Hi,
On 5/17/20 4:28 PM, Andre Przywara wrote:
> GCC 10.1 generates a warning in net/ip/csum.c about exceeding a buffer
> limit in a memcpy operation:
> ------------------
> In function ‘memcpy’,
> inlined from ‘uip_csum_udp’ at net/uip/csum.c:58:3:
> /usr/include/aarch64-linux-gnu/bits/string_fortified.h:34:10: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
> 34 | return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> In file included from net/uip/csum.c:1:
> net/uip/csum.c: In function ‘uip_csum_udp’:
> include/kvm/uip.h:132:6: note: at offset 0 to object ‘sport’ with size 2 declared here
> 132 | u16 sport;
> ------------------
When I apply this patch, I get unrecognized characters:
In function <E2><80><98>memcpy<E2><80><99>,
inlined from <E2><80><98>uip_csum_udp<E2><80><99> at net/uip/csum.c:58:3:
/usr/include/aarch64-linux-gnu/bits/string_fortified.h:34:10: error: writing 1
byte into a region of size 0 [-Werror=stringop-overflow=]
34 | return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from net/uip/csum.c:1:
net/uip/csum.c: In function <E2><80><98>uip_csum_udp<E2><80><99>:
include/kvm/uip.h:132:6: note: at offset 0 to object
<E2><80><98>sport<E2><80><99> with size 2 declared here
132 | u16 sport;
I looked at the patch source, and they're there also
>
> This warning originates from the code taking the address of the "sport"
> member, then using that with some pointer arithmetic in a memcpy call.
> GCC now sees that the object is only a u16, so copying 12 bytes into it
> cannot be any good.
> It's somewhat debatable whether this is a legitimate warning, as there
> is enough storage at that place, and we knowingly use the struct and
> its variabled-sized member at the end.
>
> However we can also rewrite the code, to not abuse the "&" operation of
> some *member*, but take the address of the struct itself.
> This makes the code less dodgy, and indeed appeases GCC 10.
>
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> Reported-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
I've tested the patch and the compile errors have gone away for x86 and arm64.
Tested virtio-net on both architectures and it works just like before:
Tested-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> ---
> net/uip/csum.c | 26 ++++++++++++--------------
> 1 file changed, 12 insertions(+), 14 deletions(-)
>
> diff --git a/net/uip/csum.c b/net/uip/csum.c
> index 7ca8bada..607c9f1c 100644
> --- a/net/uip/csum.c
> +++ b/net/uip/csum.c
> @@ -37,7 +37,7 @@ u16 uip_csum_udp(struct uip_udp *udp)
> struct uip_pseudo_hdr hdr;
> struct uip_ip *ip;
> int udp_len;
> - u8 *pad;
> + u8 *udp_hdr = (u8*)udp + offsetof(struct uip_udp, sport);
Super minor nitpick: there should be a space between u8 and *.
>
> ip = &udp->ip;
>
> @@ -50,13 +50,12 @@ u16 uip_csum_udp(struct uip_udp *udp)
> udp_len = uip_udp_len(udp);
>
> if (udp_len % 2) {
> - pad = (u8 *)&udp->sport + udp_len;
> - *pad = 0;
> - memcpy((u8 *)&udp->sport + udp_len + 1, &hdr, sizeof(hdr));
> - return uip_csum(0, (u8 *)&udp->sport, udp_len + 1 + sizeof(hdr));
> + udp_hdr[udp_len] = 0; /* zero padding */
> + memcpy(udp_hdr + udp_len + 1, &hdr, sizeof(hdr));
> + return uip_csum(0, udp_hdr, udp_len + 1 + sizeof(hdr));
> } else {
> - memcpy((u8 *)&udp->sport + udp_len, &hdr, sizeof(hdr));
> - return uip_csum(0, (u8 *)&udp->sport, udp_len + sizeof(hdr));
> + memcpy(udp_hdr + udp_len, &hdr, sizeof(hdr));
> + return uip_csum(0, udp_hdr, udp_len + sizeof(hdr));
> }
>
> }
> @@ -66,7 +65,7 @@ u16 uip_csum_tcp(struct uip_tcp *tcp)
> struct uip_pseudo_hdr hdr;
> struct uip_ip *ip;
> u16 tcp_len;
> - u8 *pad;
> + u8 *tcp_hdr = (u8*)tcp + offsetof(struct uip_tcp, sport);
>
> ip = &tcp->ip;
> tcp_len = ntohs(ip->len) - uip_ip_hdrlen(ip);
> @@ -81,12 +80,11 @@ u16 uip_csum_tcp(struct uip_tcp *tcp)
> pr_warning("tcp_len(%d) is too large", tcp_len);
>
> if (tcp_len % 2) {
> - pad = (u8 *)&tcp->sport + tcp_len;
> - *pad = 0;
> - memcpy((u8 *)&tcp->sport + tcp_len + 1, &hdr, sizeof(hdr));
> - return uip_csum(0, (u8 *)&tcp->sport, tcp_len + 1 + sizeof(hdr));
> + tcp_hdr[tcp_len] = 0; /* zero padding */
> + memcpy(tcp_hdr + tcp_len + 1, &hdr, sizeof(hdr));
> + return uip_csum(0, tcp_hdr, tcp_len + 1 + sizeof(hdr));
> } else {
> - memcpy((u8 *)&tcp->sport + tcp_len, &hdr, sizeof(hdr));
> - return uip_csum(0, (u8 *)&tcp->sport, tcp_len + sizeof(hdr));
> + memcpy(tcp_hdr + tcp_len, &hdr, sizeof(hdr));
> + return uip_csum(0, tcp_hdr, tcp_len + sizeof(hdr));
> }
> }
The patch looks functionally identical to the original version, and slightly
easier to understand:
Reviewed-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
