On Thu, Aug 15, 2019 at 02:35:35PM +0200, Greg KH wrote:
> On Thu, Aug 15, 2019 at 08:29:10PM +0800, Ming Lei wrote:
> > On Thu, Aug 15, 2019 at 02:24:19PM +0200, Greg KH wrote:
> > > On Thu, Aug 15, 2019 at 08:15:18PM +0800, Ming Lei wrote:
> > > > It is reported that sysfs buffer overflow can be triggered in case
> > > > of too many CPU cores(>841 on 4K PAGE_SIZE) when showing CPUs in
> > > > one hctx.
> > > >
> > > > So use snprintf for avoiding the potential buffer overflow.
> > > >
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > Cc: Mark Ray <mark.ray@xxxxxxx>
> > > > Fixes: 676141e48af7("blk-mq: don't dump CPU -> hw queue map on driver load")
> > > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > > > ---
> > > > block/blk-mq-sysfs.c | 30 ++++++++++++++++++------------
> > > > 1 file changed, 18 insertions(+), 12 deletions(-)
> > > >
> > > > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> > > > index d6e1a9bd7131..e75f41a98415 100644
> > > > --- a/block/blk-mq-sysfs.c
> > > > +++ b/block/blk-mq-sysfs.c
> > > > @@ -164,22 +164,28 @@ static ssize_t blk_mq_hw_sysfs_nr_reserved_tags_show(struct blk_mq_hw_ctx *hctx,
> > > > return sprintf(page, "%u\n", hctx->tags->nr_reserved_tags);
> > > > }
> > > >
> > > > +/* avoid overflow by too many CPU cores */
> > > > static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
> > > > {
> > > > - unsigned int i, first = 1;
> > > > - ssize_t ret = 0;
> > > > -
> > > > - for_each_cpu(i, hctx->cpumask) {
> > > > - if (first)
> > > > - ret += sprintf(ret + page, "%u", i);
> > > > - else
> > > > - ret += sprintf(ret + page, ", %u", i);
> > > > -
> > > > - first = 0;
> > > > + unsigned int cpu = cpumask_first(hctx->cpumask);
> > > > + ssize_t len = snprintf(page, PAGE_SIZE - 1, "%u", cpu);
> > > > + int last_len = len;
> > > > +
> > > > + while ((cpu = cpumask_next(cpu, hctx->cpumask)) < nr_cpu_ids) {
> > > > + int cur_len = snprintf(page + len, PAGE_SIZE - 1 - len,
> > > > + ", %u", cpu);
> > > > + if (cur_len >= PAGE_SIZE - 1 - len) {
> > > > + len -= last_len;
> > > > + len += snprintf(page + len, PAGE_SIZE - 1 - len,
> > > > + "...");
> > > > + break;
> > > > + }
> > > > + len += cur_len;
> > > > + last_len = cur_len;
> > > > }
> > > >
> > > > - ret += sprintf(ret + page, "\n");
> > > > - return ret;
> > > > + len += snprintf(page + len, PAGE_SIZE - 1 - len, "\n");
> > > > + return len;
> > > > }
> > > >
> > >
> > > What????
> > >
> > > sysfs is "one value per file". You should NEVER have to care about the
> > > size of the sysfs buffer. If you do, you are doing something wrong.
> > >
> > > What excatly are you trying to show in this sysfs file? I can't seem to
> > > find the Documenatation/ABI/ entry for it, am I just missing it because
> > > I don't know the filename for it?
> >
> > It is /sys/block/$DEV/mq/$N/cpu_list, all CPUs in this hctx($N) will be
> > shown via sysfs buffer. The buffer size is one PAGE, how can it hold when
> > there are too many CPUs(close to 1K)?
>
> Looks like I only see 1 cpu listed on my machines in those files, what
> am I doing wrong?
It depends on machine. The issue is reported on one machine with 896 CPU
cores, when 4K buffer can only hold 841 cores.
>
> Also, I don't see cpu_list in any of the documentation files, so I have
> no idea what you are trying to have this file show.
>
> And again, "one value per file" is the sysfs rule. "all cpus in the
> system" is not "one value" :)
I agree, and this file shouldn't be there, given each CPU will have one
kobject dir under the hctx dir.
We may kill the 'cpu_list' attribute, is there anyone who objects?
Thanks,
Ming
