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Re: [PATCH v2 01/17] block/throttle-groups: throttle_group_co_io_limits_

From: Eric Blake
Subject: Re: [PATCH v2 01/17] block/throttle-groups: throttle_group_co_io_limits_intercept(): 64bit bytes
Date: Tue, 28 Apr 2020 17:09:38 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0

On 4/27/20 3:23 AM, Vladimir Sementsov-Ogievskiy wrote:
The function is called from 64bit io handlers, and bytes is just passed
to throttle_account() which is 64bit too (unsigned though). So, let's
convert intermediate argument to 64bit too.

My audit for this patch:

Caller has 32-bit, this patch now causes widening which is safe:
block/block-backend.c: blk_do_preadv() passes 'unsigned int'
block/block-backend.c: blk_do_pwritev_part() passes 'unsigned int'
block/throttle.c: throttle_co_pwrite_zeroes() passes 'int'
block/throttle.c: throttle_co_pdiscard() passes 'int'

Caller has 64-bit, this patch fixes potential bug where pre-patch could narrow, except it's easy enough to trace that callers are still capped at 2G actions:
block/throttle.c: throttle_co_preadv() passes 'uint64_t'
block/throttle.c: throttle_co_pwritev() passes 'uint64_t'

Implementation in question: block/throttle-groups.c throttle_group_co_io_limits_intercept() takes 'unsigned int bytes' and uses it:
argument to util/throttle.c throttle_account(uint64_t)

All safe: it patches a latent bug, and does not introduce any 64-bit gotchas once throttle_co_p{read,write}v are relaxed, and assuming throttle_account() is not buggy.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  include/block/throttle-groups.h | 2 +-
  block/throttle-groups.c         | 2 +-
  2 files changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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