nbd: use shifts rather than multiplies

commit fad7cd3310db ("nbd: add the check to prevent overflow in
__nbd_ioctl()") raised an issue from the fallback helpers added in
commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
add fallback code")

ERROR: modpost: "__divdi3" [drivers/block/nbd.ko] undefined!

As Stephen Rothwell notes:
The added check_mul_overflow() call is being passed 64 bit values.
COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW is not set for this build (see
include/linux/overflow.h).

Specifically, the helpers for checking whether the results of a
multiplication overflowed (__unsigned_mul_overflow,
__signed_add_overflow) use the division operator when
!COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW. This is problematic for 64b
operands on 32b hosts.

This was fixed upstream by
commit 76ae847497bc ("Documentation: raise minimum supported version of
GCC to 5.1")
which is not suitable to be backported to stable.

Further, __builtin_mul_overflow() would emit a libcall to a
compiler-rt-only symbol when compiling with clang < 14 for 32b targets.

ld.lld: error: undefined symbol: __mulodi4

In order to keep stable buildable with GCC 4.9 and clang < 14, modify
struct nbd_config to instead track the number of bits of the block size;
reconstructing the block size using runtime checked shifts that are not
problematic for those compilers and in a ways that can be backported to
stable.

In nbd_set_size, we do validate that the value of blksize must be a
power of two (POT) and is in the range of [512, PAGE_SIZE] (both
inclusive).

This does modify the debugfs interface.

Cc: [e-mail]
Cc: Arnd Bergmann <[e-mail]>
Cc: Rasmus Villemoes <[e-mail]>
Link: https://github.com/ClangBuiltLinux/linux/issues/1438
Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lore.kernel.org/stable/CAHk-=[e-mail]/
Reported-by: Naresh Kamboju <[e-mail]>
Reported-by: Nathan Chancellor <[e-mail]>
Reported-by: Stephen Rothwell <[e-mail]>
Suggested-by: Kees Cook <[e-mail]>
Suggested-by: Linus Torvalds <[e-mail]>
Suggested-by: Pavel Machek <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Josef Bacik <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/nbd.c

···
 
 	atomic_t recv_threads;
 	wait_queue_head_t recv_wq;
-	loff_t blksize;
+	unsigned int blksize_bits;
 	loff_t bytesize;
 #if IS_ENABLED(CONFIG_DEBUG_FS)
 	struct dentry *dbg_dir;
 #endif
 };
+
+static inline unsigned int nbd_blksize(struct nbd_config *config)
+{
+	return 1u << config->blksize_bits;
+}
 
 struct nbd_device {
 	struct blk_mq_tag_set tag_set;
···
 
 #define NBD_MAGIC 0x68797548
 
-#define NBD_DEF_BLKSIZE 1024
+#define NBD_DEF_BLKSIZE_BITS 10
 
 static unsigned int nbds_max = 16;
 static int max_part = 16;
···
 		loff_t blksize)
 {
 	if (!blksize)
-		blksize = NBD_DEF_BLKSIZE;
+		blksize = 1u << NBD_DEF_BLKSIZE_BITS;
 	if (blksize < 512 || blksize > PAGE_SIZE || !is_power_of_2(blksize))
 		return -EINVAL;
 
 	nbd->config->bytesize = bytesize;
-	nbd->config->blksize = blksize;
+	nbd->config->blksize_bits = __ffs(blksize);
 
 	if (!nbd->task_recv)
 		return 0;
···
 		args->index = i;
 		queue_work(nbd->recv_workq, &args->work);
 	}
-	return nbd_set_size(nbd, config->bytesize, config->blksize);
+	return nbd_set_size(nbd, config->bytesize, nbd_blksize(config));
 }
 
 static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *bdev)
···
 	case NBD_SET_BLKSIZE:
 		return nbd_set_size(nbd, config->bytesize, arg);
 	case NBD_SET_SIZE:
-		return nbd_set_size(nbd, arg, config->blksize);
+		return nbd_set_size(nbd, arg, nbd_blksize(config));
 	case NBD_SET_SIZE_BLOCKS:
-		if (check_mul_overflow((loff_t)arg, config->blksize, &bytesize))
+		if (check_shl_overflow(arg, config->blksize_bits, &bytesize))
 			return -EINVAL;
-		return nbd_set_size(nbd, bytesize, config->blksize);
+		return nbd_set_size(nbd, bytesize, nbd_blksize(config));
 	case NBD_SET_TIMEOUT:
 		nbd_set_cmd_timeout(nbd, arg);
 		return 0;
···
 	atomic_set(&config->recv_threads, 0);
 	init_waitqueue_head(&config->recv_wq);
 	init_waitqueue_head(&config->conn_wait);
-	config->blksize = NBD_DEF_BLKSIZE;
+	config->blksize_bits = NBD_DEF_BLKSIZE_BITS;
 	atomic_set(&config->live_connections, 0);
 	try_module_get(THIS_MODULE);
 	return config;
···
 	debugfs_create_file("tasks", 0444, dir, nbd, &nbd_dbg_tasks_fops);
 	debugfs_create_u64("size_bytes", 0444, dir, &config->bytesize);
 	debugfs_create_u32("timeout", 0444, dir, &nbd->tag_set.timeout);
-	debugfs_create_u64("blocksize", 0444, dir, &config->blksize);
+	debugfs_create_u32("blocksize_bits", 0444, dir, &config->blksize_bits);
 	debugfs_create_file("flags", 0444, dir, nbd, &nbd_dbg_flags_fops);
 
 	return 0;
···
 static int nbd_genl_size_set(struct genl_info *info, struct nbd_device *nbd)
 {
 	struct nbd_config *config = nbd->config;
-	u64 bsize = config->blksize;
+	u64 bsize = nbd_blksize(config);
 	u64 bytes = config->bytesize;
 
 	if (info->attrs[NBD_ATTR_SIZE_BYTES])
···
 	if (info->attrs[NBD_ATTR_BLOCK_SIZE_BYTES])
 		bsize = nla_get_u64(info->attrs[NBD_ATTR_BLOCK_SIZE_BYTES]);
 
-	if (bytes != config->bytesize || bsize != config->blksize)
+	if (bytes != config->bytesize || bsize != nbd_blksize(config))
 		return nbd_set_size(nbd, bytes, bsize);
 	return 0;
 }