2020-04-29 19:03:58

by Arnd Bergmann

Subject: [PATCH] netfilter: nf_osf: avoid passing pointer to local var

gcc-10 points out that a code path exists where a pointer to a stack
variable may be passed back to the caller:

net/netfilter/nfnetlink_osf.c: In function 'nf_osf_hdr_ctx_init':
cc1: warning: function may return address of local variable [-Wreturn-local-addr]
net/netfilter/nfnetlink_osf.c:171:16: note: declared here
171 | struct tcphdr _tcph;
| ^~~~~

I am not sure whether this can happen in practice, but moving the
variable declaration into the callers avoids the problem.

Fixes: 31a9c29210e2 ("netfilter: nf_osf: add struct nf_osf_hdr_ctx")
Signed-off-by: Arnd Bergmann <[email protected]>
---
net/netfilter/nfnetlink_osf.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/netfilter/nfnetlink_osf.c b/net/netfilter/nfnetlink_osf.c
index 9f5dea0064ea..916a3c7f9eaf 100644
--- a/net/netfilter/nfnetlink_osf.c
+++ b/net/netfilter/nfnetlink_osf.c
@@ -165,12 +165,12 @@ static bool nf_osf_match_one(const struct sk_buff *skb,
static const struct tcphdr *nf_osf_hdr_ctx_init(struct nf_osf_hdr_ctx *ctx,
const struct sk_buff *skb,
const struct iphdr *ip,
- unsigned char *opts)
+ unsigned char *opts,
+ struct tcphdr *_tcph)
{
const struct tcphdr *tcp;
- struct tcphdr _tcph;

- tcp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(struct tcphdr), &_tcph);
+ tcp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(struct tcphdr), _tcph);
if (!tcp)
return NULL;

@@ -205,10 +205,11 @@ nf_osf_match(const struct sk_buff *skb, u_int8_t family,
int fmatch = FMATCH_WRONG;
struct nf_osf_hdr_ctx ctx;
const struct tcphdr *tcp;
+ struct tcphdr _tcph;

memset(&ctx, 0, sizeof(ctx));

- tcp = nf_osf_hdr_ctx_init(&ctx, skb, ip, opts);
+ tcp = nf_osf_hdr_ctx_init(&ctx, skb, ip, opts, &_tcph);
if (!tcp)
return false;

@@ -265,10 +266,11 @@ bool nf_osf_find(const struct sk_buff *skb,
const struct nf_osf_finger *kf;
struct nf_osf_hdr_ctx ctx;
const struct tcphdr *tcp;
+ struct tcphdr _tcph;

memset(&ctx, 0, sizeof(ctx));

- tcp = nf_osf_hdr_ctx_init(&ctx, skb, ip, opts);
+ tcp = nf_osf_hdr_ctx_init(&ctx, skb, ip, opts, &_tcph);
if (!tcp)
return false;

--
2.26.0

2020-04-29 19:14:59

by Florian Westphal

Subject: Re: [PATCH] netfilter: nf_osf: avoid passing pointer to local var

Arnd Bergmann <[email protected]> wrote:
> gcc-10 points out that a code path exists where a pointer to a stack
> variable may be passed back to the caller:
>
> net/netfilter/nfnetlink_osf.c: In function 'nf_osf_hdr_ctx_init':
> cc1: warning: function may return address of local variable [-Wreturn-local-addr]
> net/netfilter/nfnetlink_osf.c:171:16: note: declared here
> 171 | struct tcphdr _tcph;
> | ^~~~~
>
> I am not sure whether this can happen in practice, but moving the
> variable declaration into the callers avoids the problem.

LGTM, thanks Arnd.

Reviewed-by: Florian Westphal <[email protected]>

2020-04-29 19:19:14

by Pablo Neira Ayuso

Subject: Re: [PATCH] netfilter: nf_osf: avoid passing pointer to local var

On Wed, Apr 29, 2020 at 09:00:41PM +0200, Arnd Bergmann wrote:
> gcc-10 points out that a code path exists where a pointer to a stack
> variable may be passed back to the caller:
>
> net/netfilter/nfnetlink_osf.c: In function 'nf_osf_hdr_ctx_init':
> cc1: warning: function may return address of local variable [-Wreturn-local-addr]
> net/netfilter/nfnetlink_osf.c:171:16: note: declared here
> 171 | struct tcphdr _tcph;
> | ^~~~~
>
> I am not sure whether this can happen in practice, but moving the
> variable declaration into the callers avoids the problem.

Applied, thanks.