Re: [PATCH] NFS: nfsroot: replace strlcat() with snprintf()

[David Laight <david.laight.linux@gmail.com>]

On Tue, 18 Aug 2026 14:05:41 -0700
Justin Stitt <justinstitt@google.com> wrote:

> Hi,
> 
> On Tue, Aug 18, 2026 at 1:55 AM David Laight
> <david.laight.linux@gmail.com> wrote:
> >
> > On Fri, 14 Aug 2026 00:28:45 -0700
> > Mariia Nikitash <mariianikitash@google.com> wrote:
> >  
> > > In preparation for removing the deprecated strlcat() API[1], replace its
> > > uses in root_nfs_cat() with snprintf().
> > >
> > > Build the separator and source string in a single call using the
> > > remaining space in the destination buffer. snprintf() returns the length
> > > it would have written excluding the terminating NUL, so comparing the
> > > return value against the remaining buffer space preserves the existing
> > > truncation check.
> > >
> > > Link: https://github.com/KSPP/linux/issues/370 [1]
> > > Signed-off-by: Mariia Nikitash <mariianikitash@google.com>
> > > ---
> > >  fs/nfs/nfsroot.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/fs/nfs/nfsroot.c b/fs/nfs/nfsroot.c
> > > index 432612d22437..e951fe731679 100644
> > > --- a/fs/nfs/nfsroot.c
> > > +++ b/fs/nfs/nfsroot.c
> > > @@ -173,12 +173,12 @@ static int __init root_nfs_cat(char *dest, const char *src,
> > >                              const size_t destlen)
> > >  {
> > >       size_t len = strlen(dest);
> > > +     size_t remaining = destlen - len;
> > > +     const char *sep = "";
> > >
> > >       if (len && dest[len - 1] != ',')
> > > -             if (strlcat(dest, ",", destlen) >= destlen)
> > > -                     return -1;
> > > -
> > > -     if (strlcat(dest, src, destlen) >= destlen)
> > > +             sep = ",";
> > > +     if (snprintf(dest + len, remaining, "%s%s", sep, src) >= remaining)
> > >               return -1;  
> >
> > I think I'd have gone for:
> >         size_t len = strlen(dest);
> >         if (len && dest[len - 1] != ',' && ++len < destlen)
> >                 dest[len - 1] = ',';
> >         if (strscpy(dest + len, src, destlen - len) < 0)
> >                 return -1;  
> 
> Can we run into underflow issues if `len == destlen` during the
> increment. imagine @len is 10 and @destlen is also 10. We end up
> incrementing len and our strscpy receives `10 - 11` wrapping to
> SIZE_MAX.

len can only be less than destlen, so strscpy() can only see a zero
length - which is fine,

This whole code could be rewritten to avoid the strlen().
The function is only used twice to append data to a single static string.
Tweaking the initialisation would allow sizeof() be used to set
the initial length.
The initialisation also means that the ',' is always needed.

The second call appends data from an snprintf() into a temporary buffer.
It wouldn't be hard to directly add the data to the actual buffer.

David

> 
> Maybe this isn't reachable as @destlen may always be larger than what
> `strlen()` can give us but that increment looks suspicious.
> 
> >
> > Although it would be better as an 'add_option()' function.
> > I suspect the it used to be just strcat().
> > (similarly for root_nfs_copy() which is a pointless wrapper on strscpy()).
> >
> > Much more worth while would be fixing the sprintf() for NFS_ROOT.
> >
> > David
> >  
> > >       return 0;
> > >  }  
> >  
> 
> Justin