linux-kernel - Re: [PATCH] nfsd: Fix overflow causing non-working mounts on 1 TB machines

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Date:   Wed, 3 Jul 2019 11:56:34 -0400
From:   "J. Bruce Fields" <bfields@...hat.com>
To:     Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Chris Tracy <ctracy@...r.scu.edu>, linux-nfs@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, it+linux-nfs@...gen.mpg.de
Subject: Re: [PATCH] nfsd: Fix overflow causing non-working mounts on 1 TB
 machines

Good catch!  And thanks for the detailed explanation.  Applying for 5.2
and stable.

On Wed, Jul 03, 2019 at 02:54:43PM +0200, Paul Menzel wrote:
> Date: Wed, 3 Jul 2019 13:28:15 +0200
> 
> Since commit 10a68cdf10 (nfsd: fix performance-limiting session
> calculation) (Linux 5.1-rc1 and 4.19.31), shares from NFS servers with
> 1 TB of memory cannot be mounted anymore. The mount just hangs on the
> client.
> 
> The gist of commit 10a68cdf10 is the change below.
> 
>     -avail = clamp_t(int, avail, slotsize, avail/3);
>     +avail = clamp_t(int, avail, slotsize, total_avail/3);
> 
> Here are the macros.
> 
>     #define min_t(type, x, y)       __careful_cmp((type)(x), (type)(y), <)
>     #define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)
> 
> `total_avail` is 8,434,659,328 on the 1 TB machine. `clamp_t()` casts
> the values to `int`, which for 32-bit integers can only hold values
> −2,147,483,648 (−2^31) through 2,147,483,647 (2^31 − 1).
> 
> `avail` (in the function signature) is just 65536, so that no overflow
> was happening. Before the commit the assignment would result in 21845,
> and `num = 4`.
> 
> When using `total_avail`, it is causing the assignment to be
> 18446744072226137429 (printed as %lu), and `num` is then 4164608182.
> 
> My next guess is, that `nfsd_drc_mem_used` is then exceeded, and the
> server thinks there is no memory available any more for this client.
> 
> Updating the arguments of `clamp_t()` and `min_t()` to `unsigned long`
> fixes the issue.
> 
> Now, `avail = 65536` (before commit 10a68cdf10 `avail = 21845`), but
> `num = 4` remains the same.
> 
> Fixes: 10a68cdf10 (nfsd: fix performance-limiting session calculation)
> Cc: stable@...r.kernel.org
> Signed-off-by: Paul Menzel <pmenzel@...gen.mpg.de>
> ---
> 
> 1.  No, idea if `min_t()` arguments also need updating.
> 2.  Instead of `unsigned long`, should `size_t` be used?
> 
>  fs/nfsd/nfs4state.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 618e66078ee5..1a0cdeb3b875 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1563,7 +1563,7 @@ static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca)
>  	 * Never use more than a third of the remaining memory,
>  	 * unless it's the only way to give this client a slot:
>  	 */
> -	avail = clamp_t(int, avail, slotsize, total_avail/3);
> +	avail = clamp_t(unsigned long, avail, slotsize, total_avail/3);
>  	num = min_t(int, num, avail / slotsize);
>  	nfsd_drc_mem_used += num * slotsize;
>  	spin_unlock(&nfsd_drc_lock);
> -- 
> 2.22.0
>