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

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Message-ID: <CAGXu5j+CQLk6u8iMmOfbvk5=B5PdDvc8qkkHBAjpXquhYgWTwg@mail.gmail.com>
Date:   Sat, 26 Jan 2019 06:06:02 +1300
From:   Kees Cook <keescook@...omium.org>
To:     Mathieu Malaterre <malat@...ian.org>
Cc:     Nick Desaulniers <nick.desaulniers@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] writeback: tracing: Copy only up to 31 characters in
 strncpy() calls

On Fri, Jan 25, 2019 at 9:18 PM Mathieu Malaterre <malat@...ian.org> wrote:
>
> On Fri, Jan 25, 2019 at 5:26 AM Nick Desaulniers
> <nick.desaulniers@...il.com> wrote:
> >
> > On Fri, Jan 18, 2019 at 11:32 AM Mathieu Malaterre <malat@...ian.org> 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@...il.com>
> > > Link: https://lore.kernel.org/patchwork/patch/910830/
> > > Signed-off-by: Mathieu Malaterre <malat@...ian.org>
> > > ---
> > >  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

Please use strspy() -- strlcpy() will read the source beyond the length limit.
https://www.kernel.org/doc/htmldocs/kernel-api/API-strscpy.html

-- 
Kees Cook