Re: [PATCH] writeback: tracing: Copy only up to 31 characters in strncpy() calls

From: Mathieu Malaterre
Date: Fri Jan 25 2019 - 03:18:48 EST

On Fri, Jan 25, 2019 at 5:26 AM Nick Desaulniers
<nick.desaulniers@xxxxxxxxx> wrote:
>
> On Fri, Jan 18, 2019 at 11:32 AM Mathieu Malaterre <malat@xxxxxxxxxx> wrote:
> >
> > In the past an attempt was made to remove a set of warnings triggered by
> > gcc 8.x and W=1 by changing calls to strncpy() into strlcpy(). This was
> > rejected as one of the desired behavior is to keep initializing the rest
> > of the destination string whenever the source string is less than the
> > size.
> >
> > However the code makes it clear that this is not a desired behavior to
> > copy the entire 32 characters into the destination buffer, since it is
> > expected that the string is NUL terminated. So change the maximum number
> > of characters to be copied to be the size of the destination buffer
> > minus 1.
> >
> > Break long lines and make this patch go through `checkpatch --strict` with
> > no errors.
> >
> > This commit removes the following warnings:
> >
> > include/trace/events/writeback.h:69:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:99:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:179:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:223:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:277:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:299:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:324:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:375:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:586:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> > include/trace/events/writeback.h:660:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
> >
> > Cc: Nick Desaulniers <nick.desaulniers@xxxxxxxxx>
> > Link: https://lore.kernel.org/patchwork/patch/910830/
> > Signed-off-by: Mathieu Malaterre <malat@xxxxxxxxxx>
> > ---
> > include/trace/events/writeback.h | 30 ++++++++++++++++++++----------
> > 1 file changed, 20 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> > index 32db72c7c055..7bc58980f84f 100644
> > --- a/include/trace/events/writeback.h
> > +++ b/include/trace/events/writeback.h
> > @@ -67,7 +67,8 @@ TRACE_EVENT(writeback_dirty_page,
> >
> > TP_fast_assign(
> > strncpy(__entry->name,
> > - mapping ? dev_name(inode_to_bdi(mapping->host)->dev) : "(unknown)", 32);
> > + mapping ? dev_name(inode_to_bdi(mapping->host)->dev) :
> > + "(unknown)", sizeof(__entry->name) - 1);
>
> Does strncpy guarantee that destination will be NULL terminated if
> (sizeof(src) > sizeof(dst)) || (32 > sizeof(dst))?

No, that's the point here:

https://www.kernel.org/doc/htmldocs/kernel-api/API-strncpy.html

...
The result is not NUL-terminated if the source exceeds count bytes.

In the case where the length of src is less than that of count, the
remainder of dest will be padded with NUL.
...

One should use strlcpy for the use case you describe:

https://www.kernel.org/doc/htmldocs/kernel-api/API-strlcpy.html

> > __entry->ino = mapping ? mapping->host->i_ino : 0;
> > __entry->index = page->index;
> > ),
> > @@ -97,7 +98,8 @@ DECLARE_EVENT_CLASS(writeback_dirty_inode_template,
> >
> > /* may be called for files on pseudo FSes w/ unregistered bdi */
> > strncpy(__entry->name,
> > - bdi->dev ? dev_name(bdi->dev) : "(unknown)", 32);
> > + bdi->dev ? dev_name(bdi->dev) : "(unknown)",
> > + sizeof(__entry->name) - 1);
> > __entry->ino = inode->i_ino;
> > __entry->state = inode->i_state;
> > __entry->flags = flags;
> > @@ -177,7 +179,8 @@ DECLARE_EVENT_CLASS(writeback_write_inode_template,
> >
> > TP_fast_assign(
> > strncpy(__entry->name,
> > - dev_name(inode_to_bdi(inode)->dev), 32);
> > + dev_name(inode_to_bdi(inode)->dev),
> > + sizeof(__entry->name) - 1);
> > __entry->ino = inode->i_ino;
> > __entry->sync_mode = wbc->sync_mode;
> > __entry->cgroup_ino = __trace_wbc_assign_cgroup(wbc);
> > @@ -221,7 +224,8 @@ DECLARE_EVENT_CLASS(writeback_work_class,
> > ),
> > TP_fast_assign(
> > strncpy(__entry->name,
> > - wb->bdi->dev ? dev_name(wb->bdi->dev) : "(unknown)", 32);
> > + wb->bdi->dev ? dev_name(wb->bdi->dev) : "(unknown)",
> > + sizeof(__entry->name) - 1);
> > __entry->nr_pages = work->nr_pages;
> > __entry->sb_dev = work->sb ? work->sb->s_dev : 0;
> > __entry->sync_mode = work->sync_mode;
> > @@ -274,7 +278,8 @@ DECLARE_EVENT_CLASS(writeback_class,
> > __field(unsigned int, cgroup_ino)
> > ),
> > TP_fast_assign(
> > - strncpy(__entry->name, dev_name(wb->bdi->dev), 32);
> > + strncpy(__entry->name, dev_name(wb->bdi->dev),
> > + sizeof(__entry->name) - 1);
> > __entry->cgroup_ino = __trace_wb_assign_cgroup(wb);
> > ),
> > TP_printk("bdi %s: cgroup_ino=%u",
> > @@ -296,7 +301,8 @@ TRACE_EVENT(writeback_bdi_register,
> > __array(char, name, 32)
> > ),
> > TP_fast_assign(
> > - strncpy(__entry->name, dev_name(bdi->dev), 32);
> > + strncpy(__entry->name, dev_name(bdi->dev),
> > + sizeof(__entry->name) - 1);
> > ),
> > TP_printk("bdi %s",
> > __entry->name
> > @@ -321,7 +327,8 @@ DECLARE_EVENT_CLASS(wbc_class,
> > ),
> >
> > TP_fast_assign(
> > - strncpy(__entry->name, dev_name(bdi->dev), 32);
> > + strncpy(__entry->name, dev_name(bdi->dev),
> > + sizeof(__entry->name) - 1);
> > __entry->nr_to_write = wbc->nr_to_write;
> > __entry->pages_skipped = wbc->pages_skipped;
> > __entry->sync_mode = wbc->sync_mode;
> > @@ -372,7 +379,8 @@ TRACE_EVENT(writeback_queue_io,
> > ),
> > TP_fast_assign(
> > unsigned long *older_than_this = work->older_than_this;
> > - strncpy(__entry->name, dev_name(wb->bdi->dev), 32);
> > + strncpy(__entry->name, dev_name(wb->bdi->dev),
> > + sizeof(__entry->name) - 1);
> > __entry->older = older_than_this ? *older_than_this : 0;
> > __entry->age = older_than_this ?
> > (jiffies - *older_than_this) * 1000 / HZ : -1;
> > @@ -584,7 +592,8 @@ TRACE_EVENT(writeback_sb_inodes_requeue,
> >
> > TP_fast_assign(
> > strncpy(__entry->name,
> > - dev_name(inode_to_bdi(inode)->dev), 32);
> > + dev_name(inode_to_bdi(inode)->dev),
> > + sizeof(__entry->name) - 1);
> > __entry->ino = inode->i_ino;
> > __entry->state = inode->i_state;
> > __entry->dirtied_when = inode->dirtied_when;
> > @@ -658,7 +667,8 @@ DECLARE_EVENT_CLASS(writeback_single_inode_template,
> >
> > TP_fast_assign(
> > strncpy(__entry->name,
> > - dev_name(inode_to_bdi(inode)->dev), 32);
> > + dev_name(inode_to_bdi(inode)->dev),
> > + sizeof(__entry->name) - 1);
> > __entry->ino = inode->i_ino;
> > __entry->state = inode->i_state;
> > __entry->dirtied_when = inode->dirtied_when;
> > --
> > 2.19.2
> >