Re: [PATCH v5] Fix misaligned accesses to fields in HEADER struct defined in <arpa/nameser_compat.h>

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 20/03/2022 18:46, John David Anglin wrote:
> resolv: Fix unaligned accesses to fields in HEADER struct
> 
> The structure HEADER is normally aligned to a word boundary but
> sometimes it needs to be accessed when aligned on a byte boundary.
> This change defines a new typedef, UHEADER, with alignment 1.
> It is used to ensure the fields are accessed with byte loads and
> stores when necessary.
> 
> V4: Change to res_mkquery.c deleted.  Small whitespace fix.
> 
> V5: Move UHEADER typedef to resolv/resolv-internal.h.  Replace all
> HEADER usage with UHEADER in resolv/res_send.c.
> 
> Dave

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
> 
> diff --git a/resolv/res_query.c b/resolv/res_query.c
> index 5d0a68dc81..3b5c604261 100644
> --- a/resolv/res_query.c
> +++ b/resolv/res_query.c
> @@ -112,8 +112,8 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  		     int *nanswerp2, int *resplen2, int *answerp2_malloced)
>  {
>  	struct __res_state *statp = ctx->resp;
> -	HEADER *hp = (HEADER *) answer;
> -	HEADER *hp2;
> +	UHEADER *hp = (UHEADER *) answer;
> +	UHEADER *hp2;
>  	int n, use_malloc = 0;
>  
>  	size_t bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * QUERYSIZE;
> @@ -217,7 +217,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  
>  	if (answerp != NULL)
>  	  /* __res_context_send might have reallocated the buffer.  */
> -	  hp = (HEADER *) *answerp;
> +	  hp = (UHEADER *) *answerp;
>  
>  	/* We simplify the following tests by assigning HP to HP2 or
>  	   vice versa.  It is easy to verify that this is the same as
> @@ -228,7 +228,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  	  }
>  	else
>  	  {
> -	    hp2 = (HEADER *) *answerp2;
> +	    hp2 = (UHEADER *) *answerp2;
>  	    if (n < (int) sizeof (HEADER))
>  	      {
>  	        hp = hp2;
> @@ -338,7 +338,7 @@ __res_context_search (struct resolv_context *ctx,
>  {
>  	struct __res_state *statp = ctx->resp;
>  	const char *cp;
> -	HEADER *hp = (HEADER *) answer;
> +	UHEADER *hp = (UHEADER *) answer;
>  	char tmp[NS_MAXDNAME];
>  	u_int dots;
>  	int trailing_dot, ret, saved_herrno;
> diff --git a/resolv/res_send.c b/resolv/res_send.c
> index 5d6be4b82d..d6c85fd7a2 100644
> --- a/resolv/res_send.c
> +++ b/resolv/res_send.c
> @@ -245,7 +245,7 @@ static void
>  mask_ad_bit (struct resolv_context *ctx, void *buf)
>  {
>    if (!(ctx->resp->options & RES_TRUSTAD))
> -    ((HEADER *) buf)->ad = 0;
> +    ((UHEADER *) buf)->ad = 0;
>  }
>  
>  int
> @@ -560,9 +560,9 @@ send_vc(res_state statp,
>  	int *terrno, int ns, u_char **anscp, u_char **ansp2, int *anssizp2,
>  	int *resplen2, int *ansp2_malloced)
>  {
> -	const HEADER *hp = (HEADER *) buf;
> -	const HEADER *hp2 = (HEADER *) buf2;
> -	HEADER *anhp = (HEADER *) *ansp;
> +	const UHEADER *hp = (UHEADER *) buf;
> +	const UHEADER *hp2 = (UHEADER *) buf2;
> +	UHEADER *anhp = (UHEADER *) *ansp;
>  	struct sockaddr *nsap = __res_get_nsaddr (statp, ns);
>  	int truncating, connreset, n;
>  	/* On some architectures compiler might emit a warning indicating
> @@ -697,7 +697,7 @@ send_vc(res_state statp,
>  		thisansp = ansp2;
>  		thisresplenp = resplen2;
>  	}
> -	anhp = (HEADER *) *thisansp;
> +	anhp = (UHEADER *) *thisansp;
>  
>  	*thisresplenp = rlen;
>  	/* Is the answer buffer too small?  */
> @@ -718,7 +718,7 @@ send_vc(res_state statp,
>  			*thisansp = newp;
>  			if (thisansp == ansp2)
>  			  *ansp2_malloced = 1;
> -			anhp = (HEADER *) newp;
> +			anhp = (UHEADER *) newp;
>  			/* A uint16_t can't be larger than MAXPACKET
>  			   thus it's safe to allocate MAXPACKET but
>  			   read RLEN bytes instead.  */
> @@ -925,8 +925,8 @@ send_dg(res_state statp,
>  	int *terrno, int ns, int *v_circuit, int *gotsomewhere, u_char **anscp,
>  	u_char **ansp2, int *anssizp2, int *resplen2, int *ansp2_malloced)
>  {
> -	const HEADER *hp = (HEADER *) buf;
> -	const HEADER *hp2 = (HEADER *) buf2;
> +	const UHEADER *hp = (UHEADER *) buf;
> +	const UHEADER *hp2 = (UHEADER *) buf2;
>  	struct timespec now, timeout, finish;
>  	struct pollfd pfd[1];
>  	int ptimeout;
> @@ -1168,7 +1168,7 @@ send_dg(res_state statp,
>  		   MSG_TRUNC which is only available on Linux.  We
>  		   can abstract out the Linux-specific feature in the
>  		   future to detect truncation.  */
> -		HEADER *anhp = (HEADER *) *thisansp;
> +		UHEADER *anhp = (UHEADER *) *thisansp;
>  		socklen_t fromlen = sizeof(struct sockaddr_in6);
>  		assert (sizeof(from) <= fromlen);
>  		*thisresplenp = __recvfrom (pfd[0].fd, (char *) *thisansp,
> diff --git a/resolv/resolv-internal.h b/resolv/resolv-internal.h
> index 2fa57d394f..9d2e832d68 100644
> --- a/resolv/resolv-internal.h
> +++ b/resolv/resolv-internal.h
> @@ -27,6 +27,13 @@
>  #define RES_F_CONN      0x00000002 /* Socket is connected.  */
>  #define RES_F_EDNS0ERR  0x00000004 /* EDNS0 caused errors.  */
>  
> +/* The structure HEADER is normally aligned on a word boundary.  In
> +   some code, we need to access this structure when it may be aligned
> +   on a byte boundary.  To avoid unaligned accesses, we need a typedef
> +   with alignment one.  This ensures the fields are accessed with byte
> +   loads and stores.  */
> +typedef HEADER __attribute__ ((__aligned__(1))) UHEADER;
> +
>  /* Legacy function.  This needs to be removed once all NSS modules
>     have been adjusted.  */
>  static inline bool