Re: [PATCH] nbd: use shifts rather than multiplies

[Nick Desaulniers]

On Mon, Sep 20, 2021 at 4:25 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx
> Cc: Arnd Bergmann <arnd@xxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1438
> Link: https://lore.kernel.org/all/20210909182525.372ee687@xxxxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/stable/CAHk-=whiQBofgis_rkniz8GBP9wZtSZdcDEffgSLO62BUGV3gg@xxxxxxxxxxxxxx/
> Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Pavel Machek <pavel@xxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Hi Josef,
Do you have cycles to review this patch, or is this something I should
ask Jens or Linus about picking up?

> ---
> This patch is kind of a v3 for solving this problem.
> v2: https://lore.kernel.org/stable/20210914002318.2298583-1-ndesaulniers@xxxxxxxxxx/
> v1: https://lore.kernel.org/stable/20210913203201.1844253-1-ndesaulniers@xxxxxxxxxx/
> But is quite different in approach. Instead of trying to fix up the
> overflow routines in stable, we amend the code in nbd.c as per Linus
> (against mainline) with fixes from Kees to use the named overflow
> checker.
>
>  drivers/block/nbd.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 5170a630778d..1183f7872b71 100644
> --- 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;
>
> @@ -146,7 +151,7 @@ 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;
> @@ -317,12 +322,12 @@ static int nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
>                 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;
> @@ -1337,7 +1342,7 @@ static int nbd_start_device(struct nbd_device *nbd)
>                 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)
> @@ -1406,11 +1411,11 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>         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;
> @@ -1476,7 +1481,7 @@ static struct nbd_config *nbd_alloc_config(void)
>         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;
> @@ -1604,7 +1609,7 @@ static int nbd_dev_dbg_init(struct nbd_device *nbd)
>         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;
> @@ -1826,7 +1831,7 @@ nbd_device_policy[NBD_DEVICE_ATTR_MAX + 1] = {
>  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])
> @@ -1835,7 +1840,7 @@ static int nbd_genl_size_set(struct genl_info *info, struct nbd_device *nbd)
>         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;
>  }
>
> base-commit: 4c17ca27923c16fd73bbb9ad033c7d749c3bcfcc
> --
> 2.33.0.464.g1972c5931b-goog
>


-- 
Thanks,
~Nick Desaulniers