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From: "André Przywara" <andre.przywara@arm.com>
To: Alexandru Elisei <alexandru.elisei@arm.com>,
	Will Deacon <will@kernel.org>,
	Julien Thierry <julien.thierry.kdev@gmail.com>
Cc: kvm@vger.kernel.org
Subject: Re: [PATCH kvmtool] net: uip: Fix GCC 10 warning about checksum calculation
Date: Mon, 18 May 2020 14:01:30 +0100	[thread overview]
Message-ID: <ff1a0be5-dfa2-3702-a828-89de1d954f17@arm.com> (raw)
In-Reply-To: <53980bb2-fb9b-886e-2a2a-c4301f50e4fe@arm.com>

On 18/05/2020 12:29, Alexandru Elisei wrote:
> 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

Mmh, I see that GCC uses UTF-8 to render fancy ticks - for whatever
reason ;-) Removed them to avoid issues.

But where did you see those broken characters? Not sure if a non-UTF-8
capable terminal is still considered a thing in 2020?
I mean, even my Slackware is not ISO-8859-15 anymore ;-)

Fixed the rest and sent a v2.

Cheers,
Andre

>>
>> 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@arm.com>
>> Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
> 
> 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@arm.com>
> 
>> ---
>>  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@arm.com>
> 


      reply	other threads:[~2020-05-18 13:02 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-17 15:28 [PATCH kvmtool] net: uip: Fix GCC 10 warning about checksum calculation Andre Przywara
2020-05-18 11:29 ` Alexandru Elisei
2020-05-18 13:01   ` André Przywara [this message]

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