Re: [PATCH] resolv/res_query: Add note indicating that alloca usage is safe.

[Joe Simmons-Talbott <josimmon@redhat.com>]

On Mon, Aug 28, 2023 at 10:50:21AM -0300, Adhemerval Zanella Netto wrote:
> 
> 
> On 05/07/23 15:13, Joe Simmons-Talbott via Libc-alpha wrote:
> > The buffer size is small (< 1024) and fixed sized so alloca is safe
> > here.
> > ---
> >  resolv/res_query.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/resolv/res_query.c b/resolv/res_query.c
> > index 049de91b95..0e0e7be624 100644
> > --- a/resolv/res_query.c
> > +++ b/resolv/res_query.c
> > @@ -117,6 +117,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> >  	int n, use_malloc = 0;
> >  
> >  	size_t bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * QUERYSIZE;
> > +	/* alloca is safe here since bufsize < 1024 and fixed sized. */
> >  	u_char *buf = alloca (bufsize);
> 
> The bufsize on current Linux build is:
> 
>    size_t bufsize = (type == 439963904 ? 2 : 1) * (12 + 4 + 255 + 1);
> 
> So with upper bound as 544 (2 * (12 + 4 + 255 + 1)).  However, it might
> increase to 2 * PACKETSIZE later with malloc.  This is exactly the scenarion
> scratch_buffer was created, so maybe we should use it.  Below a complete
> untested patch:

Thanks for the patch.  I've tested it with 'make check' on
x86_64-linux-gnu.  I'm not sure how to handle this and don't feel
comfortable submitting it on your behalf or as my own work but am happy
to give a Reviewed-by once there's a commit message.

Thanks,
Joe
> 
> diff --git a/resolv/res_query.c b/resolv/res_query.c
> index 049de91b95..b234db83c1 100644
> --- a/resolv/res_query.c
> +++ b/resolv/res_query.c
> @@ -80,6 +80,7 @@
>  #include <stdlib.h>
>  #include <string.h>
>  #include <shlib-compat.h>
> +#include <scratch_buffer.h>
>  
>  #if PACKETSZ > 65536
>  #define MAXPACKET	PACKETSZ
> @@ -114,11 +115,14 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  	struct __res_state *statp = ctx->resp;
>  	UHEADER *hp = (UHEADER *) answer;
>  	UHEADER *hp2;
> -	int n, use_malloc = 0;
> -
> -	size_t bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * QUERYSIZE;
> -	u_char *buf = alloca (bufsize);
> -	u_char *query1 = buf;
> +	int n;
> +
> +	/* It requires 2 times QUERYSIZE for type == T_QUERY_A_AND_AAAA.  */
> +	struct scratch_buffer buf;
> +	scratch_buffer_init (&buf);
> +	_Static_assert (2 * QUERYSIZE <= sizeof (buf.__space.__c),
> +			"scratch_buffer too small");
> +	u_char *query1 = buf.data;
>  	int nquery1 = -1;
>  	u_char *query2 = NULL;
>  	int nquery2 = 0;
> @@ -129,14 +133,14 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  	if (type == T_QUERY_A_AND_AAAA)
>  	  {
>  	    n = __res_context_mkquery (ctx, QUERY, name, class, T_A, NULL,
> -				       query1, bufsize);
> +				       query1, buf.length);
>  	    if (n > 0)
>  	      {
>  		if ((statp->options & (RES_USE_EDNS0|RES_USE_DNSSEC)) != 0)
>  		  {
>  		    /* Use RESOLV_EDNS_BUFFER_SIZE because the receive
>  		       buffer can be reallocated.  */
> -		    n = __res_nopt (ctx, n, query1, bufsize,
> +		    n = __res_nopt (ctx, n, query1, buf.length,
>  				    RESOLV_EDNS_BUFFER_SIZE);
>  		    if (n < 0)
>  		      goto unspec_nomem;
> @@ -146,20 +150,20 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  		/* Align the buffer.  */
>  		int npad = ((nquery1 + __alignof__ (HEADER) - 1)
>  			    & ~(__alignof__ (HEADER) - 1)) - nquery1;
> -		if (n > bufsize - npad)
> +		if (n > buf.length - npad)
>  		  {
>  		    n = -1;
>  		    goto unspec_nomem;
>  		  }
>  		int nused = n + npad;
> -		query2 = buf + nused;
> +		query2 = buf.data + nused;
>  		n = __res_context_mkquery (ctx, QUERY, name, class, T_AAAA,
> -					   NULL, query2, bufsize - nused);
> +					   NULL, query2, buf.length - nused);
>  		if (n > 0
>  		    && (statp->options & (RES_USE_EDNS0|RES_USE_DNSSEC)) != 0)
>  		  /* Use RESOLV_EDNS_BUFFER_SIZE because the receive
>  		     buffer can be reallocated.  */
> -		  n = __res_nopt (ctx, n, query2, bufsize,
> +		  n = __res_nopt (ctx, n, query2, buf.length,
>  				  RESOLV_EDNS_BUFFER_SIZE);
>  		nquery2 = n;
>  	      }
> @@ -169,7 +173,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  	else
>  	  {
>  	    n = __res_context_mkquery (ctx, QUERY, name, class, type, NULL,
> -				       query1, bufsize);
> +				       query1, buf.length);
>  
>  	    if (n > 0
>  		&& (statp->options & (RES_USE_EDNS0|RES_USE_DNSSEC)) != 0)
> @@ -181,27 +185,25 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  		  advertise = anslen;
>  		else
>  		  advertise = RESOLV_EDNS_BUFFER_SIZE;
> -		n = __res_nopt (ctx, n, query1, bufsize, advertise);
> +		n = __res_nopt (ctx, n, query1, buf.length, advertise);
>  	      }
>  
>  	    nquery1 = n;
>  	  }
>  
> -	if (__glibc_unlikely (n <= 0) && !use_malloc) {
> +	if (__glibc_unlikely (n <= 0)) {
>  		/* Retry just in case res_nmkquery failed because of too
>  		   short buffer.  Shouldn't happen.  */
> -		bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * MAXPACKET;
> -		buf = malloc (bufsize);
> -		if (buf != NULL) {
> -			query1 = buf;
> -			use_malloc = 1;
> +		if (scratch_buffer_set_array_size (&buf,
> +						   T_QUERY_A_AND_AAAA ? 2 : 1,
> +						   MAXPACKET)) {
> +			query1 = buf.data;
>  			goto again;
>  		}
>  	}
>  	if (__glibc_unlikely (n <= 0))       {
>  		RES_SET_H_ERRNO(statp, NO_RECOVERY);
> -		if (use_malloc)
> -			free (buf);
> +		scratch_buffer_free (&buf);
>  		return (n);
>  	}
>  
> @@ -224,8 +226,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
>  				    answerp2_malloced);
>  	  }
>  
> -	if (use_malloc)
> -		free (buf);
> +	scratch_buffer_free (&buf);
>  	if (n < 0) {
>  		RES_SET_H_ERRNO(statp, TRY_AGAIN);
>  		return (n);
>