[PATCH] resolv/res_query: Add note indicating that alloca usage is safe.
Joe Simmons-Talbott
josimmon@redhat.com
Mon Aug 28 14:58:25 GMT 2023
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);
>
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