[PATCH] s390: Fix strrchr() implementation

[PATCH] s390: Fix strrchr() implementation

[Roberto Sassu]

Access the string at len - 1 instead of len.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 arch/s390/lib/string.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/s390/lib/string.c b/arch/s390/lib/string.c
index cfcdf76d6a95..162a391788ad 100644
--- a/arch/s390/lib/string.c
+++ b/arch/s390/lib/string.c
@@ -261,12 +261,12 @@ char *strrchr(const char *s, int c)
 {
        size_t len = __strend(s) - s;
 
-       if (len)
-	       do {
-		       if (s[len] == (char) c)
-			       return (char *) s + len;
-	       } while (--len > 0);
-       return NULL;
+	if (len)
+		do {
+			if (s[len - 1] == (char) c)
+				return (char *) s + len - 1;
+		} while (--len > 0);
+	return NULL;
 }
 EXPORT_SYMBOL(strrchr);
 #endif
-- 
2.32.0

Re: [PATCH] s390: Fix strrchr() implementation

[Heiko Carstens]

On Tue, Oct 05, 2021 at 09:26:21AM +0200, Roberto Sassu wrote:
> Access the string at len - 1 instead of len.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  arch/s390/lib/string.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/lib/string.c b/arch/s390/lib/string.c
> index cfcdf76d6a95..162a391788ad 100644
> --- a/arch/s390/lib/string.c
> +++ b/arch/s390/lib/string.c
> @@ -261,12 +261,12 @@ char *strrchr(const char *s, int c)
>  {
>         size_t len = __strend(s) - s;
>  
> -       if (len)
> -	       do {
> -		       if (s[len] == (char) c)
> -			       return (char *) s + len;
> -	       } while (--len > 0);
> -       return NULL;
> +	if (len)
> +		do {
> +			if (s[len - 1] == (char) c)
> +				return (char *) s + len - 1;
> +		} while (--len > 0);
> +	return NULL;

You missed to tell what this is supposed to fix. The patch however is
incorrect: the terminating null byte is considered part of the
string. With your patch strrchr(somestring, 0) would not work
correctly anymore.

However our strrchr implementation is indeed broken, since for an
empty string and searching for the null byte would incorrectly return
NULL. Luckily there is not a single invocation in the kernel which
doing that.

RE: [PATCH] s390: Fix strrchr() implementation

[Roberto Sassu]

> From: Heiko Carstens [mailto:hca@linux.ibm.com]
> Sent: Tuesday, October 5, 2021 1:13 PM
> On Tue, Oct 05, 2021 at 09:26:21AM +0200, Roberto Sassu wrote:
> > Access the string at len - 1 instead of len.
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > ---
> >  arch/s390/lib/string.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/s390/lib/string.c b/arch/s390/lib/string.c
> > index cfcdf76d6a95..162a391788ad 100644
> > --- a/arch/s390/lib/string.c
> > +++ b/arch/s390/lib/string.c
> > @@ -261,12 +261,12 @@ char *strrchr(const char *s, int c)
> >  {
> >         size_t len = __strend(s) - s;
> >
> > -       if (len)
> > -	       do {
> > -		       if (s[len] == (char) c)
> > -			       return (char *) s + len;
> > -	       } while (--len > 0);
> > -       return NULL;
> > +	if (len)
> > +		do {
> > +			if (s[len - 1] == (char) c)
> > +				return (char *) s + len - 1;
> > +		} while (--len > 0);
> > +	return NULL;
> 
> You missed to tell what this is supposed to fix. The patch however is
> incorrect: the terminating null byte is considered part of the
> string. With your patch strrchr(somestring, 0) would not work
> correctly anymore.

Hi Heiko

yes, sorry. I didn't consider that.

> However our strrchr implementation is indeed broken, since for an
> empty string and searching for the null byte would incorrectly return
> NULL. Luckily there is not a single invocation in the kernel which
> doing that.

Ok. However, the main reason of this patch is that s[0] is not
evaluated, when len > 0. I will provide a new version of the patch.

Thanks

Roberto

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