linux-kernel - Re: [PATCH v1 2/5] relayfs: introduce dump of relayfs statistics function

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Message-ID: <CAL+tcoDk2TFwAWPwBN+dQQ+guxe71F_R1rFX_f9wozjPpujBAQ@mail.gmail.com>
Date: Tue, 13 May 2025 09:48:15 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: axboe@...nel.dk, rostedt@...dmis.org, mhiramat@...nel.org, 
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org, 
	linux-block@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>, Yushan Zhou <katrinzhou@...cent.com>
Subject: Re: [PATCH v1 2/5] relayfs: introduce dump of relayfs statistics function

On Tue, May 13, 2025 at 8:51 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Mon, 12 May 2025 10:49:32 +0800 Jason Xing <kerneljasonxing@...il.com> wrote:
>
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > In this version, only support dumping the counter for buffer full and
> > implement the framework of how it works. Users MUST pass a valid @buf
> > with a valid @len that is required to be larger than RELAY_DUMP_BUF_MAX_LEN
> > to acquire which information indicated by @flags to dump.
> >
> > RELAY_DUMP_BUF_MAX_LEN shows the maximum len of the buffer if users
> > choose to dump all the values.
> >
> > Users can use this buffer to do whatever they expect in their own kernel
> > module, say, print to console/dmesg or write them into the relay buffer.
> >
> > ...
> >
> > +/**
> > + *   relay_dump - dump statistics of the specified channel buffer
> > + *   @chan: the channel
> > + *   @buf: buf to store statistics
> > + *   @len: len of buf to check
> > + *   @flags: select particular information to dump
> > + */
> > +void relay_dump(struct rchan *chan, char *buf, int len, int flags)
>
> `size_t' is probably a more appropriate type for `len'.
>
> > +{
> > +     unsigned int i, full_counter = 0;
> > +     struct rchan_buf *rbuf;
> > +     int offset = 0;
> > +
> > +     if (!chan || !buf || flags & ~RELAY_DUMP_MASK)
> > +             return;
> > +
> > +     if (len < RELAY_DUMP_BUF_MAX_LEN)
> > +             return;
>
> So we left the memory at *buf uninitialized but failed to tell the
> caller this.  The caller will then proceed to use uninitialized memory.
>
> It's a programming error, so simply going BUG seems OK.

Are you suggesting that I should remove the above check because
developers should take care of the length of the buffer to write
outside of the relay_dump function? or use this instead:
WARN_ON_ONCE(len < RELAY_DUMP_BUF_MAX_LEN);
?

>
> > +     if (chan->is_global) {
> > +             rbuf = *per_cpu_ptr(chan->buf, 0);
> > +             full_counter = rbuf->stats.full;
> > +     } else {
> > +             for_each_possible_cpu(i) {
>
> I'm thinking that at this stage in the patch series, this should be
> for_each_online_cpu(), then adjust that in patch [5/5].

Point taken. Will change it.

>
> > +                     if ((rbuf = *per_cpu_ptr(chan->buf, i)))
> > +                             full_counter += rbuf->stats.full;
> > +     }
> > +
> > +     if (flags & RELAY_DUMP_BUF_FULL)
> > +             offset += snprintf(buf, sizeof(unsigned int), "%u", full_counter);
>
> This seems strange.  sizeof(unsigned int) has nothing to do with the
> number of characters which are consumed by expansion of "%u"?

Sorry, my bad. It should be:
offset += snprintf(buf, len, "%u", full_counter);

Passing 'len' as the second parameter.

Thanks,
Jason

>
> > +
> > +     snprintf(buf + offset, 1, "\n");
> > +}
> > +EXPORT_SYMBOL_GPL(relay_dump);
> > +
> >  /**
> >   *   relay_file_open - open file op for relay files
> >   *   @inode: the inode
>