linux-kernel - Re: [PATCH v2] writeback: fix -Wstringop-truncation warnings

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Date:   Thu, 25 Jul 2019 11:50:05 -0400
From:   Qian Cai <cai@....pw>
To:     David Laight <David.Laight@...LAB.COM>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Cc:     "tobin@...nel.org" <tobin@...nel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tj@...nel.org" <tj@...nel.org>,
        "dchinner@...hat.com" <dchinner@...hat.com>,
        "fengguang.wu@...el.com" <fengguang.wu@...el.com>,
        "jack@...e.cz" <jack@...e.cz>, "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] writeback: fix -Wstringop-truncation warnings

On Thu, 2019-07-25 at 15:04 +0000, David Laight wrote:
> From: Qian Cai
> > Sent: 25 July 2019 15:39
> > 
> > There are many of those warnings.
> > 
> > In file included from ./arch/powerpc/include/asm/paca.h:15,
> >                  from ./arch/powerpc/include/asm/current.h:13,
> >                  from ./include/linux/thread_info.h:21,
> >                  from ./include/asm-generic/preempt.h:5,
> >                  from ./arch/powerpc/include/generated/asm/preempt.h:1,
> >                  from ./include/linux/preempt.h:78,
> >                  from ./include/linux/spinlock.h:51,
> >                  from fs/fs-writeback.c:19:
> > In function 'strncpy',
> >     inlined from 'perf_trace_writeback_page_template' at
> > ./include/trace/events/writeback.h:56:1:
> > ./include/linux/string.h:260:9: warning: '__builtin_strncpy' specified
> > bound 32 equals destination size [-Wstringop-truncation]
> >   return __builtin_strncpy(p, q, size);
> >          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Fix it by using the new strscpy_pad() which was introduced in the
> > commit 458a3bf82df4 ("lib/string: Add strscpy_pad() function") and will
> > always be NUL-terminated instead of strncpy(). Also, changes strlcpy()
> > to use strscpy_pad() in this file for consistency.
> > 
> > Fixes: 455b2864686d ("writeback: Initial tracing support")
> > Fixes: 028c2dd184c0 ("writeback: Add tracing to balance_dirty_pages")
> > Fixes: e84d0a4f8e39 ("writeback: trace event writeback_queue_io")
> > Fixes: b48c104d2211 ("writeback: trace event bdi_dirty_ratelimit")
> > Fixes: cc1676d917f3 ("writeback: Move requeueing when I_SYNC set to
> > writeback_sb_inodes()")
> > Fixes: 9fb0a7da0c52 ("writeback: add more tracepoints")
> > Signed-off-by: Qian Cai <cai@....pw>
> > ---
> > 
> > v2: Use strscpy_pad() to address the possible data leaking concern from
> > Steve [1].
> >     Replace strlcpy() as well for consistency.
> > 
> > [1] https://lore.kernel.org/lkml/20190716170339.1c44719d@gandalf.local.home/
> > 
> >  include/trace/events/writeback.h | 39 +++++++++++++++++++++--------------
> > ----
> >  1 file changed, 21 insertions(+), 18 deletions(-)
> > 
> > diff --git a/include/trace/events/writeback.h
> > b/include/trace/events/writeback.h
> > index aa7f3aeac740..41092d63a8de 100644
> > --- a/include/trace/events/writeback.h
> > +++ b/include/trace/events/writeback.h
> > @@ -66,8 +66,10 @@
> >  	),
> > 
> >  	TP_fast_assign(
> > -		strncpy(__entry->name,
> > -			mapping ? dev_name(inode_to_bdi(mapping->host)-
> > >dev) : "(unknown)", 32);
> > +		strscpy_pad(__entry->name,
> > +			    mapping ?
> > +			    dev_name(inode_to_bdi(mapping->host)->dev) :
> > +			    "(unknown)", 32);
> 
> Shouldn't the 32 be 'sizeof (something)' ??

Maybe could do a sizeof(__entry->name) as it is defined as,

	TP_STRUCT__entry (
		__array(char, name, 32)
		__field(unsigned long, ino)
		__field(pgoff_t, index)

But, that might be a follow-up patch and does not seem belong here.

> 
> Oh, and a horrid line break.

That line is too long needs to break up. Open up to suggestions though.