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[185.231.240.75]) by mx.google.com with ESMTPS id j15si7429149lji.192.2019.05.06.02.28.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 06 May 2019 02:28:37 -0700 (PDT) Received-SPF: pass (google.com: domain of ktkhai@virtuozzo.com designates 185.231.240.75 as permitted sender) client-ip=185.231.240.75; Authentication-Results: mx.google.com; spf=pass (google.com: domain of ktkhai@virtuozzo.com designates 185.231.240.75 as permitted sender) smtp.mailfrom=ktkhai@virtuozzo.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=virtuozzo.com Received: from [172.16.25.169] by relay.sw.ru with esmtp (Exim 4.91) (envelope-from ) id 1hNZv4-0002jm-38; Mon, 06 May 2019 12:28:22 +0300 Subject: Re: [PATCH v3 2/2] prctl_set_mm: downgrade mmap_sem to read lock To: =?UTF-8?Q?Michal_Koutn=c3=bd?= , gorcunov@gmail.com Cc: akpm@linux-foundation.org, arunks@codeaurora.org, brgl@bgdev.pl, geert+renesas@glider.be, ldufour@linux.ibm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mguzik@redhat.com, mhocko@kernel.org, rppt@linux.ibm.com, vbabka@suse.cz References: <0a48e0a2-a282-159e-a56e-201fbc0faa91@virtuozzo.com> <20190502125203.24014-1-mkoutny@suse.com> <20190502125203.24014-3-mkoutny@suse.com> From: Kirill Tkhai Message-ID: <961c4d8a-982f-720b-490b-dfb4dae7be25@virtuozzo.com> Date: Mon, 6 May 2019 12:28:21 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190502125203.24014-3-mkoutny@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 02.05.2019 15:52, Michal Koutný wrote: > The commit a3b609ef9f8b ("proc read mm's {arg,env}_{start,end} with mmap > semaphore taken.") added synchronization of reading argument/environment > boundaries under mmap_sem. Later commit 88aa7cc688d4 ("mm: introduce > arg_lock to protect arg_start|end and env_start|end in mm_struct") > avoided the coarse use of mmap_sem in similar situations. But there > still remained two places that (mis)use mmap_sem. > > get_cmdline should also use arg_lock instead of mmap_sem when it reads the > boundaries. > > The second place that should use arg_lock is in prctl_set_mm. By > protecting the boundaries fields with the arg_lock, we can downgrade > mmap_sem to reader lock (analogous to what we already do in > prctl_set_mm_map). > > v2: call find_vma without arg_lock held > v3: squashed get_cmdline arg_lock patch > > Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct") > Cc: Yang Shi > Cc: Mateusz Guzik > CC: Cyrill Gorcunov > Co-developed-by: Laurent Dufour > Signed-off-by: Laurent Dufour > Signed-off-by: Michal Koutný Reviewed-by: Kirill Tkhai > --- > kernel/sys.c | 10 ++++++++-- > mm/util.c | 4 ++-- > 2 files changed, 10 insertions(+), 4 deletions(-) > > diff --git a/kernel/sys.c b/kernel/sys.c > index 5e0a5edf47f8..14be57840511 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2122,9 +2122,14 @@ static int prctl_set_mm(int opt, unsigned long addr, > > error = -EINVAL; > > - down_write(&mm->mmap_sem); > + /* > + * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for > + * a) concurrent sys_brk, b) finding VMA for addr validation. > + */ > + down_read(&mm->mmap_sem); > vma = find_vma(mm, addr); > > + spin_lock(&mm->arg_lock); > prctl_map.start_code = mm->start_code; > prctl_map.end_code = mm->end_code; > prctl_map.start_data = mm->start_data; > @@ -2212,7 +2217,8 @@ static int prctl_set_mm(int opt, unsigned long addr, > > error = 0; > out: > - up_write(&mm->mmap_sem); > + spin_unlock(&mm->arg_lock); > + up_read(&mm->mmap_sem); > return error; > } > > diff --git a/mm/util.c b/mm/util.c > index 43a2984bccaa..5cf0e84a0823 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -758,12 +758,12 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen) > if (!mm->arg_end) > goto out_mm; /* Shh! No looking before we're done */ > > - down_read(&mm->mmap_sem); > + spin_lock(&mm->arg_lock); > arg_start = mm->arg_start; > arg_end = mm->arg_end; > env_start = mm->env_start; > env_end = mm->env_end; > - up_read(&mm->mmap_sem); > + spin_unlock(&mm->arg_lock); > > len = arg_end - arg_start; > >