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Date: Mon, 29 Jul 2019 12:04:49 +0200
From: Jan Kara <jack@...e.cz>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Qian Cai <cai@....pw>, mingo@...hat.com, tj@...nel.org,
dchinner@...hat.com, fengguang.wu@...el.com, jack@...e.cz,
linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] trace/writeback: fix Wstringop-truncation warnings
On Tue 16-07-19 17:03:39, Steven Rostedt wrote:
> On Fri, 12 Jul 2019 12:14:47 -0400
> Qian Cai <cai@....pw> wrote:
>
> > 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 strlcpy() which will always be NUL-terminated instead of
> > strncpy(). strlcpy() has already been used at some places in this file.
> >
> > 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>
> > ---
> > include/trace/events/writeback.h | 20 ++++++++++----------
> > 1 file changed, 10 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> > index aa7f3aeac740..8e3b3c4fd964 100644
> > --- a/include/trace/events/writeback.h
> > +++ b/include/trace/events/writeback.h
> > @@ -66,7 +66,7 @@
> > ),
> >
> > TP_fast_assign(
> > - strncpy(__entry->name,
> > + strlcpy(__entry->name,
> > mapping ? dev_name(inode_to_bdi(mapping->host)->dev) : "(unknown)", 32);
>
>
> Not sure this is an issue or not, but although the fix looks legit (in
> case a string is more that 31 bytes), strlcpy() does not pad the rest
> of the string like strncpy() does. This means we can possibly leak data
> through the ring buffer.
>
> This may not be an issue as ftrace can only be used by a super user
> account, but this code can also be used by perf. If it is possible for
> a non admin account to enable these events through perf, then there is
> a case of data leak.
>
> Again, it may not be a big issue, but I'm just letting people know.
>
> Note, this needs to go through the maintainer of the writeback.h, who
> are those that created it, not the tracing maintainers.
Yeah, I'd prefer to use strscpy_pad() for the copying as I'd be calmer if
we don't leak uninitialized memory to userspace. Thanks!
Honza
>
> -- Steve
>
>
> > __entry->ino = mapping ? mapping->host->i_ino : 0;
> > __entry->index = page->index;
> > @@ -110,7 +110,7 @@
> > struct backing_dev_info *bdi = inode_to_bdi(inode);
> >
> > /* may be called for files on pseudo FSes w/ unregistered bdi */
> > - strncpy(__entry->name,
> > + strlcpy(__entry->name,
> > bdi->dev ? dev_name(bdi->dev) : "(unknown)", 32);
> > __entry->ino = inode->i_ino;
> > __entry->state = inode->i_state;
> > @@ -190,7 +190,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > ),
> >
> > TP_fast_assign(
> > - strncpy(__entry->name,
> > + strlcpy(__entry->name,
> > dev_name(inode_to_bdi(inode)->dev), 32);
> > __entry->ino = inode->i_ino;
> > __entry->sync_mode = wbc->sync_mode;
> > @@ -234,7 +234,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > __field(unsigned int, cgroup_ino)
> > ),
> > TP_fast_assign(
> > - strncpy(__entry->name,
> > + strlcpy(__entry->name,
> > wb->bdi->dev ? dev_name(wb->bdi->dev) : "(unknown)", 32);
> > __entry->nr_pages = work->nr_pages;
> > __entry->sb_dev = work->sb ? work->sb->s_dev : 0;
> > @@ -288,7 +288,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > __field(unsigned int, cgroup_ino)
> > ),
> > TP_fast_assign(
> > - strncpy(__entry->name, dev_name(wb->bdi->dev), 32);
> > + strlcpy(__entry->name, dev_name(wb->bdi->dev), 32);
> > __entry->cgroup_ino = __trace_wb_assign_cgroup(wb);
> > ),
> > TP_printk("bdi %s: cgroup_ino=%u",
> > @@ -310,7 +310,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > __array(char, name, 32)
> > ),
> > TP_fast_assign(
> > - strncpy(__entry->name, dev_name(bdi->dev), 32);
> > + strlcpy(__entry->name, dev_name(bdi->dev), 32);
> > ),
> > TP_printk("bdi %s",
> > __entry->name
> > @@ -335,7 +335,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > ),
> >
> > TP_fast_assign(
> > - strncpy(__entry->name, dev_name(bdi->dev), 32);
> > + strlcpy(__entry->name, dev_name(bdi->dev), 32);
> > __entry->nr_to_write = wbc->nr_to_write;
> > __entry->pages_skipped = wbc->pages_skipped;
> > __entry->sync_mode = wbc->sync_mode;
> > @@ -386,7 +386,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > ),
> > TP_fast_assign(
> > unsigned long *older_than_this = work->older_than_this;
> > - strncpy(__entry->name, dev_name(wb->bdi->dev), 32);
> > + strlcpy(__entry->name, dev_name(wb->bdi->dev), 32);
> > __entry->older = older_than_this ? *older_than_this : 0;
> > __entry->age = older_than_this ?
> > (jiffies - *older_than_this) * 1000 / HZ : -1;
> > @@ -597,7 +597,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > ),
> >
> > TP_fast_assign(
> > - strncpy(__entry->name,
> > + strlcpy(__entry->name,
> > dev_name(inode_to_bdi(inode)->dev), 32);
> > __entry->ino = inode->i_ino;
> > __entry->state = inode->i_state;
> > @@ -671,7 +671,7 @@ static inline unsigned int __trace_wbc_assign_cgroup(struct writeback_control *w
> > ),
> >
> > TP_fast_assign(
> > - strncpy(__entry->name,
> > + strlcpy(__entry->name,
> > dev_name(inode_to_bdi(inode)->dev), 32);
> > __entry->ino = inode->i_ino;
> > __entry->state = inode->i_state;
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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