[PATCH 5.14 066/172] nbd: use shifts rather than multiplies

Greg Kroah-Hartman
2021-10-04T13:35:54Z

From: Nick Desaulniers <ndesaulniers@google.com>

commit 41e76c6a3c83c85e849f10754b8632ea763d9be4 upstream.

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: stable@vger.kernel.org
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://github.com/ClangBuiltLinux/linux/issues/1438
Link: https://lore.kernel.org/all/20210909182525.372ee687@canb.auug.org.au/
Link: https://lore.kernel.org/stable/CAHk-=whiQBofgis_rkniz8GBP9wZtSZdcDEffgSLO62BUGV3gg@mail.gmail.com/
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Suggested-by: Kees Cook <keescook@chromium.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210920232533.4092046-1-ndesaulniers@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/block/nbd.c | 29 +++++++++++++++++------------
1 file changed, 17 insertions(+), 12 deletions(-)

--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -97,13 +97,18 @@ struct nbd_config {

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;

@@ -147,7 +152,7 @@ static struct dentry *nbd_dbg_dir;

#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;
@@ -350,12 +355,12 @@ static int nbd_set_size(struct nbd_devic
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;
@@ -1370,7 +1375,7 @@ static int nbd_start_device(struct nbd_d
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)
@@ -1439,11 +1444,11 @@ static int __nbd_ioctl(struct block_devi
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;
@@ -1509,7 +1514,7 @@ static struct nbd_config *nbd_alloc_conf
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;
@@ -1637,7 +1642,7 @@ static int nbd_dev_dbg_init(struct nbd_d
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;
@@ -1841,7 +1846,7 @@ nbd_device_policy[NBD_DEVICE_ATTR_MAX +
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])
@@ -1850,7 +1855,7 @@ static int nbd_genl_size_set(struct genl
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;
}