Re: [PATCH] nss_dns: Check for proper A/AAAA address alignment

[Florian Weimer <fweimer at redhat dot com>]

* Carlos O'Donell:

> On 5/16/19 1:18 PM, Florian Weimer wrote:
>> 2019-05-16  Florian Weimer  <fweimer@redhat.com>
>> 
>> 	* resolv/nss_dns/dns-host.c (getanswer_r): Be more explicit about
>> 	struct in_addr/struct in6_addr alignment.
>> 
>
> Can we use standard macros to compute alignmnet and differences, it
> makes the code more easy to read at a glance.

I want to convert this to struct alloc_buffer eventually, then this will
go away anyway.  I'm trying to post smaller patches towards this goal.
These changes are really hard to review unfortunately.

As a stop-gap measure, I've changed the code to use PTR_ALIGN_UP.

>> diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
>> index 9c15f25f28..9c40051aff 100644
>> --- a/resolv/nss_dns/dns-host.c
>> +++ b/resolv/nss_dns/dns-host.c
>> @@ -947,8 +947,15 @@ getanswer_r (struct resolv_context *ctx,
>>  	      linebuflen -= nn;
>>  	    }
>>  
>> -	  linebuflen -= sizeof (align) - ((u_long) bp % sizeof (align));
>> -	  bp += sizeof (align) - ((u_long) bp % sizeof (align));
>> +	  /* Provide sufficient alignment for both address
>> +	     families.  */
>> +	  enum { align = 4 };
>> +	  _Static_assert ((align % __alignof__ (struct in_addr)) == 0,
>> +			  "struct in_addr alignment");
>> +	  _Static_assert ((align % __alignof__ (struct in6_addr)) == 0,
>> +			  "struct in6_addr alignment");
>
> OK.
>
>> +	  linebuflen -= align - ((u_long) bp % align);
>> +	  bp += align - ((u_long) bp % align);
>
> Is the use case common enough to add something to libc-pointer-arith.h
> to handle linebuflen adjustment?

Yes, see struct alloc_buffer.

> align_diff = ALIGN_UP_DIFF (bp, align);
> linebuflen -= align_diff;
> bp += align_diff;
>
> If not then can we still use ALIGN_UP? It makes it immediately
> obvious the intent was to align the pointer upwards and adjust
> the length of the remaining buffer.

This lacks overflow checking.  It is not a good coding pattern in my
opinion.

Thanks,
Florian

nss_dns: Check for proper A/AAAA address alignment

2019-05-24  Florian Weimer  <fweimer@redhat.com>

	* resolv/nss_dns/dns-host.c (getanswer_r): Be more explicit about
	struct in_addr/struct in6_addr alignment.

diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
index 9c15f25f28..5af47fd10d 100644
--- a/resolv/nss_dns/dns-host.c
+++ b/resolv/nss_dns/dns-host.c
@@ -78,6 +78,7 @@
 #include <stdlib.h>
 #include <stddef.h>
 #include <string.h>
+#include <libc-pointer-arith.h>
 
 #include "nsswitch.h"
 #include <arpa/nameser.h>
@@ -947,8 +948,18 @@ getanswer_r (struct resolv_context *ctx,
 	      linebuflen -= nn;
 	    }
 
-	  linebuflen -= sizeof (align) - ((u_long) bp % sizeof (align));
-	  bp += sizeof (align) - ((u_long) bp % sizeof (align));
+	  /* Provide sufficient alignment for both address
+	     families.  */
+	  enum { align = 4 };
+	  _Static_assert ((align % __alignof__ (struct in_addr)) == 0,
+			  "struct in_addr alignment");
+	  _Static_assert ((align % __alignof__ (struct in6_addr)) == 0,
+			  "struct in6_addr alignment");
+	  {
+	    char *new_bp = PTR_ALIGN_UP (bp, align);
+	    linebuflen -= new_bp - bp;
+	    bp = new_bp;
+	  }
 
 	  if (__glibc_unlikely (n > linebuflen))
 	    goto too_small;