Re: [PATCH] mm: debug: Fix a width vs precision bug in printk

[Michal Hocko <mhocko@kernel.org>]

On Fri 23-11-18 16:08:46, Andrew Morton wrote:
> On Fri, 23 Nov 2018 23:48:06 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> 
> > On 2018/11/23 23:36, Dan Carpenter wrote:
> > > On Fri, Nov 23, 2018 at 10:01:25AM +0100, Michal Hocko wrote:
> > >> On Fri 23-11-18 10:21:35, Dan Carpenter wrote:
> > >>> We had intended to only print dentry->d_name.len characters but there is
> > >>> a width vs precision typo so if the name isn't NUL terminated it will
> > >>> read past the end of the buffer.
> > >>
> > >> OK, it took me quite some time to grasp what you mean here. The code
> > >> works as expected because d_name.len and dname.name are in sync so there
> > >> no spacing going to happen. Anyway what you propose is formally more
> > >> correct I guess.
> > >>  
> > > 
> > > Yeah.  If we are sure that the name has a NUL terminator then this
> > > change has no effect.
> > 
> > There seems to be %pd which is designed for printing "struct dentry".
> 
> ooh, who knew.  Can we use that please?

I wasn't aware of it either. I do not mind using it instead of the
opencoded variant of mine.

This should do it, right?
diff --git a/mm/debug.c b/mm/debug.c
index d18c5cea3320..68e9a9f2df16 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -80,7 +80,7 @@ void __dump_page(struct page *page, const char *reason)
 		if (mapping->host->i_dentry.first) {
 			struct dentry *dentry;
 			dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias);
-			pr_warn("name:\"%*s\" ", dentry->d_name.len, dentry->d_name.name);
+			pr_warn("name:\"%pd\" ", dentry);
 		}
 	}
 	BUILD_BUG_ON(ARRAY_SIZE(pageflag_names) != __NR_PAGEFLAGS + 1);

-- 
Michal Hocko
SUSE Labs