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

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Date:   Fri, 17 Apr 2020 10:50:01 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Marion & Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     selvin.xavier@...adcom.com, devesh.sharma@...adcom.com,
        dledford@...hat.com, leon@...nel.org, colin.king@...onical.com,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] RDMA/ocrdma: Fix an off-by-one issue in 'ocrdma_add_stat'

On Fri, Apr 17, 2020 at 03:28:21PM +0200, Marion & Christophe JAILLET wrote:
> 
> Le 14/04/2020 à 20:34, Jason Gunthorpe a écrit :
> > 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@...adoo.fr>
> > > 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
> > > +++ 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.

So it needsa *pcur = 0 in the error path?

Jason