fsverity_private.h"
+
+/* The hash algorithms supported by fs-verity */
+const struct fsverity_hash_alg fsverity_hash_algs[] = {
+	[FS_VERITY_HASH_ALG_SHA256] = {
+		.name = "sha256",
+		.digest_size = SHA256_DIGEST_SIZE,
+		.block_size = SHA256_BLOCK_SIZE,
+		.algo_id = HASH_ALGO_SHA256,
+	},
+	[FS_VERITY_HASH_ALG_SHA512] = {
+		.name = "sha512",
+		.digest_size = SHA512_DIGEST_SIZE,
+		.block_size = SHA512_BLOCK_SIZE,
+		.algo_id = HASH_ALGO_SHA512,
+	},
+};
+
+/**
+ * fsverity_get_hash_alg() - get a hash algorithm by number
+ * @inode: optional inode for logging purposes
+ * @num: the hash algorithm number
+ *
+ * Get the struct fsverity_hash_alg for the given hash algorithm number.
+ *
+ * Return: pointer to the hash alg if it's known, otherwise NULL.
+ */
+const struct fsverity_hash_alg *fsverity_get_hash_alg(const struct inode *inode,
+						      unsigned int num)
+{
+	if (num >= ARRAY_SIZE(fsverity_hash_algs) ||
+	    !fsverity_hash_algs[num].name) {
+		fsverity_warn(inode, "Unknown hash algorithm number: %u", num);
+		return NULL;
+	}
+	return &fsverity_hash_algs[num];
+}
+
+/**
+ * fsverity_prepare_hash_state() - precompute the initial hash state
+ * @alg: hash algorithm
+ * @salt: a salt which is to be prepended to all data to be hashed
+ * @salt_size: salt size in bytes
+ *
+ * Return: the kmalloc()'ed initial hash state, or NULL if out of memory.
+ */
+union fsverity_hash_ctx *
+fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
+			    const u8 *salt, size_t salt_size)
+{
+	u8 *padded_salt = NULL;
+	size_t padded_salt_size;
+	union fsverity_hash_ctx ctx;
+	void *res = NULL;
+
+	/*
+	 * Zero-pad the salt to the next multiple of the input size of the hash
+	 * algorithm's compression function, e.g. 64 bytes for SHA-256 or 128
+	 * bytes for SHA-512.  This ensures that the hash algorithm won't have
+	 * any bytes buffered internally after processing the salt, thus making
+	 * salted hashing just as fast as unsalted hashing.
+	 */
+	padded_salt_size = round_up(salt_size, alg->block_size);
+	padded_salt = kzalloc(padded_salt_size, GFP_KERNEL);
+	if (!padded_salt)
+		return NULL;
+	memcpy(padded_salt, salt, salt_size);
+
+	switch (alg->algo_id) {
+	case HASH_ALGO_SHA256:
+		sha256_init(&ctx.sha256);
+		sha256_update(&ctx.sha256, padded_salt, padded_salt_size);
+		res = kmemdup(&ctx.sha256, sizeof(ctx.sha256), GFP_KERNEL);
+		break;
+	case HASH_ALGO_SHA512:
+		sha512_init(&ctx.sha512);
+		sha512_update(&ctx.sha512, padded_salt, padded_salt_size);
+		res = kmemdup(&ctx.sha512, sizeof(ctx.sha512), GFP_KERNEL);
+		break;
+	default:
+		WARN_ON_ONCE(1);
+	}
+	kfree(padded_salt);
+	return res;
+}
+
+/**
+ * fsverity_hash_block() - hash a single data or hash block
+ * @params: the Merkle tree's parameters
+ * @data: virtual address of a buffer containing the block to hash
+ * @out: output digest, size 'params->digest_size' bytes
+ *
+ * Hash a single data or hash block.  The hash is salted if a salt is specified
+ * in the Merkle tree parameters.
+ */
+void fsverity_hash_block(const struct merkle_tree_params *params,
+			 const void *data, u8 *out)
+{
+	union fsverity_hash_ctx ctx;
+
+	if (!params->hashstate) {
+		fsverity_hash_buffer(params->hash_alg, data, params->block_size,
+				     out);
+		return;
+	}
+
+	switch (params->hash_alg->algo_id) {
+	case HASH_ALGO_SHA256:
+		ctx.sha256 = params->hashstate->sha256;
+		sha256_update(&ctx.sha256, data, params->block_size);
+		sha256_final(&ctx.sha256, out);
+		return;
+	case HASH_ALGO_SHA512:
+		ctx.sha512 = params->hashstate->sha512;
+		sha512_update(&ctx.sha512, data, params->block_size);
+		sha512_final(&ctx.sha512, out);
+		return;
+	default:
+		BUG();
+	}
+}
+
+/**
+ * fsverity_hash_buffer() - hash some data
+ * @alg: the hash algorithm to use
+ * @data: the data to hash
+ * @size: size of data to hash, in bytes
+ * @out: output digest, size 'alg->digest_size' bytes
+ */
+void fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
+			  const void *data, size_t size, u8 *out)
+{
+	switch (alg->algo_id) {
+	case HASH_ALGO_SHA256:
+		sha256(data, size, out);
+		return;
+	case HASH_ALGO_SHA512:
+		sha512(data, size, out);
+		return;
+	default:
+		BUG();
+	}
+}
+
+void __init fsverity_check_hash_algs(void)
+{
+	size_t i;
+
+	/*
+	 * Sanity check the hash algorithms (could be a build-time check, but
+	 * they're in an array)
+	 */
+	for (i = 0; i < ARRAY_SIZE(fsverity_hash_algs); i++) {
+		const struct fsverity_hash_alg *alg = &fsverity_hash_algs[i];
+
+		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);
+
+		/*
+		 * For efficiency, the implementation currently assumes the
+		 * digest and block sizes are powers of 2.  This limitation can
+		 * be lifted if the code is updated to handle other values.
+		 */
+		BUG_ON(!is_power_of_2(alg->digest_size));
+		BUG_ON(!is_power_of_2(alg->block_size));
+
+		/* Verify that there is a valid mapping to HASH_ALGO_*. */
+		BUG_ON(alg->algo_id == 0);
+		BUG_ON(alg->digest_size != hash_digest_size[alg->algo_id]);
+	}
+}