diff mbox series

resolve: Remove __res_context_query alloca usage

Message ID 20230914175407.2406866-1-adhemerval.zanella@linaro.org
State Superseded
Headers show
Series resolve: Remove __res_context_query alloca usage | expand

Commit Message

Adhemerval Zanella Sept. 14, 2023, 5:54 p.m. UTC
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.  The default scratch_buffer
should fullfill the most usual allocation requirement.

Checked on x86_64-linux-gnu and i686-linux-gnu.
---
 resolv/res_query.c | 47 +++++++++++++++++++++++-----------------------
 1 file changed, 24 insertions(+), 23 deletions(-)

Comments

Florian Weimer Sept. 14, 2023, 8:37 p.m. UTC | #1
* Adhemerval Zanella:

> @@ -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;

The alignment code shouldn't be necessary anymore.

Thanks,
Florian
Joe Simmons-Talbott Sept. 19, 2023, 8:15 p.m. UTC | #2
On Thu, Sep 14, 2023 at 02:54:07PM -0300, Adhemerval Zanella wrote:
> 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.  The default scratch_buffer
> should fullfill the most usual allocation requirement.
> 
> Checked on x86_64-linux-gnu and i686-linux-gnu.

LGTM,

Reviewed-by: Joe Simmons-Talbott <josimmon@redhat.com>
> ---
>  resolv/res_query.c | 47 +++++++++++++++++++++++-----------------------
>  1 file changed, 24 insertions(+), 23 deletions(-)
> 
> 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);
> -- 
> 2.34.1
>
Adhemerval Zanella Sept. 20, 2023, 10:09 a.m. UTC | #3
On 14/09/23 17:37, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> @@ -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;
> 
> The alignment code shouldn't be necessary anymore.

Indeed, I will update the patch.
diff mbox series

Patch

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);