Re: [PATCH] RDMA/ocrdma: Fix an off-by-one issue in 'ocrdma_add_stat'

[Marion & Christophe JAILLET]

Le 14/04/2020 Ã 20:34, Jason Gunthorpe a ÃcritÂ:
> On Sat, Mar 28, 2020 at 08:30:40AM +0100, Christophe JAILLET wrote:
> > There is an off-by-one issue when checking if there is enough space in the
> > output buffer, because we must keep some place for a final '\0'.
> >
> > While at it:
> >     - Use 'scnprintf' instead of 'snprintf' in order to avoid a superfluous
> >      'strlen'
> >     - avoid some useless initializations
> >     - avoida hard coded buffer size that can be computed at built time.
> >
> > Fixes: a51f06e1679e ("RDMA/ocrdma: Query controller information")
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> > ---
> > The '\0' comes from memset(..., 0, ...) in all callers.
> > This could be also avoided if needed.
> > ---
> >   drivers/infiniband/hw/ocrdma/ocrdma_stats.c | 9 ++++-----
> >   1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/infiniband/hw/ocrdma/ocrdma_stats.c b/drivers/infiniband/hw/ocrdma/ocrdma_stats.c
> > index 5f831e3bdbad..614a449e6b87 100644
> > --- a/drivers/infiniband/hw/ocrdma/ocrdma_stats.c
> > +++ b/drivers/infiniband/hw/ocrdma/ocrdma_stats.c
> > @@ -49,13 +49,12 @@ static struct dentry *ocrdma_dbgfs_dir;
> >   static int ocrdma_add_stat(char *start, char *pcur,
> >   				char *name, u64 count)
> >   {
> > -	char buff[128] = {0};
> > -	int cpy_len = 0;
> > +	char buff[128];
> > +	int cpy_len;
> >   
> > -	snprintf(buff, 128, "%s: %llu\n", name, count);
> > -	cpy_len = strlen(buff);
> > +	cpy_len = scnprintf(buff, sizeof(buff), "%s: %llu\n", name, count);
> >   
> > -	if (pcur + cpy_len > start + OCRDMA_MAX_DBGFS_MEM) {
> > +	if (pcur + cpy_len >= start + OCRDMA_MAX_DBGFS_MEM) {
> >   		pr_err("%s: No space in stats buff\n", __func__);
> >   		return 0;
> >   	}
> The memcpy is still kind of silly right? What about this:
>
> static int ocrdma_add_stat(char *start, char *pcur, char *name, u64 count)
> {
> 	size_t len = (start + OCRDMA_MAX_DBGFS_MEM) - pcur;
> 	int cpy_len;
>
> 	cpy_len = snprintf(pcur, len, "%s: %llu\n", name, count);
> 	if (cpy_len >= len || cpy_len < 0) {
> 		pr_err("%s: No space in stats buff\n", __func__);
> 		return 0;
> 	}
> 	return cpy_len;
> }
>
> Jason

It can looks useless, but I think that the goal was to make sure that we 
would not display truncated data. Each line is either complete or absent.

I don't have any strong opinion of what is best, but I can understand 
the current logic.

This function is not a hot spot, so useless memcpy is not a big issue.

CJ